Rejection Amplifier Circuit - HP 333A Technical Manual

TM11-6625-1576-15
Section IV
Paragraphs 4-13 to 4-25 and Figure 4-2
the emitter of A2Q3 to the collector of A2Q2. Overall
negative feedback from the emitter circuit of A2Q4 to
the source of A2Q1 results in unity gain from the im-
pedance converter.
4-13. The bias points of the transistors in the imped-
ance converter are selected to minimize instrument
induced distortion. A2Q 1, 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.

4-14. REJECTION AMPLIFIER CIRCUIT.

4-15. The rejection amplifier circuit (see Figures 6-3
and 6-6) consists of the preamplifier (A3Q1) thru A3Q3),
the Wien bridge resistive leg and auto control loop
(A5Q1 thru A5Q9 with associated lamp and photocell),
the reactive leg and auto control loop (A5Q10 thru
A5Q 18 with associated lamp and photocell), and the
bridge amplifier (A3Q4 thru A3Q6).
4-16. PREAMPLIFIER CIRCUIT.
4-17. The signal from the impedance converter is ap-
plied to the preamplifier, which is used during SET
LEVEL and DISTORTION measuring operations. Neg-
ative feedback from the junction of A3R1O and A3R11 is
applied to the junction of A3R2 and A3C2 to establish
the operating point for A3Q1. Negative feedback from
the emitter of A3Q3 is applied to the emitter of A3Q1
to stabilize the preamplifier. The preamplifier, like
the impedance converter, is designed for high open
loop gain and low closed loop gain to minimize instru-
ment induced distortion.
4-18. WIEN BRIDGE CIRCUIT.
4-19. 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, S1, 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 fundamen-
tal frequency of the input signal by setting the FRE-
QUENCY RANGE selector, S4, for the applicable fre-
quency range, and tuning the capacitors C4A through
C4D. The bridge circuit is balanced by adjusting 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 photo-
cells are derived by detecting the bridge output using
the input signal as a reference.
4-20. When the Wien bridge is not tuned exactly to the
frequency to be nulled, a portion of the fundamental
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.
4-2
Figure 4-2. Bridge Waveforms
4-21. Figure 4-2a is a sinusoid input to the Wien bridge.
If the resistive leg of the bridge i
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 180° 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 differ-
ence is that the reactive signal is 90° out of phase with
the resistive signal.
4-22. 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 which 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
insensitive to input differing from the reference by 90° .
4-23. In an independent, but similar control loop, the
bridge input signal is shifted 90° and used as the
reference signal for the detector.
develops control voltages to null the reactive leg of
the bridge, but is insensitive to signals of the form
in Figure 4-2b which are caused by small tuning
errors of the resistive branch.
4-24. The result is that the two control loops derive
information from a common source and develop two
independent control signals for nulling the two legs
of the bridge. These control voltages are used to
vary the brilliance of lamps, which in turn causes
resistance changes in photocells which form part of
the Wien bridge.
4-25. Refer to Figure 4-3 for the phase relationship
of the bridge error signal and reference voltage at the
base of A5Q4. The shaded portions of the error sig-
nals (b and c) indicate that part of the error signal
which contributes to the dc lamp control voltage. As
indicated in d, any error signal that is 90° out of
phase with the reference does not affect the dc lamp
control voltage.
Model 333A/334A
slightly unbalanced,
S
This detector