HP 651B - Page 9

Section IV
Model 651B
4-11. The complementary symmetry circuit is used
to provide power gain and to increase the dynamic
voltage range of the oscillator; also, the low output
impedance of the complementary symmetry circuit
prevents the oscillator output circuit from being loaded
by the RC bridge. The complementary symmetry
circuit transistors are forward-biased by diodes
A2CR2, A2CR3, and A2CR4, and with no signal ap¬
plied, are conducting slightly to reduce cross-over
distortion in the output signal.
4-12. The output of the oscillator circuit drives the
power amplifier with a constant voltage set by the
AMP LITUDE COARSE and FINE controls, R2 and R3.
The voltage level applied to the power amplifier is
held constant by the action of the peak detector circuit.
4-13. AUTOMATIC GAIN CONTROL .
4-14. The output of the oscillator circuit is super¬
imposed on a negative reference bias at the base of
A2Q7. This bias voltage is determined by the setting
of the amplitude controls. The peak detector, A2Q7,
will conduct only on the positive peaks when the nega¬
tive bias is overcome. The average dc voltage across
A2C7, A2C8 or A2C9 biases the two diodes A2CR6,
A2CR7 so it determines the impedance of the negative
feedback side of theWein bridge. Thus the amplitude
of the oscillations is automatically controlled. A2CR5
and A2CR9 provide temperature compensation for the
bias voltage on A2Q7, andA2CR8prevents the reverse
breakdown of A2Q7. A2R17 is adjustable to compen¬
sate for differences in the operating characteristics
of diodes A2CR6 and A2CR7, minimizing distortion
in the negative feedback and subsequently in the oscil¬
lator output,
4-15. POWER AMPLIFIER .
4-16. The power amplifier circuit increases the power
gain of thesignalreceivedfromthe oscillator circuit.
The operation of the differential amplifier A2Q8 and
A2Q9, emitter follower A2Q10, and complementary
symmetry circuit A2Q11 and A2Q12 is similar to the
corresponding st^es in the oscillator circuit. The
negative feedback voltage from the output of the com¬
plementary symmetry circuit is applied to the dif¬
ferential amplifier at a fixed level to stabilize the
power amplifier output signal. The power amplifier
output is continuously monitored by the monitor cir¬
cuit before the signal is applied to the output attenuator
circuit.
4-17. MONITOR CIRCUIT .
4-18. The monitor circuit monitors the signal level
applied to the output attenuator circuit and provides
a signal to the output monitor Ml, which indicates
the amplitude of the output in RMS volts and dBm.
The amplifier A1Q9 serves both as an impedance
converter between the monitor circuit and the power
amplifier output circuit, and as a current source to
provide full-scale monitor indications. The high in¬
put impedance of A1Q9 prevents the power amplifier
from being loaded with the low impedance of the output
monitor. Ml. The emitter follower, A1Q8, provides
a positive feedback voltage which is applied between
resistors A1R18 and A1R19, in the collector lead of
amplifier A1Q9. The application of the feedback volt¬
age at this point is used to increase the effective re¬
sistance of tlie collector circuit, which results in the
amplifier A1Q9 appearing as a high impedance current
source to the monitor. Diode AICRIO provides a
small amount of forward bias to rectifier diodes
A1CR8 and A1CR9, which keeps the diodes out of the
non-linear region, thus increasing monitor accuracy
atone-tenth full-scale readings. The 10 MHz adjust¬
ment, A1C15, compensates for small variations in
circuit capacitance so the monitor will have a flat
frequency response. The monitor calibration resistor,
A1R23, provides an additional calibration adjustment
which is made at 400 Hz.
4-19. OUTPUT ATTENUATOR .
4-20. The output attenuator provides a means of at¬
tenuating the signal level applied to the 50 ohm and
600 ohm output connectors. The OUTPUT ATTEN¬
UATOR switch, S2, selects a combination of four
resistor networks to produce the desired level of
signal attenuation. Each step provides an attenuation
of 10 dB. The AMPLITUDE controls, R2 and R3,
vary the level of attenuation in increments between
each 10 dB step selected by the OUTPUT ATTEN¬
UATOR switch.
4-21. Output impedances other than the standard 50
and 600 ohms can be obtained by changing the value
of resistors S2R14 or S2R13. The value of the re¬
placement resistor is added to the 50 ohm oscillator
output to give the required outfit impedance.
4-22. REGULATED POWER SUPPLY .
4-23. The regulated power supply provides all volt¬
ages required by the test oscillator circuits. The
power supply consists of a +30 volt series regulated
supply anda -25 volt series regulated supply which is
referenced to the +30 volt circuit.
4-24. The +30 volt regulated supply is of the conven¬
tional series regulator type. The emitter follower
A1Q2 is used to increase the loop gain of the cir¬
cuit, thus improving voltage regulation. The +30
volt adjustment, A1R4, sets the +30 volt and -25
volt supply output level.
4-25. The -25 volt regulated supply is of the conven¬
tional series regulator type and operates the same as
the +30 volt supply. A current limiter, A1Q7, has
been added to limit the load current to a set value.
When the load current exceeds the set value, the cur¬
rent limiter conducts, causing the series regulator
Q2 to reduce the output voltage level until the load
causing an excessive cxirrent is removed. Diodes
AlCR6and AlCR7protect the control transistor A1Q6,
against short circuits between the two voltage sup¬
plies, or short circuits in the output of the -25 volt
supply.
4-2
01810-2