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BACKGROUND OF THE INVENTION
This invention relates to an electronic tuner for musical instruments, and
more particularly to an electronic tuner for detecting and indicating
whether or not the frequency of a sound given forth from a musical
instrument is deviated from the standard tuning frequency of the
particular musical sound and, if deviated, how much it is deviated.
In general, musical sounds or notes are defined according to frequency, so
that the frequency of any particular musical sound or note should not
differ with musical instruments. However, it is difficult to maintain the
musical instruments in their correctly tuned condition. For example,
pianos, guitars (except steel guitars) and the like can be played with
correct musical intervals for a certain period of time once they are
tuned. On the other hand notes given off, by string instruments such as a
violin, a steel guitar, etc. and wind instruments appreciably differ with
players.
Accordingly, it is necessary to tune the instruments used in orchestras,
brass bands and so on. Tuners are employed for such tuning. Heretofore,
various types of tuners have been manufactured and sold and one that has
been in relatively wide use is a tuner commercially known under the name
of "Strobo CONN", manufactured by Conn Inc. of U.S.A. In this tuner, 12
windows are formed in the surface of a panel and, behind these windows,
strobe discs are disposed which are coupled together by means of gears and
rotate in predetermined ratios to one another. And these strobo discs are
positioned so that they can be partly seen through the windows,
respectively. One surface of each strobe disc has formed thereon black and
white striped patterns at predetermined intervals in the rotational
direction of the disc and the striped patterns are irradiated by light of
a discharge tube which is turned on and off at the frequency of a
particular musical sound. When the striped patterns are seen as if
stopped, it is judged that the sound is correct. The 12 windows
respectively correspond to the notes C to B of one octave and have formed
thereon striped patterns arranged in integral multiple relationships, with
which tuning of notes of different octaves is achieved.
This type of tuner is so constructed as to drive the plurality of discs
with one motor, and hence has such disadvantages as complexity in
construction and expensiveness.
In a modified form of this type of tuner, the number of windows are reduced
to one and instead the number of revolutions of the motor is changed by a
changeover switch in a stairstep manner in accordance with each particular
note, thereby changing the strobe frequency. In some cases, a cathode ray
tube is employed as the indicating means. Namely, on the screen of the
cathode ray tube, a strip-like bright line is normally displayed at a
sweep speed of the frequency corresponding to each note. An electron beam
is brightness modulated by the sound given off by a musical instrument and
when the frequency of the sound is synchronized with the sweep speed
defined by the switch, the bright line becomes a broken line and is seen
as if stopped. When the frequency of the musical sound is a little
deviated from the defined frequency, the broken line moves to right or
left. Depending upon whether the broken line moves to right or left, it is
judged whether the frequency of the musical sound is deviated upwardly or
downwardly. However, this type of tuner employs the cathode ray tube, and
hence is expensive.
Another conventional type of tuner employs a lamp as the indicating means
so as to reduce the manufacturing cost. In this tuner, a plurality of
lamps are aligned in line and normally turned on and off one after another
at high speed, that is scanned in such a manner as if they are all lighted
simultaneously. When a musical sound is given forth, if its frequency is
equal to the scanning speed selected by a changeover switch, only one
lamp, for example, the center one is lighted. Where the frequency of the
musical sound is deviated upwardly or downwardly, the lamps are lighted in
a sequential order from right to left or left to right and the direction
of the lighting indicates the direction of the frequency deviation. This
tuner has an advantage of low manufacturing cost.
With these conventional tuners, however, the direction of the frequency
deviation is indicated first and, in order to detect the amount of
frequency deviation, it is necessary, for example, in the case of the
tuner employing the strobe disc, to adjust a motor speed adjusting knob
until the striped pattern of the window corresponding to the note of the
musical sound stops and then to read a rotary scale of the knob. In the
case of the tuner employing the cathode ray tube, too, it is necessary to
adjust a sweep speed fine control knob to stop the displayed broken line.
And, also in the case of the tuner using the lamps, it is required to
adjust a lamp switching speed adjusting knob to stop the lamp lighting
position at the center.
