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Description  |
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TECHNICAL FIELD
This invention relates to oscillators, and more particularly, to
oscillators which are voltage and/or temperature compensated.
BACKGROUND ART
It is typically desirable for oscillators to have frequency stability with
supply voltage variation as well as temperature variation. Of course, much
work has been done in providing such stability. The additional circuitry
required to obtain the desired compensation can be quite extensive.
Additionally, different types of oscillators have varying purposes with
varying degrees of complexity for frequency stability. One oscillator use
is for charge pumps. A charge pump is typically for obtaining a voltage of
the opposite polarity or greater magnitude than that of the suppy voltage.
An oscillator is required to drive the charge pump. Although frequency
stability is important for such an application, typically circuit
complexity is even more important. Consequently, compensation for
temperature or supply voltage variation in order to be useful must be
relatively simple.
SUMMARY OF THE INVENTION
An object of the subject invention is to provide an improved oscillator.
Another object of the invention is to provide an improved oscillator having
temperature compensation.
Yet another object of the invention is to provide an improved oscillator
having power supply voltage variation compensation.
These and other objects of the invention are achieved in an oscillator
having inverter and delay stages coupled between first and second
reference terminals. In one aspect of the invention a depletion transistor
has a first current electrode coupled to a power supply terminal, a
control electrode coupled to the first reference terminal, and a second
current electrode coupled to the second reference terminal. In another
aspect of the invention, the delay of each delay stage is controlled by a
control signal. The control signal is provided by a temperature
compensation circuit. The temperature compensation circuit has a control
transistor which provides the control signal at a voltage which is
inversely proportional to the threshold voltage of the control transistor.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE is a circuit diagram of an oscillator circuit according to
a preferred embodiment of the invention.
DESCRIPTION OF THE INVENTION
Shown in the FIGURE is a compensated oscillator 10 comprised generally of
an oscillator circuit 11, a voltage compensation circuit 12, and a
temperature compensation circuit 13. Oscillator circuit 11 is comprised
generally of inverters 16, 17, and 18 and RC circuits 21, 22, and 23.
Voltage compensation circuit 12 is comprised of a transistor 26.
Temperature compensation circuit 13 is comprised of transistors 27, 28,
and 29. Inverter 16 is comprised of transistors 31 and 32. Inverter 17 is
comprised of transistors 33 and 34. Inverter 18 is comprised of
transistors 35 and 26. RC circuit 21 is comprised of a transistor 41 and a
capacitor 42. RC circuit 22 is comprised of a transistor 43 and a
capacitor 44. RC circuit 23 is comprised of a transistor 45 and a
capacitor 46. All of the transistors 26, 27, 28, 29, 31, 32, 33, 34, 35,
36, 41, 43, and 45 are N channel insulated gate field effect transistors.
Transistors 26, 29, 31, 33, 35, 41, 43, and 45 are preferably depletion
transistors having a threshold voltage of -3.2 to -3.6 volts. Transistors
27, 28, 32, 34, and 36 are preferably enhancement transistors having a
threshold voltage of 0.6 to 0.9 volt. Capacitors 42, 44, and 46 are
preferably formed, in conventional fashion, from depletion transistors.
Transistor 32 has a gate as an input for inverter 16, a source connected to
ground, and a drain as an output of inverter 16. Transistor 31 has a drain
connected to a node 51, and a gate and source connected to the drain of
transistor 32. Transistor 41 has a drain as an input of RC circuit 21
connected to the output of inverter 16, a gate connected to a node 52, and
a source as an output of RC circuit 21. Capacitor 42 has a first terminal
connected to the source of transistor 41, and a second terminal connected
to ground. Transistor 34 has a gate as an input of inverter 17 connected
to the output of RC circuit 21, a source connected to ground, and a drain
as an output of inverter 17. Transistor 33 has a drain connected to node
51, and a gate and a source connected to the drain of transistor 34.
Transistor 43 has a drain as an input of RC circuit 22 connected to the
output of inverter 17, a gate connected to node 52, and a source as an
output of RC circuit 22. Capacitor 44 has a first terminal connected to
the source of transistor 43, and a second terminal connected to ground.
Transistor 36 has a gate as an input of inverter 18 connected to the
output of RC circuit 22, a source connected to ground, and a drain as an
output of inverter 18. Transistor 35 has a drain connected to node 51, and
a gate and source connected to the drain of transistor 36. Transistor 45
has a drain as an input of RC circuit 23 connected to the output of
inverter 18, a gate connected to node 52, and a source as an output of RC
circuit 23 connected to the input of inverter 16. Capacitor 46 has a first
terminal connected to the source of transistor 45, and a second terminal
connected to ground. Transistor 26 has a drain connected to a positive
power supply terminal Vcc for receiving, for example, 5 volts, a gate
connected to ground, and a source connected to node 51. Transistor 27 has
a gate and a drain connected to node 51, and a source. Transistor 28 has a
gate and a drain connected to the source of transistor 27, and a source as
an output of temperature compensation circuit 13 connected to node 52.
