c. Input Impedance.
As illustrated in figure 3, the Video Amplifier
input impedance decreases at higher frequencies (above 1 kHz).
This
decrease is due to the capacitive loading effect of the amplifier input.
Thus when a signal source of fixed impedance is connected to the input, a
meter at the signal source will not indicate the correct input level. To
avoid this error, monitor the input level with an RF Voltmeter to ensure a
satisfactory input level.
Figure 3.
Input impedance vs. input frequency.
3.
CIRCUIT DESCRIPTION.
The Model 5261A is composed of four functional circuits; a
preamplifier, attenuator, video amplifier and output amplifier.
These
circuits combine to amplify ac signals as small as 1 millivolt and provide a
usable output level to the counter input circuit.
All dc voltages to
operate the Model 5261A are obtained from the counter via the plug-in
adapter.
a. Preamplifier.
The preamplifier
circuit (figure 4) consists of a
nuvistor cathode follower (Q1) driving an
emitter follower (Q2) to provide a
high impedance input and a low impedance
output. Diodes CR1 and CR2 limit
input signal peaks to prevent damage to
Q1.
Resistor R4 limits Q1 gate
current during overload. Capacitors C2,
C3, C4 and C5 filter power supply
b. Attenuator. The resistive attenuator (figure 5) reduces the input
signal and establishes the overall gain of the video amplifier. The amount
of attenuation inserted is controlled by the Sensitivity switch.
RC
networks C2/R10 and C1/R9 are selected to improve frequency response.
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