In the practical tuning of a musical instrument, the player is required to
tune the instrument in accordance with the amount of the frequency
deviation obtained by himself while handling the instrument at the same
time. Consequently, it is inconvenient for him to adjust the adjusting
knobs, too. Further, in the case of musical sounds of high frequencies,
the frequency difference is likely to be large, so that, if such a sound
is out of tone, the flow of the striped pattern of the strobe disc, the
broken line displayed on the cathode ray tube or the lamp indication is
very fast and the direction of the frequency deviation is difficult to
judge. In the case of low-frequency sounds, the frequency difference is
not so large, and consequently even if such a sound is not correct, the
flow of the striped pattern of the strobe disc, the broken line on the
cathode ray tube or the lamp indication is slow, and hence its indication
cannot be recognized immediately.
One object of this invention is to provide an electronic tuner which is
adapted for a direct-reading indication of the amount of frequency
deviation of a musical sound.
Another object of this invention is to provide an electronic tuner which
furnishes a direct-reading indication of the amount of frequency deviation
of a musical sound requiring the player only to produce the sound from his
musical instrument.
Another object of this invention is to provide an electronic tuner which is
designed to indicate the ratio between the frequency of a musical sound
and that of a reference oscillator and wherein, whether the frequency of
the sound is high or low, the amount of deviation can be directly
indicated by a scale graduated in percent.
Another object of this invention is to provide an electronic tuner which
employs a switch for changing over the oscillation frequency of a
reference oscillator to the frequencies of each scale, and hence enables
tuning for 12 notes of each scale.
Another object of this invention is to provide an electronic tuner in which
higher harmonics that are an even-number times a reference signal supplied
to a phase comparator are superimposed upon the reference signal to enable
tuning at one set position for each particular note of several octaves.
Another object of this invention is to provide an electronic tuner which is
capable of correctly indicating the frequency of a musical sound even if
the oscillation frequency of a reference oscillator drifts due to a
temperature change or the like.
Another object of this invention is to provide an electronic tuner which is
capable of selectively changing a standard frequency for tuning to 440,
435 and 445Hz.
Still another object of this invention is to provide an electronic tuner
which employs digital indicating means to facilitate the reading of an
indication.
SUMMARY OF THE INVENTION
The electronic tuner according to this invention has housed in its case a
microphone for converting a musical sound into an electric signal, a
low-frequency amplifier for amplifying the converted musical sound, a
voltage-controlled variable frequency oscillator having its oscillation
frequency controlled by a control voltage, a phase comparator, a low-pass
filter connected to the output side of the phase comparator and an
indicator for indicating the output voltage from the low-pass filter. The
musical sound signal derived from the low-frequency amplifier and the
oscillation signal of the reference oscillator are compared in phase with
each other and the output from the phase comparator is applied through the
low-pass filter to the indicator and a frequency control terminal of the
reference oscillator. By the compared output, the oscillation frequency of
the reference oscillator is synchronized with the frequency of the input
musical sound signal, and the compared output, obtained when they are
synchronized with each other, is indicated by the indicator.
The voltage-controlled oscillator, the phase comparator and the low-pass
filter make up a phase lock loop (usually called PLL) and a control
voltage necessary for its phase locking operation is indicated by the
indicator, by which the ratio between the frequency of the input musical
sound signal and the oscillation frequency of the reference oscillator is
indicated. Accordingly, in the present invention, a direct-reading
indication of the ratio between the frequency of the input musical sound
signal and the oscillation frequency of the reference oscillator can be
provided on the indicator only by giving forth the sound from the musical
instrument. In addition, since the phase lock loop is used, the frequency
selecting characteristic is extremely sharp and a slight frequency
difference of any sound can be discriminated to enable accurate tuning.
