Method and apparatus which adds distortion to a signal to compensate for distortion added at a later stage by a nonlinear element

What is claimed is:

1. An apparatus for providing distortion to an input signal, comprising:

a first coupler which separates a first alternating current component from the input signal, to provide a first signal representing the first alternating current component and a second signal representing the input signal having the first alternating current component separated therefrom;

an adjusting unit which adjusts the signal level of the first and second signals;

a first distortion generating unit which provides nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal; and

a second coupler which combines the signal level adjusted second signal with the distorted first signal, to produce a corresponding output signal.

2. An apparatus as in claim 1, further comprising:

a transmission line which provides the input signal to the adjusting unit and the first coupler, the adjusting unit having an impedance which matches the impedance of the transmission line.

3. An apparatus as in claim 2, wherein the adjusting unit comprises a PIN attenuator for adjusting the signal level of the first and second signals.

4. An apparatus as in claim 1, wherein the adjusting unit comprises a PIN attenuator for adjusting the signal level of the first and second signals.

5. An apparatus as in claim 1, wherein

the adjusting unit adjusts a power level of power provided to the first distortion generating unit by adjusting the signal level of the first and second signals, an increase in the signal level of the second signal causing a decrease in the signal level of the first signal, and a decrease in the signal level of the second signal causing an increase in the signal level of the first signal, and

the adjusting unit adjusts said power level of power provided to the first distortion generating unit so that the provided power has frequency characteristics which oppose frequency characteristics of the distortion provided by the first distortion generating unit.

6. An apparatus as in claim 5, wherein

the distortion provided by the first distortion generating unit increases as the frequency of the first signal increases, and

the adjusting unit adjusts the power level provided to the first distortion generating unit so that the provided power decreases when the distortion provided by the first distortion generating unit increases, the decrease in power causing the first distortion generating unit to decrease the amount of provided distortion.

7. An apparatus as in claim 1, wherein the first distortion generating unit comprises:

a nonlinear element which receives a bias current and generates the nonlinear distortion in accordance with the received bias current; and

a current generator which generates and controls the bias current.

8. An apparatus as in claim 7, wherein the nonlinear element is a field-effect transistor having a drain and source connected together.

9. An apparatus as in claim 5, wherein the first distortion generating unit comprises:

a nonlinear element which receives a bias current and generates the nonlinear distortion in accordance with the received bias current; and

a current generator which generates and controls the bias current.

10. An apparatus as in claim 9, wherein the nonlinear element is a field-effect transistor having a drain and source connected together.

11. An apparatus as in claim 7, wherein the nonlinear element is a diode.

12. An apparatus as in claim 9, wherein the nonlinear element is a diode.

13. An apparatus as in claim 7, wherein the nonlinear element has an input terminal and an output terminal, and the first distortion generating unit further comprises:

a first inductor connected to the input terminal of the nonlinear element; and

a second inductor connected to the output terminal of the nonlinear element.

14. An apparatus as in claim 1, wherein the first and second couplers each comprise a capacitor.

15. An apparatus as in claim 1, wherein the first and second couplers are capacitors connecting the adjusting unit to the first distortion generating unit.

16. An apparatus as in claim 1, further comprising:

an attenuator which adjusts the signal level of the input signal before the first alternating current component is separated from the input signal by the first coupler.

17. An apparatus as in claim 1, further comprising:

a third coupler which separates a second alternating current component from the input signal, to provide a third signal representing the second alternating current component, the second signal representing the input signal with the first and second alternating current components separated therefrom, and the adjusting unit adjusts the signal level of the first, second and third signals;

a second distortion generating unit which provides nonlinear distortion to the signal level adjusted third signal, to produce a distorted third signal; and

a fourth coupler which combines the signal level adjusted second signal with the distorted third signal.

18. An apparatus as in claim 17, wherein the first and second distortion generating units together operate to cancel even-ordinal distortion and thereby produce only odd-ordinal nonlinear distortion.

