(Transformer Coupling) POWER AMPLIFIERS (Transformer Coupling)

Class-A The theoretical maximum efficiency of a basic RC- coupled class-A amplifier is limited to 25%. In practical circuit, the efficiency is less than 25%. Used for output power of about 1 W only. Transformer coupling can increase the maximum efficiency to 50% Disadvantage of transformer coupling – expensive & bulky.

Transformer-coupled common-emitter amplifier Neglecting transformer resistance and assuming RE is small;

For ideal transformer;

Turn ratio is designed for maximum symmetrical swing, hence; The maximum average power delivered to load equals maximum average power delivered to the primary of the transformer (VCC and ICQ are maximum possible amplitudes of signal)

The average power supplied by the VCC source is; The maximum possible efficiency is;

EXAMPLE

Solution DC analysis

Using Thevenin’s theorem;

Taking the VBB – B – E loop; Substituting values;

Using reflected resistance, the circuit becomes as shown

The corresponding ac equivalent circuit; Using hybrid- model, the circuit may be represented as follows;

The voltage gain; And the amplitude of the output voltage across the transformer primary is;

At the transformer secondary; n1 : n2 = 3 : 1

The average power to the load; The average power supplied by the source; The power conversion efficiency;

Transformer-coupled emitter-follower amplifier The maximum power conversion efficiency = 50%

EXAMPLE Known: Calculate a for PL = 5 W Calculate 

Solution DC analysis

Replacing the Tx with the equiv. impedance The AC equivalent circuit

The equivalent circuit (hybrid- model); The voltage gain is;

The amplitude of the output voltage is;

For 5 W output power; Hence; The transformer turn ratio;

As calculated previously; Neglecting power in R1 and R2, the power supplied by the source is; The power conversion efficiency is;