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Section IV
Model 654A
feedback signals is amplified by the differential amplifier
(A2Q2 and A2Q3) and applied, through emitter follower
A2Q4, to the complementary symmetry pair A2Q5 and
A 2Q6.
The
very
low
output
impedance
of
the
complementary pair acts as a constant voltage source for
the Buffer Amplifier and also allows feedback to be applied
to the bridge without loading the output of the oscillator.
A2Q5 and A2Q6 are forward biased through A2CR7 and
A2CR8 so as to conduct slightly when no signal is applied;
this eliminates crossover distortion of the output signal.
4-10. A2Q7 acts as a peak detector. Part of the oscillator
signal is superimposed on a negative bias at the base of
A2Q7; A2Q7 conducts only when the positive peaks of the
signal overcome the negative bias. The negative dc output
of A2Q7 is filtered by A2C2, A2C3 and A2C4 and used to
bias the diodes A2CR1 and A2CR2 so as to vary the
impedance of the negative feedback side of the bridge to
control the amplitude of oscillations. A2R8 is switched into
the circuit on the X100 through X1M ranges and A2C5 on
the X10 through X1K ranges for extra filtering. A2CR3
limits any reverse voltage transients across the polarized
capacitors A2C3, A2C4 and A2C5. A2R9* is selected to
draw current from the bridge so as to balance the current
through A2CR1 and A2CR2 and improve distortion in the
output signal.
4-11. BUFFER AMPLIFIER. ( Schematic 1)
4-12. The
Buffer
Amplifier (A2Q8
through
A2Q10)
provides isolation between the oscillator and the Balanced
Amplifier and is also used as the point at which the 654A
signal is leveled. A2Q8 and A2Q9 form a differential
amplifier whose output, from the collector of A2Q9, is
amplified by A2Q10 and fed to the Balanced Amplifier.
Part of the signal output from A2Q10 is applied as negative
feedback through A2R39 to the base of A2Q8 to stabilize
the amplifier gain over its 10 Hz to 10 MHz frequency
range. The signal from the oscillator is divided down at the
input
to the
Buffer Amplifier by a resistive divider
consisting of A2R33 and the resistive element of the
photosensitive resistor (A2DSV1). The resistance of the
resistive element is determined by the current through the
lamp, which is controlled by the Automatic Leveling
Circuits (Paragraph 4-19 and following). The signal level at
the input of the Buffer Amplifier varies with changes in the
resistance of the photosensitive resistor to control the level
of the 654A output signal.
4-13. COUNTER EMITTER FOLLOWER. ( Schematic 1)
4-14. The Counter Emitter Follower (A3Q1) serves as
isolation between the Oscillator circuit, and the rear panel
COUNTER OUTPUT (J2). The output frequency is the
same as the Variable Test Oscillator frequency. Signal
amplitude at the COUNTER OUTPUT is approximately
0.1V rms into a 50 ohm load.
4-15. BALANCED AMPLIFIER. ( Schematic 2, Figure 7-3)
4-16. The single-ended sinusoidal output from the Buffer
Amplifier is amplified and converted into a balanced signal
in the Balanced Amplifier. The Amplifier consists of three
cascaded differential amplifiers (A2Q11 through A2Q16)
and two complementary symmetry pairs (A2Q18, A2Q19
and A2Q20, A2Q21). The balanced output signal from the
complementary
symmetry
pairs
is
developed
across
summing resistors A2R74 and A2R75 and then applied
through A2R76 and C9, and through A2R77 and C10 to
the attenuators (S4). The input sinusoidal signal at the base
of A2Q11 is converted into two signals of equal amplitude,
but 180° out of phase, taken from the collectors of A2Q11
and A2Q12. The two signals then follow parallel paths,
while maintaining the 180° phase relationship, through the
Balanced Amplifier. One signal path is through A2Q11,
A2Q13 and A2Q15 to the complementary symmetry pair
A2Q18 and A2Q19; the other signal path is through
A2Q12, A2Q14 and A2Q16 to complementary symmetry
pair A2Q20 and A2Q21. The complementary pairs are
biased through diodes A2CR15, A2CR16 and A2CR17,
A2CR18 so that the transistors conduct slightly when no
signal is applied; this eliminates crossover distortion of the
signal at the output of the Balanced Amplifier.
4-17. The gain of the Balanced Amplifier is stabilized by
means of negative feedback. A2R51 couples the output
signal from the top of A2R74 back to the base of A2Q11
and A2R53 couples the opposite signal from the bottom
end of A2R75 back to the base of A2Q12. Note that the
application of feedback together with the high gain of the
Amplifier and the very low signal source impedance (from
the Buffer Amplifier) causes the Balanced Amplifier to
function as an operational amplifier with differential input
and output; as a result, the overall gain of the Balanced
Amplifier is determined by the ratio of A2R51 to A2R44
and A2R53 to A2R54. Capacitors A2C15 and A2C18
improve the high frequency response of the feedback paths.
4-18. To maintain proper balance of the output signal, the
junction of summing resistors A2R74 and A2R75 is held at
virtual ground by means of negative feedback to the third
differential amplifier (A2Q15, A2Q16). The differential
pair A2Q22 and A2Q23 compares the voltage at the
junction of A2R74 and A2R75 with ground (the base of
A2Q23
is connected directly to ground). Any signal
unbalance or common mode signal across the two resistors
moves their junction away from ground; this voltage at the
junction is amplified by A2Q22 and A2Q23 and applied to
the base of A2Q17 to change its collector current. A2Q17
is the source of current for A2Q15 and A2Q16 so that the
change in current through them will be such as to restore
the balance between the two sinusoidal signals and return
the junction of A2R74 and A2R75 back to ground. A2C32
is adjusted for equal signal voltage across A2R74 and
A2R75. A2R47, A2C16, A2C23, A2C26, A2C27, A2C28
and A2C29 all serve as frequency shaping elements to
improve the frequency response and to insure stability of
the Balanced Amplifier over its frequency range of 10 Hz to
10 MHz.
4-2
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