19. An apparatus as in claim 17, further comprising:

an attenuator which adjusts the signal level of the input signal before the first and second alternating current component are separated from the input signal.

20. An apparatus as in claim 19, wherein the first and second distortion generating units together operate to cancel even-ordinal distortion and thereby produce only odd-ordinal nonlinear distortion.

21. An apparatus as in claim 1, wherein the first and second couplers, the adjusting unit and the first distortion generating unit together form a second and third order distortion unit which provides second and third order distortion to the input signal, to produce a second and third order distorted output signal, the apparatus further comprising:

an attenuator which adjusts the signal level of the input signal before the first alternating current component is separated from the input signal, to allow the second and third order distortion unit to produce a second and third order distorted output signal having a suppressed third order component; and

a third order distortion generating unit which provides third order distortion to the second and third order distorted output signal.

22. An apparatus as in claim 21, wherein the third order distortion generating unit comprises:

an amplifier which amplifies the second and third order distorted output signal;

couplers which separate a first alternating current component and a second alternating current component from the amplified second and third order distorted output signal, to provide a first signal representing the first alternating current component, a second signal representing the second alternating current component, and a third signal representing the second and third order distorted output signal having the first and second alternating current components separated therefrom;

an adjusting unit which adjusts the signal level of the first, second, and third signals;

a first distortion generating unit which provides nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal;

a second distortion generating unit which provides nonlinear distortion to the signal level adjusted second signal, to produce a distorted second signal; and

couplers which combines the signal level adjusted third signal with the distorted first and second signals, to produce a corresponding output signal.

23. An apparatus for providing distortion to an input signal, comprising:

a first attenuator which attenuates the signal level of the input signal;

a first coupler which separates an alternating current component from the attenuated input signal, to provide a first signal representing the alternating current component and a second signal representing the input signal having the alternating current component separated therefrom;

a second attenuator which attenuates the signal level of the first and second signals;

a distortion generating unit which provides nonlinear distortion to the signal level attenuated first signal, to produce a distorted first signal; and

a second coupler which combines the signal level attenuated second signal with the distorted first signal.

24. A method for providing distortion to an input signal, comprising the steps of:

separating a first alternating current component from the input signal, to provide a first signal representing the first alternating current component and a second signal representing the input signal having the first alternating current component separated therefrom;

adjusting the signal level of the first and second signals;

providing nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal; and

combining the signal level adjusted second signal with the distorted first signal, to produce a corresponding output signal.

25. A method as in claim 24, further comprising the steps of:

separating a second alternating current component from the input signal, to provide a third signal representing the second alternating current component, the second signal representing the input signal with the first and second alternating current components separated therefrom;

adjusting the signal level of the third signal;

providing nonlinear distortion to the signal level adjusted third signal, to produce a distorted third signal; and

combining the signal level adjusted second signal with the distorted third signal.

26. A method as in claim 25, wherein the steps of providing nonlinear distortion to the signal level adjusted first signal and providing nonlinear distortion to the signal level adjusted third signal, together operate to cancel even-ordinal distortion and thereby produce only odd-ordinal nonlinear distortion.

27. A method for providing distortion to an input signal, comprising the steps of:

generating second order distortion and providing the generated second order distortion to the input signal while suppressing third order distortion, to produce a second order distorted output signal; and

generating distortion and cancelling even-ordinal components of the generated distortion to generate third order distortion, and providing the third order distortion to the second order distorted output signal, to produce an output signal having second and third order distortion.

28. A method claim 27, wherein the step of generating second order distortion comprises the steps of:

attenuating the signal level of the input signal;

separating an alternating current component from the attenuated input signal, to provide a first signal representing the alternating current component and a second signal representing the input signal having the alternating current component separated therefrom;

adjusting the signal level of the first and second signals;

providing nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal; and

combining the signal level adjusted second signal with the distorted first signal, to produce the second order distorted output signal.

