HP 209A Operating And Service Manual page 15

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.
Al CRI 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 Al Q6.
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 Al Q8. 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 C15 and its
parallel capacitors. As the amplifier output amplitude
increases, Al Q9 conducts more and Al C15 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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