Moreover, the tuner of this invention does not employ any mechanical parts
unlike Strobo CONN, and hence is stable, highly reliable, long-lived and
inexpensive.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system diagram showing one example of an electronic tuner
constructed in accordance with the present invention;
FIG. 2 is a system diagram illustrating another example of this invention;
FIGS. 3A-3G show a series of wave-form diagrams for explaining its
operation;
FIG. 4 is a system diagram showing another example of this invention;
FIG. 5 is a circuit diagram illustrating the detailed construction of the
example of FIG. 2;
FIG. 6 is a front view showing one example of the external appearance of
the tuner of this invention;
FIG. 7 is its side view;
FIG. 8 is a system diagram illustrating another example of the tuner of
this invention employing temperature compensating means;
FIG. 9 is a front view showing one example of a scale of an indicator for
use in the tuner of this invention;
FIG. 10 is a system diagram showing another example of this invention
employing a digital indicator; and
FIG. 11 is a system diagram for explaining the construction of the digital
indicator.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, reference numeral 1 indicates a microphone for converting an
incoming musical sound or tone into an electric signal. The input musical
sound signal converted by the microphone 1 into the electric signal is
amplified by a low-frequency amplifier 2, the amplified output of which is
supplied to a phase lock loop 3. As is known in the art, the phase lock
loop 3 is composed of a phase comparator 4, a low-pass filter 5 for
smoothing the output signal from the phase comparator 4 and a
voltage-controlled reference oscillator 6 (hereinafter indicated by VCO6)
whose oscillation frequency is controlled by a deviation voltage signal
derived at the output side of the low-pass filter 5. The input musical
sound signal received by the microphone 1 and the oscillation signal of
the oscillator VCO6 are compared in phase by the phase comparator 4. By
the resulting compared output, the oscillation frequency of the oscillator
VCO6 is controlled and the oscillation frequency and phase of the
oscillator VCO6 are respectively locked at the frequency and in the phase
of the input musical sound signal. And the deviation voltage value
necessary for the locking is indicated by an indicator 8. The deviation
voltage is in proportion to the frequency ratio between the input musical
sound signal and the oscillation signal and the indicator 8 provides an
indication of a value corresponding to the frequency ratio between the
input musical sound signal and the oscillation signal of the oscillator
VCO6.
The oscillator VCO6 is provided with a note switching circuit 9. In the
note switching circuit 9 of this example, the resistance values of
resistors 11.sub.1 to 11.sub.12 of a time constant circuit for determining
the oscillation frequency of the oscillator VCO6 are selected so that by
selectively changing over the resistors 11.sub.1 to 11.sub.12 with a
switch 12, the oscillation frequency of the oscillator VCO6 may be changed
over to any one of the standard tuning frequencies of 12 notes C,
C.music-sharp., D, D.music-sharp., E, F, F.music-sharp., G,
G.music-sharp., A, A.music-sharp. and B. That is, the switching position
of the switch 12 is selected depending upon the sound to be tuned and the
indicator 8 indicates the amount of deviation of the musical sound from a
correct note at that position. A double deflection zero center type meter
can be employed as the meter 8. The meter is graduated in in percent
(which indicates the whole tone with 200 graduations) and, in this case,
positive and negative graduations are respectively provided to the right
and left of the zero position. A deflection of the pointer to the positive
side indicates that the frequency of the input musical sound signal is
higher than the standard tuning frequency and a deflection to the negative
side indicates that the frequency of the input musical sound is lower than
the standard tuning frequency. In practice, full scales of .+-.50 percent
are provided to the right and left of zero and the meter is adapted to
deflect to the full scale when the input musical sound signal is deviated
.+-.1/4 tone from the standard value.
As is seen from the fact that the phase lock loop has an extremely narrow
frequency selecting characteristic about the oscillation frequency of the
oscillator VCO6 as is well-known in the art, the frequency of each sound
of the input musical sound signal can be discriminated by the phase lock
loop from the others with high accuracy. Accordingly, with the use of the
phase lock loop as the frequency comparing means of the tuner, as in the
present invention, there are no possibilities of the tuner responding to
other sounds than that set by the tuner itself. Further, if the
oscillation frequency of the oscillator VCO6 is accurately set at the
standard tuning frequency of each scale note, it is possible to detect a
frequency deviation of the musical sound signal from the standard value
with accuracy. Moreover, since the frequency deviation of the musical
sound signal from the standard tuning frequency is directly indicated on
the indicator 8, there is no need for a player using the tuner to make any
tune adjustments, such as an adjustment of strobe frequency changing means
for finding the amount of frequency deviation as in a conventional tuner.
This permits a player of a musical instrument to devote himself to tuning
of the musical instrument, and hence enables rapid tuning.
Tuning of only one sound is insufficient for the tuning of a musical
instrument and it is necessary to tune the musical instrument over one
octave or all over the scale notes obtainable with the musical instrument,
as shown in the FIG. 1 example. To this end, the note switching circuit 9
is provided in association with the oscillator VCO6 for selectively
switching the respective scales. For changing the scale notes to be tuned,
switching of the oscillation frequency of the oscillator VCO6 actuating
the switch 12 of the note switching circuit 9 and playing of the musical
instrument are achieved alternately with each other. Accordingly, there
are some occasions when the scale note selectively designated by the
switch 12 and the sound produced by the musical instrument do not
correspond to each other for some reason. For example, there is the
likelihood that although the switch 12 is positioned to select the note C,
the player produces a sound D in the mistaken belief that the switch 12 is
actually connected to select the note D. In such a case, since the
difference between the oscillation frequency of the oscillator VCO6 and
the frequency of the sound of the musical instrument is large, the signal
supplied to the low-pass filter 5 becomes of high-frequency components.