29. A method as in claim 28, wherein the step of generating distortion and cancelling even-ordinal components of the generated distortion to generate third order distortion comprises the steps of:

amplifying the second order distorted output signal;

separating a first alternating current component and a second alternating current component from the amplified second order distorted output signal, to provide a first signal representing the first alternating current component, a second signal representing the second alternating current component, and a third signal representing the second order distorted output signal having the first and second alternating current components separated therefrom;

adjusting the signal level of the first, second, and third signals;

providing nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal;

providing nonlinear distortion to the signal level adjusted second signal, to produce a distorted second signal; and

combining the signal level adjusted third signal with the distorted first and second signals, to produce a corresponding output signal.

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CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims priority to, Japanese patent application 08-052136, filed Mar. 8, 1996, in Japan, and which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for adding distortion to a signal, to thereby compensate for distortion affecting the signal at a later stage by a nonlinear element.

2. Description of the Related Art

Nonlinear elements, such as amplifiers and laser circuits, have nonlinear characteristics which can produce nonlinear distortion. Devices are available which compensate for such nonlinear distortion.

For example, a predistorter system can be used to compensate for nonlinear distortion. Generally, with a predistorter system, a nonlinear circuit is positioned before the nonlinear element which produces the distortion. The nonlinear circuit has opposite characteristics than those of the nonlinear element, to thereby compensate for the distortion produced by the nonlinear element.

FIGS. 1(A), 1(B), 1(C), 1(D) and 1(E) are diagrams illustrating the operation of such a conventional predistorter system. Referring now to FIG. 1(A), a nonlinear element 1202 produces nonlinear distortion. A distortion compensating unit 1201 is positioned before a nonlinear element 1202, and has nonlinear characteristics opposed to that of nonlinear element 1202. An input signal is processed by distortion compensating unit 1201 and is then provided to nonlinear element 1202. Nonlinear element 1202 then produces a distortion compensated output signal. Alternatively, distortion compensating unit 1201 can be positioned after nonlinear element 1202.

FIG. 1(B) is a graph illustrating an input signal, FIG. 1(C) is a graph illustrating the input/output characteristics of distortion compensating unit 1201, FIG. 1(D) is a graph illustrating input/output characteristics of non-linear element 1202, and FIG. 1(E) is a graph illustrating an output signal. As can be seen from FIGS. 1(C) and 1(D), the nonlinear characteristics of distortion compensating unit 1201 are opposite the nonlinear characteristics of nonlinear element 1202. As a result, as can be seen from FIGS. 1(B) and 1(E), a distortion compensated output signal is produced.

FIG. 2 is a diagram illustrating a conventional distortion compensating unit 1201. Referring now to FIG. 2, an input signal is provided to a directional coupler 1301. Direction coupler 1301 branches the input signal into a first signal and a second signal. The first signal is provided to a delay element 1302, and the second signal is provided to a distortion component generating unit 1303. Distortion component generating unit 1303 generates a distortion component for the input signal. The distortion component is then adjusted in amplitude and phase by an attenuator 1304 and a phase adjustor 1305, respectively. The adjusted distortion component is provided to a directional coupler 1306.

On the other hand, the signal received by delay element 1302 is time delayed and provided to directional coupler 1306. The delay time corresponds to the processing time of distortion component generating unit 1303, attenuator 1304, and phase adjustor 1305. Directional coupler 1306 couples the time delayed signal with the distortion component.

However, the distortion compensating unit illustrated in FIG. 2 requires a phase shifting 90.degree. coupler and a 180.degree. coupler for canceling a primary signal element in distortion component generating unit 1303 and phase adjustor 1305. As a result, the distortion generating unit is relatively expensive to produce and is relatively large-scale. Moreover, a 90.degree. coupler can only be used for a limited frequency band. Therefore, the distortion compensating unit cannot be used to compensate for distortion over a broad band.

