A.F. Class B Amplifiers

Author: J.B. Hoag

Fig. 23 I. A Class B amplifier delivers a larger output for a given amount of distortion than a Class A or push-pull amplifier. The tapped windings are in the same direction in both transformers

The circuit diagram of an audio frequency, double-ended Class B amplifier (Fig. 23 I) is the same as that of a push-pull amplifier. The C-battery voltage is adjusted so that the tubes operate at, or slightly above, cutoff. Sharp cutoff tubes are used.

Figs. 23 J and K. Principle of operation of Class B amplifiers

Figure 23 J shows the action taking place in tube number 1. This curve is redrawn in Fig. 23 K, together with a similar but inverted curve for tube 2.

The principle of operation is as follows: when the grid of tube 1 goes positive, the grid of tube 2 goes negative and the current in the plate circuit of 2 remains at its previous value (zero). But the plate current of the upper tube increases, giving an output voltage which is, say, positive at the top. On the second half-cycle, the grid of the upper tube goes negative; but, since its plate current is already zero, there is no change in the upper circuit. The lower grid, however, becoming positive, gives an increase in its plate current and this, in turn, induces a voltage in the secondary of the output transformer. The polarity of this voltage is the reverse of that when the upper tube was the conductor, i.e., a negative voltage is induced at the top of the output. Thus tube 1 handles all of the positive half-cycles, tube 2 handles all of the negative half-cycles, and the output transformer combines them to form a sinusoidal wave, as at the right of Fig. 23 K.

It is clear from the discussion above that a double-ended Class B amplifier can handle nearly twice the input voltage of a push-pull amplifier, for a given amount of distortion. It is also obvious that this is approximately four times greater than the permissible input voltage for a one-tube amplifier having the same amount of distortion. There is, of course, a certain amount of distortion in the Class B amplifiers due to the slight curvature of the characteristic curve near the cutoff. Also, Class B amplifiers eliminate the even harmonics, as in the case of push-pull amplifiers.

As an added advantage, Class B amplifiers prove to be more efficient in operation than Class A. This is because the d.c. plate currents are practically zero in both tubes before the signal arrives. The energy of the plate battery is consumed only during the time of amplifying a signal. Inasmuch as power is proportional to the square of current, the power output of Class B amplifiers is proportional to the square of the exciting grid voltage.

As compared with Class A or Class C amplifiers, Class B amplifiers have intermediate power outputs, plate efficiencies (of the order of 50 per cent), and power amplification.