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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 EB without any intervening impedance, so that for alternating currents it is essentially grounded. Grid voltage eg must be great enough to include the output E0 in addition to the normal grid-to-cathode voltage at Emin. However, the grid power is still the same as it would be were the cathode grounded. This circuit is used when the output impedance ZL 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 rp 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 gm(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).

Last Update: 2011-02-17