In addition, the distortion compensating unit illustrated in FIG. 2 can only be used to compensate for third order distortion since second order distortion becomes out of the band.

FIG. 3 is a diagram illustrating an additional, conventional distortion compensating unit 1201. Referring now to FIG. 3, an input signal f is received by a directional coupler 1401. Direction coupler 1401 branches the input signal into a first signal which is provided to an even-ordinal distortion generating unit 1423, and a second signal which is provided to an odd-ordinal distortion generating unit 1424.

The first signal is received by a 180.degree. coupler 1402 of even-ordinal distortion generating unit 1423. 180.degree. coupler 1402 branches the first signal into separate signals by shifting the phase of the first signal by 180.degree.. The two signals output from 180.degree. coupler 1402 are provided to diodes 1403 and 1407, respectively, and provided with distortion. A directional coupler 1411 combines the signals from diodes 1403 and 1407. Since the signals provided to diodes 1403 and 1407 are 180.degree. shifted in phase, an odd-ordinal distortion component generated by diodes 1403 and 1407 is canceled and an even-ordinal distortion signal (Af.sup.2 +Bf.sup.4 . . .) is output from directional coupler 1411. The Af.sup.2 and Bf.sup.4 correspond to the high frequencies of A sin2.omega.t and B sin4.omega.t, respectively.

The second signal branched by directional coupler 1401 is received by a directional coupler 1412 of odd-ordinal distortion generating unit 1424. Directional coupler 1412 branches the second signal into two separate signals which are provided to diodes 1413 and 1417, respectively, and provided with distortion. A directional coupler 1421 combines the signals from diodes 1413 and 1417. Diodes 1413 and 1417 are set in opposite directions. As a result, an even-ordinal distortion component generated by diodes 1413 and 1417 is canceled, and an odd-ordinal distortion signal (f +Cf.sup.3 . . .) is output from directional coupler 1421.

The even-ordinal distortion signal (Af.sup.2 +Bf.sup.4 . . .) generated by even-ordinal distortion generating unit 1423 and the odd-ordinal distortion signal (f+Cf.sup.3 . . .) generated by odd-ordinal distortion generating unit 1424 are combined by a directional coupler 1422, to thereby produce a distortion signal (f+Af.sup.2 +Cf.sup.3 +Bf.sup.4 . . .).

The distortion compensating unit illustrated in FIG. 3 can compensate distortion over a band of approximately DC - 500 MHz, and can adjust the odd- and even-ordinal distortions independently.

However, the distortion compensating unit illustrated in FIG. 3 is relatively costly and large-scale, since it requires the use of expensive, large scale directional couplers 1401, 1411, 1412, 1421, and 1422. Moreover, the distortion compensating unit is limited in frequency band by directional couplers 1401, 1411, 1412, 1421, and 1422. In addition, it is difficult to finely adjust distortion in such a distortion compensating unit.

SUMMARY OF THE INVENTION

Accordingly, it is an objects of the present invention to provide a distortion compensation unit which compensates for distortion over a broad band, and provides finely adjustable distortion.

It is an additional object of the present invention to provide a distortion compensation unit which has a reduced cost and complexity.

The foregoing objects of the present invention are achieved by providing an apparatus which adds preliminary distortion to an input signal. The apparatus includes first and second couplers, an adjusting unit, and a distortion generating unit. The first coupler separates a first alternating current component from the input signal, to provide a first signal representing the first alternating current component and a second signal representing the input signal having the first alternating current component separated therefrom. The adjusting unit adjusts the signal level of the first and second signals. The first distortion generating unit provides nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal. The second coupler combines the signal level adjusted second signal with the distorted first signal, to produce a corresponding output signal.

Object of the present invention are also achieved by providing an apparatus which includes a second order distortion generating unit and a third order distortion generating unit. The second order distortion generating unit generates second order distortion and provides the generated second order distortion to the input signal while suppressing third order distortion, to produce a second order distorted output signal. The third order distortion generating unit generates distortion and cancels even-ordinal components of the generated distortion to generate third order distortion, and provides the third order distortion to the second order distorted output signal, to produce an output signal having second and third order distortion.

