Section IV
AGC
reduces the
gain
of
the amplifier to three; and
stable oscillation
is
achieved.
4-13.
BUFFER
AMPLIFIER.
4-14. The 5 volt rms sine
wave
output from the
oscillator circuit
is
coupled
to
the buffer amplifier.
The amplifier has a high open loop gain that
is
controlled by the negative feedback to provide a
gain
of
2.
This enables the circuit to have very low
distortion characteristics. The buffer amplifier uses a
complementary symmetry transistor pair
to
furnish a
10 volt
rms
output.
4-15.
SINE-SQUARE
CONVERTER.
4-16. The 5 volt rms sine
wave
output from the
oscillator circuit
is
also applied to the sine-square
converter. The sine
wave
is
coupled to a tunnel diode
which produces a small square wave output with fast
rise
and
fall
times. This small square wave signal
is
then shaped and amplified.
It
appears at the output
as
a
20
volt peak-to-peak square wave.
4-17.
DETAILED
CIRCUIT
DESCRIPTION.
4-18. For the following paragraphs, refer to the
Oscillator Schematic Diagram, Figure
7-1
.
4-19. Transistors
AIQI
through AIQ7 make up the
basic oscillator amplifier.
Al
QI
is
an
N-channel FET.
AlCRI sets up proper de bias for
Al
Q2. Diodes
AICR6, AICR7, AICR8 set up proper bias for
Al
Q4.
Capacitor
Al
C9
is
chosen to provide a stable
roll
off
at high frequencies.
Al
Q7
is
a current source
for AIQ3 and AIQ4. AICR4 and AICR5 provide
proper biasing for complementary output transistors
Al
Q5
and AlQ6.
4-20. The positive feedback arm
of
the
Wien
bridge
consists
of
tuning capacitors
AICIA
and
AIClB,
and
range switching resistors
Al
RI through
Al
RI7.
4-21. The negative feedback arm
of
the
Wien
bridge
depends upon the ratio
of
the impedance
of
Al
R28
to the total impedance
of
Al
R23, Al R24,
Al
R25,
and AlQ8. Al R25 reduces the effect
of
the FET
Al
Q8
to increase stability.
Al
Q8
proVides
AGC
for
this amplifier by varying impedance
to
obtain the
proper negative feedback.
4-2
ModeI209A
4-22. The conduction
of
FET
Al
Q8
is
controlled by
the peak detector circuit using AIQ9. AIQ9 conducts
during the most negative portion
of
each negative half
cycle, developing a negative charge in
Al
C
15
and its
parallel capacitors.
As
the amplifier output amplitude
increases, Al
Q9
conducts more and
Al
C
15
becomes
more negatively charged. This makes the FET input
voltage more negative, increasing its impedance and
increasing the negative feedback
to
reduce the output
amplitude
of
the amplifier.
4-23. Transistors AIQ13 through
AIQI8
comprise a
buffer amplifier with a gain
of
two. AIQ13 and
AIQI4
form a differential amplifier. Diodes
AICRI8
and
Al
CRI9
furnish
proper
biasing for
complementary output transistors
Al
QI7
and
Al
QI8.
When the output attenuator
Al
R79
is
fully
clockwise, the output amplitude
is
greater than 10
volts
rms.
When
the
attenuator
is
fully
counter-clockwise, the output
is
attenuated by
greater than 20
dB.
4-24. The Sine-Square Converter circuit includes
Al
QlO through
Al
QI2.
This converter circuit
operates
as
a saturating amplifier. Tunnel diode
Al
CRI2
squares the sine
wave
input, and the
Symmetry Adjust
Al
R45 determine the level where
conduction starts. This provides for adjustment
of
the
symmetry
of
the square
wave.
Zener diode
Al
CRI5
sets the voltage level
of
the negative portion
of
the
square wave.
Al
QI2
furnishes the positive portion
of
the square
wave
output, and
Al
QII
furnishes the
negative output.
4-25.
POWER
SUPPLY.
4-26. The following paragraphs refer
to
the Power
Supply Schematic, Figure 7-2.
4-27. This power supply
is
a series regulated power
supply furnishing
+21
volts and
-21
volts. Zener diode
A2CR6 serves
as
a reference for the positive power
supply, which in turn serves
as
the reference for the
negative supply. The positive supply
is
described here,
and the negative supply operates similarly.
4-28. Transistor A2QI regulates the output voltage
and
is
controlled by A2Q3. A2Q2
is
a current source
for A2Q3. Zener diode A2CR5 furnishes bias for
A2Q2, while A2R2 injects negative ripple feedback.
A2CR6 sets the emitter voltage
of
A2Q3, setting up a
reference for the supply output. A2Q4 current limits
the output
to
prevent damage
to
the supply.
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