The high-frequency components cannot pass through the low-pass filter 5,
so that the output voltage from the low-pass filter 5 does not undergo any
change and the pointer of the indicator 8 remains to register zero.
In short, even if a wrong sound is produced, the indicator 8 indicates
zero. Accordingly, even in the case of such a wrong sound being produced,
there is the fear of judging erroneously that the sound has the standard
tuning frequency. This problem can be solved, for example, by arranging
the indicator 8 so that its pointer is normally biased in one direction
and that when the musical sound signal agrees with the standard value, the
pointer then registers zero. With such an arrangement, the pointer of the
indicator 8 deflects only when the frequency of the musical sound is
inside of a certain frequency range about the standard tuning frequency of
the note selected for tuning. And when sounds other than the selected one
are produced, the pointer does not move, so that the player can
immediately recognize his error. For biasing the pointer of the indicator
8, it is sufficient only to previously deviate the oscillation frequency
of the oscillator VCO6, for example, by 50 percent downwardly (or
upwardly) of the standard value of each scale note. This can be achieved
by a method of selecting such a resistance value of each of the resistors
11.sub.1 to 11.sub.12 as to provide such a frequency or superimposing a
bias voltage on a deviation signal supplied to the oscillator VCO6.
Accordingly, in the case where the oscillation frequency of the oscillator
VCO6 is deviated, for example, about 50 percent downwardly of the standard
value, the pointer of the indicator 8 normally deflects to the position of
-50 percent. And when the tuner is supplied with a musical sound signal
having the standard tuning frequency of the note being selected, the
indicator 8 registers zero. In the case of a musical sound signal having a
frequency deviation of 50 percent higher than the standard value, the
indicator 8 indicates +50 percent and in the case of a musical sound
signal having a frequency deviation of 50 percent lower than the standard
value, the indicator 8 indicates -50 percent. Consequently, in the case of
a frequency deviation of more than 50 percent lower than the standard
value, the pointer does not move but the frequency deviation of the
musical sound signal from the standard tuning frequency of the note being
selected is known. In practice, it is sufficient only to adopt full scales
of about .+-.70 to 80 percent and to deviate the frequency of the
oscillator VCO6 by a value corresponding to the full scale of one side.
Accordingly, by deviating the oscillation frequency of the oscillator VCO6
by a predetermined amount, as mentioned above, and by biasing the pointer
of the indicator 8 to the full-scale position of one side correspondingly,
it is possible to prevent that even a wrong sound is indicated to "have
the standard value."
It has already been described that the notes to be tuned are selectively
changed over by the switch 12. The frequencies of notes which are
respectively higher than the basic note by one and two octaves at each set
position of the switch 12, are respectively twice and four times as high
as the frequency of the corresponding basic note. Accordingly, it will be
convenient if tuning for the note of higher octaves than the basic note
can be achieved at the same set position of the switch 12. To perform
this, for example, signals of frequencies twice and four times as high as
the frequency of the basic note and, if necessary, a signal of higher
harmonic are superimposed on a signal of the frequency of the basic note,
that is, a distorted wave signal containing harmonics, is applied from the
oscillator VCO6 to the phase comparator 4, by which tuning for the note of
higher octaves than the basic note can be achieved at the same set
position of the switch 12.
FIG. 2 illustrates an embodiment of this invention which is designed for
the abovesaid purpose. In FIG. 2, parts corresponding to those in FIG. 1
are identified by the same reference numerals. (This also applies to the
other drawings.) The output from the oscillator 6 is shaped by a waveform
shaping circuit 7 such as a flip-flop circuit into a square wave. The
waveform shaping circuit 7 is composed of five flip-flop circuits 13 to
17. The flip-flop circuits 13 and 14 are connected in cascade and the
flip-flop circuit 13 is supplied with the output from the oscillator 6.