Objects of the present invention are also achieved by providing a method which includes the steps of (1) separating a first alternating current component from an input signal, to provide a first signal representing the first alternating current component and a second signal representing the input signal having the first alternating current component separated therefrom; (2) adjusting the signal level of the first and second signals; (3) providing nonlinear distortion to the signal level adjusted first signal, to produce a distorted first signal; and (4) combining the signal level adjusted second signal with the distorted first signal, to produce a corresponding output signal.

Objects of the present invention are also achieved by providing a method which includes the steps of (1) generating second order distortion and providing the generated second order distortion to an input signal while suppressing third order distortion, to produce a second order distorted output signal; and (2) generating distortion and cancelling even-ordinal components of the generated distortion to generate third order distortion, and providing the third order distortion to the second order distorted output signal, to produce an output signal having second and third order distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 (prior art) is a diagram illustrating a conventional predistorter system.

FIG. 2 (prior art) is a diagram illustrating a conventional distortion compensating unit.

FIG. 3 (prior art) is a diagram illustrating a conventional distortion compensating unit.

FIG. 4 is a diagram illustrating a distortion compensating unit, according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a distortion compensating unit, according to an embodiment of the present invention.

FIGS. 6(A) and 6(B) are graphs illustrating the operation of the distortion compensating unit illustrated in FIG. 5, according to an embodiment of the present invention.

FIGS. 7(A) and 7(B) are graphs illustrating the operation of the distortion compensating unit illustrated in FIG. 5, according to an embodiment of the present invention.

FIGS. 8(A) and 8(B) are graphs illustrating the operation of the distortion compensating unit illustrated in FIG. 5, according to an embodiment of the present invention.

FIG. 9 is a diagram illustrating a distortion compensating unit according to an additional embodiment of the present invention.

FIG. 10 is a diagram illustrating a distortion compensating unit, according to a further embodiment of the present invention.

FIG. 11(A) is a diagram illustrating a distortion compensating unit according to an embodiment of the present invention.

FIGS. 11(B), 11(C), 11(D) and 11(E) are graphs illustrating characteristics of the distortion compensating unit illustrated in FIG. 11(A), according to an embodiment of the present invention.

FIG. 12 is a diagram illustrating a distortion compensating unit according to an additional embodiment of the present invention.

FIG. 13 is a diagram illustrating a distortion compensating unit according to a further embodiment of the present invention.

FIG. 14 is a graph illustrating a change in distortion of second and third order distortion depending on the input level of an input signal, of the distortion compensating unit illustrated in FIG. 13, according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

FIG. 4 is a diagram illustrating a distortion compensating unit 50, according to an embodiment of the present invention. Referring now to FIG. 4, distortion compensation unit 50 has an input and an output. A transmission line 2000 is connected to the input and the output, for providing an input signal to distortion compensating unit 50, and for receiving a distortion compensated signal at the output of distortion compensating unit 50. An attenuating unit 102 is connected between the input and the output of distortion compensating unit 50.

An input signal is provided to attenuating unit 102 via transmission line 2000. Attenuating unit 102 attenuates the input signal and produces a corresponding attenuated signal. Attenuating unit 102 includes, for example, a resistor (not illustrated) or a PIN attenuator circuit (not illustrated).

A first coupling unit 103 couples attenuating unit 102 to a distortion generating unit 101. First coupling unit 103 passes only alternating current components of an input signal to distortion generating unit 101. More specifically, first coupling unit 103 blocks direct current components from being transferred between distortion generating unit 101 and attenuating unit 102.

