Cathode Follower

Fig. 134. Cathode follower.

The circuit of Fig. 134 is known as a cathode follower. Here the anode is connected to the high-voltage supply E_B without any intervening impedance, so that for alternating currents it is essentially grounded. Grid voltage e_g must be great enough to include the output E₀ in addition to the normal grid-to-cathode voltage at E_min. However, the grid power is still the same as it would be were the cathode grounded. This circuit is used when the output impedance Z_L is variable or of low power factor so that normally it would be difficult to produce in it full output from the tube. The circuit has a low internal effective impedance as far as the output is concerned. It is approximately equal to the normal plate resistance r_p divided by the amplification factor μ of the tube. This is equivalent to saying that the effective internal impedance is approximately the reciprocal of the mutual conductance g_m⁽¹⁾ for class A or B amplifiers. Cathode followers have been used to drive grids of class B modulator tubes, which are highly variable loads. The circuit produces nearly constant output voltage but at the expense of increased grid swing.

If the tube feeds a low impedance load, output may be increased by coupling the load through a transformer. Frequency response in cathode output transformers is usually flat over a very wide range because of the low effective source impedance.

(1)	See "Feedback," by E. K. Sandeman, Wireless Engr., 17, 350 (August, 1940).