Radar System Engineering

SEC. 13.6] AMPLIFIERS 495
by using a smaller value of Rk, which sacrifices gain, or by using a
positive grid bias. The latter is usually to be preferred.
3. To match the impedance of a cable. In this use the cable sees R,
in parallel with the internal impedance of the circuit proper.
Correct methods of matching are shown in Fig. 131 lc.
Another special case of Fig. 13”1la is one in which the plate and
cathode resistors are made equal, thus providing equal signals of both
polarities from a unipolar input signal. Since the feedback ratio is unity
for each output signal, this “split load” or” phase-splitting” amplifier has
many of the desirable properties of the cathode follower, but the singlesided
gain is always less and the internal impedance is greater.
Amplifiers for Dejection-coil Currents.—Special problems are involved
in producing rapidly changing deflection currents such as those involved
in range sweeps, since the voltage across the coil may become very large.
For a linearly increasing current (as in a range sweep) this voltage is
given by
di
di
‘z+ Ri=La+ Rat’
where a is a constant. Since di/dt is also a constant the waveform consists
of a step plus a linear increase. The sweep is usually produced by
an increasing current since, except in special cases, this results in the
minimum average current. As a result, the coil voltage drop during
the sweep results in a decrease in the plate potential of the driver tube.
The drop across the coil reaches its greatest value at the end of the fastest
sweep, and the power-supply potential must be designed to accommodate
this case. Such a sweep need not dissipate much power since the average
current is low, but unfortunately the same power supply is used on high–
duty–ratio sweeps where the average current is high. The total power
dissipation due to the sweep circuit can be minimized by using a large
number of turns on the deflecting coil and a correspondingly high supply
voltage, since this reduces the losses in the driver tube and in the power
supply itself. Usually, however, other circuits derive their power from
the same supply, and a high voltage results in unnecessary dissipation in
them. In consequence, the system is often designed in such a way that
the plate of the driver tube operates on the margin of insufficient voltage
on fast sweeps. Even if this were not the case, the amplifier would
tend to discriminate against the higher frequencies because of the higher
impedance offered by the coils to these frequencies.
As a result of these factors good range sweeps can be obtained only
with the use of greater negative feedback than would be necessary
merely to correct for nonlinearities in the tube in a normal application.