Class A Push-Pull Amplifier

A class A push-pull amplifier circuit is shown in Fig.1. A push-pull amplifier means that current flows in the output of the active device (each transistor) for the entire of the input cycle.

Circuit Arrangement:

The circuitry of a typical push-pull amplifier uses two identical transistors, say Q1 and Q2 is shown in Fig. 1. The emitter terminals of two transistors Q1 and Q2 connected together. The input signal is applied to the inputs of two transistors through centre tapped step up transformer T1 which provides opposite polarity signals to the two transistor inputs. The collector terminal of both the transistors are connected to end terminals of centre tapped primary of output transformer T2. The power supply Vcc is connected between the emitter terminals and the centre tap of primary of primary of output transformer. Resistors R1 and R2 are used to provide the proper bias for the circuit. The load Rl is connected across the secondary of the output transformer T2. The turn ratio (2N1:N2) of the output transformer is chosen so as to match the load with the output impedance of the amplifier and, therefore, transfer maximum power. The quiescent currents of the two transistors, which are equal in magnitude, flow in opposite directions through each half of primary of the output transformer T2, so no saturation of the magnetic core occurs.

Fig. 1. Class A Push-Pull Amplifier

Circuit Operation:

When the base current of one transistor is being driven positive with respect to the quiescent point Q, the collector current increases, thus causing a decrease in collector potential relative to ground. At the same time, however, a reverse action takes place in the base circuit of the second transistor, i.e., base current decreases causing a drop in the collector current with a consequent rise in collector potential w.r.t. ground. This means that the ac current flowing through the transformer primary winding is in the same direction. As I1 increases (i.e. pulls), the current I2 decreases (i.e. pushes). Hence the name push-pull amplifier. The overall operation results in ac voltage induced in the secondary of the output transformer and thus ac power is delivered to the load. The difference of two collector currents is illustrated in Fig. 2.

Fig. 2

Advantages:

• Because of absence of even harmonics in the output of the push-pull amplifier, such a circuit gives more output per active device for a given amount of distortion or less distortion for a given power output power transistor.
• As already mentioned earlier, the dc components of the collector current oppose each other magnetically in the transformer core. This eliminates any tendency toward core saturation and consequent non-linear distortion that may arise from the curvature of the transformer magnetization curve.
• Another advantage of this system is that the effects of ripple voltage that may be continued in the power supply because of inadequate filtering will be balanced out. This cancellation results because the currents produced by the ripple voltage are in opposite directions in the transformer winding, and so will not appear in the load. Of course, the power supply hum will also act on the voltage amplifier stages, and so will be part of the input to power stage. This hum will not be eliminated by the push-pull circuit.

Disadvantages:

The drawbacks of push-pull amplifiers are:

(i) requirement of two identical or matched pair transistors

(ii) need of use of driver stage to furnish two equal and opposite voltages at the input and

(iii) need of bulky and expensive transformers.