Transformer Coupled Class A Power Amplifiers

TRANSFORMER COUPLED CLASS A POWER AMPLIFIERS

Instead of connecting the load directly, the output is connected to the load through a transformer as shown in Fig.5.7. This is used for impedance matching and is used for low impedance s like loudspeakers. By adjusting the turns ratio (N₁/N₂), the output impedance is matched with the load impedance.

Almost all the supply voltage will drop across the transistor at the operating point. Hence, the dc load line is almost vertical. For the ac signal, the load resistance seen on the primary side of the transformer is

This acts as R_C of the transistor circuit. The ac load line is known in Fig. 5.8.

The voltage induced in the transformer primary in the direction to aid the supply voltage, so that V_CC becomes 2 V_CC.

Therefore,

The transformer coupling increases the maximum possible efficiency of the amplifier circuit to 50%.

Advantages

1. Maximum Power transfer occurs.

2. DC biasing current doesnot flow through the load so power is saved.

3. Higher efficiency than direct coupled class A amp.

Disadvantages

1. Complex circuit design.

2. Bulky and expensively

3. Due to saturation of transformer core, secondary induced voltage is zero and primary current becomes very large.