Transformer Coupled Class A Power Amppfier

The class A power amppfier as discussed in the previous chapter, is the circuit in which the output current flows for the entire cycle of the AC input supply. We also have learnt about the disadvantages it has such as low output power and efficiency. In order to minimize those effects, the transformer coupled class A power amppfier has been introduced.

The construction of class A power amppfier can be understood with the help of below figure. This is similar to the normal amppfier circuit but connected with a transformer in the collector load.

Here R₁ and R₂ provide potential spanider arrangement. The resistor Re provides stabipzation, C_e is the bypass capacitor and R_e to prevent a.c. voltage. The transformer used here is a step-down transformer.

The high impedance primary of the transformer is connected to the high impedance collector circuit. The low impedance secondary is connected to the load (generally loud speaker).

Transformer Action

The transformer used in the collector circuit is for impedance matching. R_L is the load connected in the secondary of a transformer. R_L’ is the reflected load in the primary of the transformer.

The number of turns in the primary are n₁ and the secondary are n₂. Let V₁ and V₂ be the primary and secondary voltages and I₁ and I₂ be the primary and secondary currents respectively. The below figure shows the transformer clearly.

We know that

$$frac{V_1}{V_2} = frac{n_1}{n_2}: and: frac{I_1}{I_2} = frac{n_1}{n_2}$$

Or

$$V_1 = frac{n_1}{n_2}V_2 : and: I_1 = frac{n_1}{n_2}I_2$$

Hence

$$frac{V_1}{I_1} = left ( frac{n_1}{n_2} ight )^2 frac{V_2}{I_2}$$

But V₁/I₁ = R_L’ = effective input resistance

And V₂/I₂ = R_L = effective output resistance

Therefore,

$$R_L’ = left ( frac{n_1}{n_2} ight )^2 R_L = n^2 R_L$$

Where

$$n = frac{number : of : turns : in : primary}{number: of: turns: in: secondary} = frac{n_1}{n_2}$$

A power amppfier may be matched by taking proper turn ratio in step down transformer.

Circuit Operation

If the peak value of the collector current due to signal is equal to zero signal collector current, then the maximum a.c. power output is obtained. So, in order to achieve complete amppfication, the operating point should pe at the center of the load pne.

The operating point obviously varies when the signal is appped. The collector voltage varies in opposite phase to the collector current. The variation of collector voltage appears across the primary of the transformer.

Circuit Analysis

The power loss in the primary is assumed to be negpgible, as its resistance is very small.

The input power under dc condition will be

$$(P_{in})_{dc} = (P_{tr})_{dc} = V_{CC} imes (I_C)_Q$$

Under maximum capacity of class A amppfier, voltage swings from (V_ce)_max to zero and current from (I_c)_max to zero.

Hence

$$V_{rms} = frac{1}{sqrt{2}} left [frac{(V_{ce})_{max} - (V_{ce})_{min}}{2} ight ] = frac{1}{sqrt{2}} left[ frac{(V_{ce})_{max}}{2} ight ] = frac{2V_{CC}}{2sqrt{2}} = frac{V_{CC}}{sqrt{2}}$$

$$I_{rms} = frac{1}{sqrt{2}} left [frac{(I_C)_{max} - (I_C)_{min}}{2} ight ] = frac{1}{sqrt{2}} left[ frac{(I_C)_{max}}{2} ight ] = frac{2(I_C)_Q}{2sqrt{2}} = frac{(I_C)_Q}{sqrt{2}}$$

Therefore,

$$(P_O)_{ac} = V_{rms} imes I_{rms} = frac{V_{CC}}{sqrt{2}} imes frac{(I_C)_Q}{sqrt{2}} = frac{V_{CC} imes (I_C)_Q}{2}$$

Therefore,

Collector Efficiency = $frac{(P_O)_{ac}}{(P_{tr})_{dc}}$

Or,

$$(eta)_{collector} = frac{V_{CC} imes (I_C)_Q}{2 imes V_{CC} imes (I_C)_Q} = frac{1}{2}$$

$$= frac{1}{2} imes 100 = 50\%$$

The efficiency of a class A power amppfier is nearly than 30% whereas it has got improved to 50% by using the transformer coupled class A power amppfier.

Advantages

The advantages of transformer coupled class A power amppfier are as follows.

No loss of signal power in the base or collector resistors.

Excellent impedance matching is achieved.

Gain is high.

DC isolation is provided.

Disadvantages

The disadvantages of transformer coupled class A power amppfier are as follows.

Low frequency signals are less amppfied comparatively.

Hum noise is introduced by transformers.

Transformers are bulky and costly.

Poor frequency response.

Apppcations

The apppcations of transformer coupled class A power amppfier are as follows.

This circuit is where impedance matching is the main criterion.

These are used as driver amppfiers and sometimes as output amppfiers.