Cathode Follower

TM 11-6660-204-25

age becomes greater than that on the cathode, the

2-5. Cathode Follower

diode conducts and allows current to be fed to the

(fig. 23)

anode from R303. The upper limit of 155 volts on

Cathode follower V302 A is one-half of a minia-

the capacitor (or anode), which is the condition of

ture,

twin-triode tube. Its purpose is to

the circuit before application of the signal is con-

prevent loading of the noise-suppression diode. Its

trolled by a voltage divider string, consisting of

operation is explained with switch S301 assumed

resistor R303, the diode, resistors R307, R396 and

to beat the IN position.

the upper part of R304.

a. The signal from the plate of tube V301B is

d. Assume that a strong signal wave, similar to

fed directly to the grid of cathode follower

that shown at J304 (fig. 2-1), is applied, The ini-

V302A. The plate of tube V302A is connected di-

tial swing is negative, and capacitor C302 dis-

rectly to the regulated +300 volts. The output

charges through resistor R303 and the diode. At

signal is developed across cathode resistor R308

the frequencies involved, the voltage decline on

and is returned to the --175-volt regulated sup-

capacitor C302 closely follows the signal voltage

ply, which is the same as that of the input circuit,

wave. After the completion of the downward

Although the cathode of tube V320A is connected

swing of the signal voltage, the cathode signal

to --175 volts and the grid connected to a positive

potential starts rising. Immediately, conduction in

voltage source, the currest through cathode resis-

the diode is cut off because the relatively long-

tor R308 produce a positive cathode potential,

time constant of capacitor C302, resistors R307,

with respect to ground, that is greater than the

R306, and R304 prevents the voltage from rising

grid potential, so the tube is biased negatively.

on the capacitor (and anode) as the signal voltage

The output signal from the cathode of tube

rises on the cathode; the anode thus remains neg-

V302A is fed to the clipper amplifier through cou-

ative with respect to its cathode during this pe-

pling capacitor C303.

riod. This results in an exponential rise of voltage

b. When the input signal goes negative, the

on the capacitor as shown by the waveform (fig.

plate current decreases and causes the voltage

2-1) at J307; this voltage rises independently of

across cathode resistor R308 to decrease. As the

the signal until such time in the next cycle when

signal swings positive, the plate current increases

the signal voltage on the cathode again drops to a

and causes the voltage to increase across resistor

point where it is less than that on the anode, then

R308, which results in a waveshape across R308

the diode conducts starting a new cycle,

(fig. 2-3) that is similar to the input signal of

e. The output signal from the diode is free of

tube V302A. The amplitude of the output wave-

noise voltage because the positive portion is not

form is slightly smaller than the input, because

affected by the actual signal. It is this portion of

the cathode-follower introduces a slight loss. Jack

the signal wave that triggers the univibrator;

J308 is provided for test purposes where measure-

ments can be made for both the output of V302A

therefore, the noise that gets through the diode on

and the input to tube V302B.

the negative portion of the wave is of no conse-

quence (within reasonable limits).

2-6. Clipper Amplifier

f. Jack J306 is a convenience terminal for

(fig. 2-3)

checking the voltage applied to the noise-suppres-

Clipper amplifier V302B is one-half of a minia-

sion circuit. This voltage is adjusted by poten-

ture,

twin-triode tube. Its purpose is to

tiometer R304 which determines the signal ampli-

distort the sawtooth waveform received from the

tude. Potentiometer R306 is the main control of

preceding stage so its output is approximately a

square wave.

the time constant resistance-capacitance (RC )

circuit, and is adjusted for linearity on the rising

a. The output signal of cathode follower

portion of the signal wave, (Only a small part of

V302AA is applied through coupling capacitor

C303 across grid resistor R310. The signal is then

the exponential curve is used, and this appears to

applied to the grid of tube V302B, through grid-

be linear on the test scope. ) Because there is some

current limiting resistor R309, which clips the

interaction between these controls, the adjustment

positive portions of the signal across resistor R310.

of one will affect the adjustment of the other;

The clipper output signal is developed across

adjustments are alternated until the optimum po-

plate-load resistor R311, which is connected to the

sition is found.

regulated +300 volt supply.

2-5