Distortion generating unit 101 generates distortion to be added to the input signal. Distortion generating unit comprises a nonlinear element (not illustrated), such as a diode, and generates distortion by controlling the nonlinear element. Distortion generated by distortion generating unit 101 is adjusted by changing a bias current provided to the nonlinear element. Adjusting the level of attenuation by attenuating unit 102 successfully adjusts the amount of power of the input signal provided for distortion generating unit 101, and thereby adjusts the amount of distortion provided for the input signal.

A second coupling unit 104 couples alternating current components of the distortion generated by distortion generating unit to attenuating unit 102. More specifically, second coupling unit blocks direct current components from being transferred between distortion generating unit 101 and attenuating unit 102. Thus, second coupling unit 104 combines an alternating current, distortion signal generated by distortion generating unit 101 with the output signal of attenuating unit 102. To block direct current components but allow alternating current components to pass therethrough, first and second coupling units 103 and 104 each comprise, for example, a capacitor (not illustrated).

Attenuating unit 102 maintains an impedance which is consistent with the impedance of transmission line 2000. Therefore, attenuating unit 102 allows an input signal to be transmitted over a broad band, and a part of the input signal to be provided to distortion generating unit 101 through first coupling unit 103. Further, as previously described, the transmission of direct current between distortion generating unit 101 and attenuating 23 unit 102 is interrupted by first and second coupling unit 103 and 104. As a result, a distortion operating point on a nonlinear element (not illustrated) of distortion generating unit 101 can be fixed, regardless of the operation of attenuating unit 102. Moreover, as will be discussed in more detail below, the amount of distortion provided by the nonlinear element can be adjusted by controlling a bias current of the nonlinear element.

FIG. 5 is a more detailed diagram illustrating a distortion compensating unit 55, according to an embodiment of the present invention. Referring now to FIG. 5, a distortion generating unit 201 distorts an input signal. An attenuating unit 202 adjusts the amount of power to be provided for distortion generating unit 201. Capacitors 207 and 208 prevent high frequency components from being transmitted between distortion generating unit 201 and attenuating unit 202. Thus, distortion generating unit 201, attenuating unit 202, and inductors 203 and 205 in FIG. 5 correspond, respectively, to distortion generating unit 101, attenuating unit 102 and first and second coupling units 103 and 104, respectively, in FIG. 4.

Distortion generating unit 201 includes a field-effect transistor 206 which is, for example, a GaAs field-effect transistor, a high electron mobility transistor (HEMT), or any other compound semiconductor field-effect transistor such as InP or ZnSe. Thus, field-effect transistor 206 is a nonlinear element for generating distortion. Field-effect transistor 206 is shorted between its source and drain. Distortion generating unit 201 also includes an electric current source 204 having a variable current, and inductors 203 and 205. Electric current source 204 provides a variable bias current for controlling field-effect transistor 206. Inductors 203 and 205 prevent high frequency components from being externally transmitted from distortion generating unit 201.

Attenuating unit 202 includes resistors 209, 210 and 211 for maintaining an impedance which matches the impedance of transmission line 2000. Resistors 209, 210 and 211 are connected in a .pi. configuration. More specifically, one end of resistor 210 and one end of resistor 211 is connected to ground, and resistor 209 is connected between the other ends of resistors 210 and 211. Transmission line 2000 is connected to a connection point between resistors 209 and 210, and to a connection point between resistors 209 and 211.

Transmission line 2000, at the input of distortion compensating unit 55, is connected to the gate of field-effect transistor 206 through capacitor 207. Transmission line 2000, at the output of distortion compensating unit 55, is connected to the source and drain of field-effect transistor 206 through capacitor 208. The gate of field-effect transistor 206 is connected to electric current source 204 through ground via inductor 203. An input terminal of electric current source 204 is connected to the source and drain of field-effect transistor 206 through inductor 205. An output terminal of electric current source 204 is connected to ground.

Direct current from electric current source 204 is provided to field-effect transistor 206 through inductors 203 and 205, to set a bias point of field-effect transistor 206. Capacitors 207 and 208 prevent direct current from electric current source 204 from being transmitted to attenuating unit 202.

