Radar System Engineering

494 THE RECEIVING SYSTEM—INDICATORS [SEC. 136
the plate load is reacthe, thecathode impedance should bealso, with the
same sort of frequency response. For pure resistances the feedback
ratio is Rk/(RL) and Go is RL/(Rk + R~) multiplied by the gain that
would obtain if both resistances were in the plate circuit.
In the cathode followers of Fig. 13”llb, the cathode circuit serves as
the load and the plate is connected directly to B+, resulting ina negative-feedback
amplifier of gainless than 1, characterized by great linearity,
excellent frequency response, high input impedance, and low output
impedance.
Since the entire output voltage appears across the cathode, the feedback
ratio P is unity. Therefore G is always less than 1 [Eq. (l)],
approaching, for large values of the cathode resistor, the value AL/(U+ 1),
where p is the voltage amplification constant of the tube. Since l/GO is
nearly always considerably less than O, the amplifier has a faithful
response.
The internal impedance of the circuit driling the total load is given
by 1 ) where g- is the mutual conductance and RP the plate
g. + l/Rp
resistance of the tube. This is a little less than l/g~, and is therefore in
the range from one to a few hundred ohms.
The input impedance of the tube itself (assuming no grid current)
results from the sum of the capacities between the grid and the fixed
elements (plate, screen, etc.), plus the grid-cathode capacity divided by
1 – G, where G is the gain. This impedance is in parallel with the grid
resistor and the wiring capacity. The effective impedance of the grid
resistor connection can be increased by the method shown on the right
in Fig. 13”1lb, since the drop in the grid resistor is thereby reduced.
Among the applications of the cathode follower are the following:
1. As a low-impedance source to supply the considerable power
required to drive such a load as a transformer or a deflection coil.
2. As a low-impedance source which will provide fairly uniform frequency
response even when considerable capacity is associated
with the load. The time constant and therefore the high-frequency
response will be determined by the capacity in parallel with the
internal impedance of the tube and resistance Rk. Since the tube
impedance is small, good response can be obtained at frequencies
of a few megacycles per second, even with fairly large load capacities.
However, the internal impedance of the tube at any instant
depends upon the actual tube constants at that instant. A steep
negative wavefront may cause the grid to enter into a region of low
g~ or to cut off, if the current flowing is too small to discharge the
ocmdenser with sufficient rapidity. The current can be increased