The input and output sides of the flip-flop circuit 13 and the output side
of the flip-flop circuit 14 are respectively connected to fixed contacts
24, 25 and 26 of an octave changeover switch 18. By connecting a movable
contact terminal 27 of the octave changeover switch 18 to the fixed
contacts 24, 25 and 26 one after another, a signal whose frequency changes
in a ratio of 1:2:4 can be obtained from the movable contact terminal 27.
The signal thus obtained is applied to a trigger input terminal of the
first-stage one of the flip-flop circuits 15, 16 and 17 connected in
cascade. At two output terminals of the flip-flop circuit 17 of the final
stage, there are derived such rectangular waves 28 and 29 as shown in
FIGS. 3A and 3B which are frequency divided to 1/8 and opposite in phase
to each other. If the frequency of the rectangular waves 28 and 29 is
taken as a fundamental frequency f.sub.1, a rectangular wave 31 such as
shown in FIG. 3C which has a frequency f.sub.2 twice the fundamental
frequency f.sub.1 can be obtained at the output of the flip-flop circuit
16 of the stage preceding the flip-flop circuit 17. And, at the output of
the flip-flop circuit 15, a rectangular wave 32 can be derived which has a
frequency f.sub.4 four times the fundamental frequency f.sub.1 as shown in
FIG. 3D. The rectangular waves 28 and 32, and 29 and 31 are respectively
AND'ed with each other in AND circuits 21 and 22, by which a discontinuous
rectangular wave 233 containing the fundamental frequency f.sub.1 and the
frequency f.sub.4, shown in FIG. 3E, is derived from the AND circuit 21
and a rectangular wave 234 containing the fundamental frequency f.sub.1
and the frequency f.sub.2, shown in FIG. 3F, is derived from the AND
circuit 22. The outputs from the AND circuits 21 and 22 are OR'ed with
each other in an OR circuit 23 and its output is supplied to the phase
comparator 4. That is, the phase comparator 4 is supplied with such a
signal 235 as depicted in FIG. 3G which contains the fundamental frequency
f.sub.1 and the frequencies f.sub.2 and f.sub.4. Consequently, in the
phase comparator 4, musical sound signals of the three frequencies
f.sub.1, f.sub.2 and f.sub.4 can be compared with one another at the same
time. Namely, three notes of sequentially different octaves can be tuned
at one set position of each of the note changeover switch 12 and the
octave changeover switch 18. Further, by switching the changeover switch
18, the tuning range can be shifted twice for each octave. Accordingly,
with the embodiment of FIG. 2, it is possible to tune all notes within the
range of five octaves in all.
FIG. 4 illustrates a modified form of the FIG. 2 embodiment for tuning
notes of different octaves with the switch 12 being held at one set
position. The FIG. 4 embodiment employs a plurality of phase comparators
4.sub.1, 4.sub.2, 4.sub.3, . . . and 4.sub.n, whose input terminals are
connected together to the output side of the amplifier 2. To the output
sides of the phase comparators 4.sub.1 to 4.sub.n are respectively
connected low-pass filters 5.sub.1 to 5.sub.n, the output terminals of
which are connected together to the indicator 8 and the control input
terminal of the oscillator VCO6, respectively. The waveform shaping
circuit 7 is composed of a cascade connection of n flip-flop circuits,
from which signals of frequencies f.sub.1, f.sub.2, f.sub.3, . . . and
f.sub.n are respectively derived and then applied to the phase comparators
4.sub.1 to 4.sub.n. With the construction of this example, n scales of
different octaves can be tuned at one set of the note changeover switch
12. In addition, since the signals of the frequencies ranging from f.sub.1
to f.sub.n are applied in the form of continuous waves to the phase
comparators 4.sub.1 to 4.sub.n unlike in the example of FIG. 4, the phase
lock loop 3 operates stably. Further, since the low-pass filters 5.sub.1
to 5.sub.n, each corresponding to one octave, can be provided, the
frequency draw-in range of the oscillator VCO6 can be made equal for each
octave. This prevents dispersion in the indication range of the indicator
8 according to octave. Namely, in the case where one low-pass filter 5 is
used in common to signals of frequencies different three octaves from each
other as described previously with regard to FIG. 2, the frequency draw-in
range of the oscillator 6 varies with the frequency of the input musical
sound. For example, even if the frequency draw-in range is .+-.70 percent
in the case of a signal of lower frequency, the range sometimes becomes
.+-.40 percent in the case of a signal of higher frequency. However, the
construction of FIG. 4 is free from such diadvantage.