When an input signal f is transmitted to attenuating unit 202, high frequency components of the input signal are provided to distortion generating unit 201 through capacitor 207. Field-effect transistor 206 produces a distortion component so that distortion compensating unit 55 outputs a distortion compensated output signal (f+Af.sup.2 +Bf.sup.3 . . .). Since attenuating unit 202 maintains an impedance which matches the impedance of transmission line 2000, distortion can be compensated over a broad band of several kHz through 1 GHz or more.

FIGS. 6(A) and 6(B) are graphs illustrating the operation of distortion compensating unit 55, according to an embodiment of the present invention, wherein the direct current supplied by electric current source 204 is adjusted.

More specifically, FIG. 6(A) is a graph illustrating the operation of the distortion compensating unit when the bias current provided to field-effect transistor 206 is small. Referring now to FIG. 6(A), curve 90 represents the voltage (V)-current (I) characteristics of field-effect transistor 206, curve 92 represents an input signal supplied to distortion compensating unit 55, and curve 94 represents an output signal produced by distortion compensating unit 55. As illustrated in FIG. 6(A), when the bias current is small, the input signal 92 oscillates around a bias point 301 in an area in which a nonlinear level is high. As a result, the distortion applied by field-effect transistor 206 is large.

FIG. 6(B) is a graph illustrating the operation of distortion compensating unit 55 when the bias current provided to field-effect transistor 206 is large. Referring now to FIG. 6(B), when the bias current is large, the input signal 92 oscillates around a bias point 302 in an area in which a nonlinear level is low. As a result, the distortion applied by field-effect transistor 206 is small.

Since distortion generating unit 201 and attenuating unit 202 are coupled together through capacitors 207 and 208, the transmission of direct current between distortion generating unit 201 and attenuating unit 202 is interrupted and the bias current of field-effect transistor 206 is constant. Therefore, the distortion applied by field-effect transistor 206 can be adjusted.

FIGS. 7(A), 7(B), 8(A) and 8(B) are graphs illustrating the control of distortion by adjusting the attenuation of attenuating unit 202, according to an embodiment of the present invention.

More specifically, FIGS. 7(A) and 7(B) are graphs illustrating the amount of distortion when the amount of attenuation by attenuation unit 202 is small. Referring now to FIG. 7(A), when the amount of attenuation is small, electric power 402 of the input signal provided to attenuating unit 202 increases and electric power 401 provided to distortion generating unit 201 decreases. Therefore, as illustrated in FIG. 7(B), the input level of an input signal 96 provided to distortion generating unit 201 becomes lower, and an output signal 98 of distortion generating unit 201 indicates that the amount of distortion applied by distortion generating unit 201 becomes smaller.

FIGS. 8(A) and 8(B) are graphs illustrating the amount of distortion when the amount of attenuation by attenuation unit 202 is large. Referring now to FIG. 8(A), when the amount of attenuation is large, an electric power 502 provided for attenuating unit 202 decreases and an electric power 501 provided for distortion generating unit 201 increases. Therefore, as illustrated in FIG. 8(B), the input level of the input signal 96 provided to distortion generating unit 201 becomes higher and the output signal 98 of distortion generating unit 201 indicates that the amount of distortion applied by distortion generating unit 201 becomes larger.

Thus, field-effect transistor 206 functions as a distortion generating element. However, instead of a field-effect transistor, some other type of nonlinear element, such as a diode, can be used as the distortion generating element.

According to the above embodiments of the present invention, the amount of distortion applied to an input signal can be adjusted for a high frequency component of the input signal. By using a GaAs field-effect transistor 206 shorted between the source and drain as a distortion generating element, a uniform distortion can be generated over a broad band of a few kHz through 1 GHz or more, and with a relatively simple circuit configuration.

