HP 333A Operating And Service Manual page 17

Model 333A/334A
BRIDGE
AMPLIFIER
А394 ~
A3Q6
WEIN
BRIDGE
NEGATIVE FEEDBACK
3334/334A-RO
Section ТУ
5 50 500 5000 50K 600K
AMPLIFIER GAIN db
FREQUENCY .
Figure 4-6. Rejection Amplifier Block Diagram and Typical Frequency Rejection Characteristic
in resistance less than on the two lower ranges. How-
ever, less variation in resistance is needed to tune
the leg, because the impedance in the reactive leg
becomes progressively less as the higher frequency
ranges are selected.
4-35. Any error signal that is not an integral multiple
of 90° is the result of the reactive leg of the bridge
being detuned, andthe resistive leg being unbalanced.
For example, an error signal that is 45° out of phase
(Figure 4-5e and j) will result in outputs from both
resistive and reactive detectors to tune the bridge
and reject the fundamental.
4-36, When the FUNCTION selector is set to the
VOLTMETER or SET LEVEL position, the junction
of the series and shunt reactive branches ofthe Wien
bridge is connected to circuit groundthrough A3R19
by SIBF which disables the frequency rejection
characteristic of the bridge circuit. With the bridge
circuit disabled, the rejection amplifier circuit pro-
vides one dB of gain for the fundamental frequency
and the harmonics. In the SET LEVEL operation, this
signal is used to establish the SET LEVEL refer-
ence.
4-37. BRIDGE AMPLIFTER CIRCUIT.
4-38, The bridge amplifier circuit consists of three
stages of amplification, A3Q4 through A3Q6. The
first stage of amplification, A3Q4, is a field effect
transistor which amplifies the difference signal between
the gate andthe source. The field effect transistor is
selected for minimum noise performance with the high
impedances of the Wien bridge circuit. The signal
from the drain is applied to the two stage feedback
amplifier A3Q5 and A3Q6. The output of A3Q6 is
coupled to the meter circuit by the post attenuator
S3R1 through S3R11. Negative feedback from the out-
put of the bridge amplifier is applied to the preamp-
lifier circuit to narrow the frequency rejection
characteristic. It can be noted from the rejection
characteristic (refer to Figure 4-3) for the bridge
that the rejection of harmonic voltages is not con-
stant. Typically the second harmonic is attenuated
several dB more than the third harmonic andthethird
more than the fourth. The result of the negative feed-
back is illustrated by the rejection characteristic
Shown in dashed lines on the attenuation and phase
characteristic of Figure 4-3, Figure 4-6 shows a
simplified block diagram of the rejection amplifier
with the typical frequency-rejection characteristic.
Refer to Figure 4-7, Bandwidth Versus Null Depth for
further detail on the rejection characteristic.
4-39. HIGH PASS FILTER.
(Refer to Figure 7-3).
4-40. The HIGH PASS FILTER is normally used when
the fundamental of the input signal is greater than 1
kHz. In the voltmeter mode of operation, the filter
is not used. In SET LEVEL and DISTORTION func-
tions the filter presents » 40 dB attenuation to 50 or
60 Hz hum components, but offers no attenuation to
frequencies over 1 kHz. The filter assembly, АТ,
consists of A7C1, АТС2, апа А711, The filter can
be inserted or bypassed by the HIGH PASS FILTER
switch, S9.
4-41. METER AMPLIFIER.
(Refer to Figure 7-4)
4-42. The meter amplifier consists of the post atten-
uator, the meter amplifier circuit, and the meter
rectifier circuit.
4-43, POST ATTENUATOR.
4-44, The post attenuator, S3R1 through S3R11, is a
resistive network which attenuates the input signal in
10 dB steps. The attenuator is usedin conjunction
with either the input sensitivity attenuator or the
1000:1 attenuator to limit the signal level to the
meter amplifier to 1 mV for full scale deflection on
all ranges from 1 mV to 300 V full scale. The meter
circuit sensitivity is increased to 300 рҮ for full
scale deflection on the .0003V range by switching
resistors A2R29 and A2R30 into the calibration net-
work. Resistor A2R41 and capacitor A2C29 are also
Switched into the calibration network on the .0003V
range to extend the passband of the amplifier.
4-45, METER AMPLIFIER CIRCUIT.
4-46, The meter amplifier circuit consists of a five
stage amplifier circuit, A2Q5 through A2Q9, which
develops the current for full scale meter deflection.
Negative dc feedback from the emitter circuit of
A2Q8 is applied to the base of А205 to stabilize the
dc operating point of the meter amplifier circuit and
to minimize the tendency for de drift due to ambient
temperature changes. A2R51 and A2CR8 are electric-
ally in the circuit only whenthe meter amplifier is over-
loaded. When the voltage on the emitter of A2Q9
4-5