Rejection Amplifier Circuit - HP 333A Operating And Service Manual

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Section IV
linear. The input impedance is kept linear by use of
local positive feedback from the. source of A2Ql to
the gate of A2Ql and to the protective diodes A2CR2
and A2CR3. Thus signals with a large source imped-
ance .can be measured accurately. Overall induced
distortion is further minimized by a high open loop
gain and 100% negative feedback. The high open loop
gain is achieved by local positive feedback from the
emitter of A2Q3 to the collector of A2Q2. Overall
negative feedback from the emitter circuit of A2Q4
to the source of A2Ql results in unity gain from the
impedance converter.
4-16. The bias points of the transistors in the imped-
ance converter are selected to minimize instrument
induced distortion. A2Ql, an extremely low noise,
high impedance field effect transistor, is the major
component that makes linearity of the impedance con-
verter independent of the signal source impedance.
4-17. REJECTION AMPLIFIER CIRCUIT.
(Refer to Figures 7-3 and 7-5)
4-18. The rejection amplifier circuit consists of the
preamplifier {A3Ql thru A3Q3), the Wien bridge
resistive leg and auto control loop {A5Ql thru A5Q9
with associated lamp and photocell), the reactive leg
and auto control loop {A5Ql0 thru A5Ql8 with associ-
ated lamp and photocell), and the bridge amplifier
{A3Q4 thru A3Q6).
4-19. PREAMPLIFIER CIRCUIT.
4-20. The signal from the impedance converter is
applied to the preamplifier, which is used during SET
LEVEL and DISTORTION measuring operations.
Negative feedback from the junction of A3Rl0 and
A3Rl 1 is applied to the junction of A3R2 and A3C2 to
establish the operating point for A3Ql. Negative
feedback from the emitter of A3Q3 is applied to the
emitter of A3Ql to stabilize the preamplifier, The
preamplifier, like the impedance converter, is
designed for high open loop gain and low closed loop
gain to minimize instrument induced distortion.
4-21.. WIEN BRIDGE CIRCUIT.
4-22. In the distortion measuring operation the Wien
bridge circuit is used as a rejection filter for the
fundamental frequency of the input signal. With the
FUNCTION selector, Sl, in the DISTORTION position,
the Wien bridge is connected as an interstage coupling
network between the preamplifier circuit and the
bridge amplifier circuit. The bridge is tuned to the
fundamental frequency of the input signal by setting
the FREQUENCY RANGE selector, S4, for the appli-
cable frequency range, and tuning the capacitors C4A
through C4D. The bridge circuit is balanced
by
ad-
justing the COARSE balance control, R4, and the FINE
balance control, R5. In the AUTOMATIC MODE fine
tuning and balancing are accomplished
by
photoelectric
cells which are in the resistive and reactive legs of
the Wien bridge. The error signals for driving the
photocells are derived by detecting the bridge output
using the input signal as a reference.
4-23. When the Wien bridge is not tuned exactly to
the frequency to be nulled, a portion of the fundamental
4-2
Model 333A/334A
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1
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OUTPUT OF BRIDGE
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IF RESISTIVE LEG
B-+--;_._"......1.-'&--
IS UNBALANCED
OUTPUT OF BRIDGE
IF REACTIVE LEG C-....._...;a:_.,.......,a:~ .... -
IS DETUNED
333A-B-1464
Figure 4-2. Bridge Waveforms
frequency will appear at the bridge output. The phase
of this signal depends on which leg of the bridge is not
tuned, or on the relative errors in tuning if neither is
set correctly. The magnitude of the signal is propor-
tional to the magnitude of the tuning error of either or
both legs of the bridge.
4-24. Figure 4-2a shows a sinusoid inputtothe Wien
bridge. If the resistive leg of the bridge is slightly
unbalanced, the output of the bridge is very small,
but has the waveform shown in Figure 4-2b and is in
phase with the input. As the resistive leg is tuned,
the signal approaches zero amplitude at null and then
becomes larger, but moo out of phase,
if
the null
position is passed. When the resistive leg is correctly
tuned and the reactive leg is tuned through null, a
similar waveform is produced, Figure 4-2c. The
only difference is that the reactive signal is 900 out
of phase with the resistive signal.
4-25. When the bridge output is detected using the
input signal as the reference, the error signals in
phase or 180° out of phase with the reference develop
a voltage that is used to vary the resistance in the
resistive leg of the bridge, to tune it to the correct
null position. Signals of the form in Figure 4-2c do not
develop any voltage, as the resistive detector is in-
sensitive to inputs differing from the reference by 900.
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