When distortion is compensated in a broad band of 1 GHz or more, a diode can be used as the distortion generating element. The diode should have a band of 10 GHz or more and including an even band of 1 GHz or more. Such a diode can be a Schottky diode. However, a Schottky diode is microscopic, requires a high implementation technology, and is costly. Therefore, a GaAs field-effect transistor 206 can be used to realize a distortion compensating circuit having a simple, relatively inexpensive configuration.

FIG. 9 is a diagram illustrating a distortion compensating unit 60 according to an additional embodiment of the present invention, and using a PIN attenuator circuit to adjust the attenuation of an attenuating unit.

Referring now to FIG. 9, an attenuating unit 602 receives an input signal from transmission line 2000, and attenuates the input signal. A distortion generating unit 601 distorts an alternating current component of the input signal. Inductors 603 and 605 prevent high frequency components from being externally transmitted from distortion generating unit 601. An electric current source 604 provides a bias current for a field-effect transistor 606. Capacitors 607 and 608 connect distortion generating unit 601 to attenuating unit 602.

Attenuating unit 602 include a PIN attenuator circuit 609 and an electric current source 610 which provides a bias current to a PIN diode (not illustrated) of PIN attenuator circuit 609. Thus, distortion compensating unit 60 is similar to distortion compensating unit 55 illustrated in FIG. 5, except that attenuating unit 202 of FIG. 5 is replaced with attenuating unit 602 in FIG. 9 having PIN attenuator circuit 609 and electric current source 610.

In FIG. 9, the bias point on a PIN diode of PIN attenuator circuit 609 is moved by adjusting the bias current provided by electric current source 610, thereby changing the resistance of the PIN diode to change the attenuation.

Since the signal electric power provided for distortion generating unit 601 changes with the attenuation by attenuating unit 602, the distortion added by distortion generating unit 601 can be finely adjusted. At this time, the impedance of attenuating unit 602 matches the impedance of transmission line 2000 so that distortion can be compensated over a broad band of several kHz through 1 GHz or more.

Distortion compensating unit 60 allows the power of a signal provided to distortion generating unit 601 to be changed via attenuating unit 602. More specifically, attenuating unit 602 can change the attenuation with a fixed impedance, thereby externally controlling the distortion with precision for a high frequency component of an input signal.

FIG. 10 is a diagram illustrating a distortion compensating unit 65 according to an additional embodiment of the present invention. Referring now to FIG. 10, an attenuating unit 702 attenuates an input signal received from transmission line 2000. A distortion generating unit 701 is for distorting the input signal. Inductors 703, 705, 709, 711, 718, and 721 oppose high frequency components. An electric current source 704 provides a bias current for a field-effect transistor 706. An electric current source 710 provides a bias current for a PIN diode 712. An electric current source 722 provides a bias current for a PIN diode 720. A GaAs field-effect transistor 706 generates distortion. Capacitors 707 and 708 couple distortion generating unit 701 to attenuating unit 702. Resistors 715 and 716 maintain an impedance which is consistent with the impedance of transmission line 2000. Capacitors 713 and 714 couple PIN diode 712 to transmission line 2000 using an alternating current. Capacitors 717 and 719 couple PIN diode 720 to transmission line 2000 using an alternating current. PIN diodes 712 and 720 adjust the amount of attenuation provided by attenuating unit 702.

Transmission line 2000 is connected to attenuating unit 702 through the serially-connected resistors 715 and 716. Transmission line 2000 is connected at the input of the distortion compensating unit to the anode terminal of PIN diode 712 through capacitor 713. Transmission line 2000 is connected at the output of distortion compensating unit to the cathode terminal of PIN diode 712 through capacitor 714. The anode of PIN diode 712 is connected to the source through inductor 709, while the cathode of PIN diode 712 is connected to ground through electric current source 710 and inductor 711. The anode of PIN diode 720 is connected to ground through capacitor 719 and is connected to electric current source 722 through inductor 718. The cathode of PIN diode 720 is connected to the connection points of resistors 715 and 716 through capacitor 717, and is connected to ground through the inductor 721 and electric