Transistor 29 has a drain connected to the source of transistor 28, and a
gate and a source connected to ground.
Oscillator 11 is a conventional three stage ring oscillator which provides
an output signal VO at the output of inverter 18. The frequency of signal
VO is primarily determined by the gains of transistors 41, 43, and 45, the
capacitance of capacitors 42, 44, and 46, and the gain of inverters 16-18.
These are all a matter of design choice. Each RC circuit 21-23 establishes
a delay between inverter stages 16-18 to affect the frequency. Three
inverter stages with feedback ensure that oscillator 10 will oscillate.
The frequency of oscillation is affected by the voltage at node 51.
Transistor 26 is interposed between the power supply voltage at Vcc and
node 51 to compensate for changes in voltage at Vcc. Because transistor 26
has a negative threshold voltage, transistor 26 can be conducting with a
positive voltage on its source, node 51. As node 51 rises in voltage,
transistor 26 will become non-conducting when node 51 reaches the
magnitude of the threshold voltage of transistor 26, in this case 3.2 to
3.6 volts. The voltage at node 51 is thus primarily dependent upon the
threshold voltage of transistor 26 instead of the voltage at Vcc. Although
the threshold voltage of transistor 26 can vary from -3.2 to -3.6 volt
over process variations, typical variation will be much less, for example,
0.2 volt variation or less. Consequently, most integrated circuits will
provide a voltage at node 51 as designed. Transistor 26 is chosen to be of
sufficient gain that current supplied to oscillator circuit 11 will not
cause significant voltage drop across transistor 26 above that caused by
threshold voltage. Transistor 26 thereby provides effective compensation
for supply voltage variation with minimal increase in circuit complexity.
As temperature increases, the gain of insulated gate field effect
transistors decreases. This has the effect of decreasing the frequency of
oscillation. The reduction in gain of transistors 41, 43, and 45 causes
the RC circuits 21-23 to provide more time delay between inverter stages.
Capacitors 42, 44, and 46 are charged at a slower rate due to the increase
in effective resistance of transistors 41, 43, and 45. This tends to
reduce the frequency of oscillation.
Temperature compensation circuit 13 provides a higher voltage to node 52 as
temperature increases to increase the conductivity of transistors 41, 43,
and 45. The increase in voltage on node 52 thereby compensates for the
decrease in gain of transistors 41, 43, and 45 due to temperature
increase. The amount of voltage increase required can be determined, for
example, by measuring the voltage at node 52 and the frequency at the
lowest applicable temperature, raising the temperature to the highest
applicable temperature, increasing the voltage at node 52 until the same
frequency as at the lowest temperature is obtained, measuring such
voltage, and taking the difference between the voltages at node 52 at the
lowest and highest temperatures. Such difference is the desired increase
in voltage on node 52 for temperature compensation circuit 13.
Compensation circuit 13 takes advantage of the fact that threshold voltage
of insulated gate field effect transistors decreases with temperature. For
a typical voltage range of 0.0.degree. C. to 70.0.degree. C. a typical
change in threshold voltage is 0.2 volt. Transistors 27 and 28 are
diode-connected so that the voltage at node 52 will be two threshold
voltages below the voltage at node 51. The gain of transistor 29 is much
smaller than that of transistors 27 and 28 and is present as a load to
ensure that node 52 does not float to a higher voltage than desired. The
voltage at node 51 also increases with temperature due to the threshold
voltage of transistor 26 becoming more negative with temperature.
Consequently, the voltage change at node 52 will be equal to three
threshold voltage changes, in this case 3 times 0.2 which equals 0.6 volt.
The change in voltage at node 52 is thus easily adjusted in increments of
0.2 volts by adding the desired number of diode-connected transistors
between node 51 and 52.
In the event that the desired change in voltage at node 52 is not a
multiple of 0.2 volt, transistor 29 can be increased in gain to draw more
current through transistors 27 and 28. For example, if the desired voltage
increase at node 52 is 0.5 volt, the gains of transistors 27, 28, and 29
are proportioned to each other so that there is current flow causing some
voltage drop across transistors 27 and 28 in addition to that caused by
threshold voltage. As temperature increases the voltage change at node 52
will be somewhat reduced. Another possibility is to change transistors 41,
43, and 45 to light depletion or enhancement transistors, for example, to
see if a multiple of a threshold voltage change can be obtained for the
desired voltage change at node 52. With modern modelling techniques, such
experiments are easily run. In any event, circuit 13 is useful in
compensating for temperature variation.
While the invention has been described in a preferred embodiment, it will
be apparent to those skilled in the art that the disclosed invention may
be modified in numerous ways and assume many embodiments other than that
specifically set out and described above. Accordingly, it is intended by
the appended claims to cover all modifications of the invention which fall
within the true spirit and scope of the invention.
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