Radar System Engineering

SEC.12.6] VIDEO AMPLIFIERS 451
amplifier must recover from large signals quickly, requiring that
no control-grid current be drawn on strong signals.
3. Some form of limiting must be provided. The noise voltage at
the output of the detector will be approximately 2 or 3 volts peak.
In order that the noise be clearly visible on the screen, it must be
amplified sufficiently to drive the indicator tube over something
like half its allowable control-grid voltage swing. Since signal
voltages from the detector may be 20 volts or more, the output
voltage must be limited so that the indicator will not be driven
outside its allowable range of voltage.
The simplest and most commonly used type of video amplifier is the
resistante-capacitance-coupled am-
6+
plifier shown in Fig. 12.10.
64C7
Although the gain for small sig-
c, 6AG7
nals can be simply exp~essed in terms + ‘= ~
---
:
of the tube and circuit constants, it
-L
.~. To
is usually determined by reference to
R
terminated
.-_L_- ‘ R,
the characteristic curves of the tube.
CB - line
For many reasons, the high-frequency
8[*
performance is most conveniently
expressed by giving the frequency
at which the gain is down 3 db (voltage
down to 0.707). This will be
referred to as the “ cutoff frequency. ”
For the amplifier shown thi~ occurs when
j=-!L
23rRC’
= =
R=
B+
FIG. 12,10.—Video amplifier.
where R = parallel resistance of R= and R~, and C is the total capacity to
ground which includes output and input capacities plus stray capacities.
Again the need for keeping the stray capacity to a minimum is apparent.
This is sometimes difficult when large coupling condensers or lohg leads
are involved.
There are other coupling networks that will increase the cutoff frequency
and yet preserve the same gain. Perhaps the simplest of these
is the “shunt peaking” circuit, which uses an inductance in series with
the load resistor, RP. Let the parameter M be defined by the equation
L = il{R2C. (12]
Then M can be used as a measure of the performance of the circuit.
Since a value of M = 0.25 corresponds to critical damping, it produces
qo overshoot and yet increases the cutoff frequency by a factor of 1.41.
A value of M = 0.41 is the highest that can be used without puttinga hump
(11)