FIG. 5 shows a concrete construction of the embodiment described above in
connection with FIG. 2. In FIG. 5, the microphone 1 is used as a speaker,
too, and is adapted to be changed over by ganged mode change-over switches
33 and 34 to the case of causing the indicator 8 to indicate a frequency
deviation of a musical sound and to the case where the oscillation signal
of the oscillator 6 is produced as a standard sound of each scale note to
enable the player to compare an actual musical sound with the standard
sound for detecting the frequency deviation. When the movable contact of
each of the mode changeover switches 33 and 34 is connected to either one
of its fixed contacts 35 and 36, the abovesaid standard sound is produced
from the speaker 1 and when the movable contact is connected to another
fixed contact 37, the frequency deviation of the musical sound is
indicated by the indicator 8. The difference between the fixed contacts 35
and 36 is whether the standard sound produced from the speaker 1 is loud
or not. When the contact 36 is selected, a resistor 39 is inserted between
the output of a speaker driving amplifier 38 and the speaker 1, by which
the level of the sound is attenuated about 1/2 as compared with that when
the contact 35 is selected. The combination speaker-microphone 1 is always
connected to the input side of the low-frequency amplifier 2 through a
switch contact 41 of an external microphone jack 40. To the input side of
the low-frequency amplifier 2 is connected a parallel circuit of two
diodes 42, 43 connected in opposite directions to each other. This
parallel circuit serves as a limiter with respect to an excessive input to
protect amplifier active elements, which are two field effect transistors
44 and 45 in this example. As load resistors of these field effect
transistors 44 and 45, resistance elements that the gate electrodes of
field effect transistors 46 and 47 are respectively to their source
electrodes are employed. To the gate of the field effect transistor 44 is
supplied a musical sound signal converted by the combination
speaker-microphone 1 into an electrical signal, and the source of the
transistor 44 is grounded and the drain is connected to a positive power
source line 48 through the field effect transistor 46 serving as the
resistance element and a decoupling circuit 49. The gate of the field
effect transistor 45 of the next stage is supplied with the amplified
output from the field effect transistor 44 of the preceding stage through
a capacitor 51. The source of the field effect transistor 45 is connected
to the fixed contact 37 of the mode changeover switch 34 and the drain is
connected to the positive power source line 48 through the field effect
transistor 47 serving as the resistance element. The amplified output from
the field effect transistor 45 is supplied to the phase comparator 4
through a capacitor 52. With such an arrangement, only when the mode
changeover switch 34 is connected to the fixed contact 37, the
low-frequency amplifier 2 operates to supply the phase comparator 4 with
the musical sound signal converted by the combination speaker-microphone
1. The low-frequency amplifier 2 performs as a saturation amplifier and
the musical sound signal, which is supplied to the phase comparator 4
through the capacitor 52 is rendered into a rectangular wave having a duty
ratio of 1/2.
The phase comparator 4 is comprised of a field effect transistor 53
performing a switching operation, resistors 54 and 55 and a differential
amplifier 56 and operates at a voltage +V.sub.cc that the voltage of a
battery 86 is boosted by a DC/DC converter 84 and supplied to a line 50.
The field effect transistor 53 is connected between the junction of a
series circuit of the resistors 54 and 55 and a line 57 of 1/2V.sub.cc.
The line 57 of 1/2V.sub.cc is impressed with a voltage 1/2V.sub.cc that
the voltage Vcc of the line 50 is divided by a differential amplifier 82
to 1/2. The gate of the transistor 53 is supplied with an output signal
235 (refer to FIG. 3G) from the waveform shaping circuit 7. The input
musical sound signal is applied from one end of the resistor 54 to a
non-inverting input terminal .sym. of the differential amplifier 56
through the resistors 54 and 55 and, at the same time, to an inverting
input terminal .crclbar. of the differential amplifier 56 through a
resistor 58. Between the inverting input terminal .crclbar. and the output
end of the differential amplifier 56 is connected a resistor 59 of the
same resistance value as that of the resistor 58 to provide a negative
feedback to retain the amplification degree of the differential amplifier
56 at 1.
When the field effect transistor 53 is in the on state, the differential
amplifier 56 is actuated as an inverting amplifier and when the former is
in the off | | |
