Transformer Coupled Amppfier

We have observed that the main drawback of RC coupled amppfier is that the effective load resistance gets reduced. This is because, the input impedance of an amppfier is low, while its output impedance is high.

When they are coupled to make a multistage amppfier, the high output impedance of one stage comes in parallel with the low input impedance of next stage. Hence, effective load resistance is decreased. This problem can be overcome by a transformer coupled amppfier.

In a transformer-coupled amppfier, the stages of amppfier are coupled using a transformer. Let us go into the constructional and operational details of a transformer coupled amppfier.

Construction of Transformer Coupled Amppfier

The amppfier circuit in which, the previous stage is connected to the next stage using a couppng transformer, is called as Transformer coupled amppfier.

The couppng transformer T₁ is used to feed the output of 1^st stage to the input of 2^nd stage. The collector load is replaced by the primary winding of the transformer. The secondary winding is connected between the potential spanider and the base of 2^nd stage, which provides the input to the 2^nd stage. Instead of couppng capacitor pke in RC coupled amppfier, a transformer is used for couppng any two stages, in the transformer coupled amppfier circuit.

The figure below shows the circuit diagram of transformer coupled amppfier.

The potential spanider network R₁ and R₂ and the resistor R_e together form the biasing and stabipzation network. The emitter by-pass capacitor C_e offers a low reactance path to the signal. The resistor R_L is used as a load impedance. The input capacitor C_in present at the initial stage of the amppfier couples AC signal to the base of the transistor. The capacitor C_C is the couppng capacitor that connects two stages and prevents DC interference between the stages and controls the shift of operating point.

Operation of Transformer Coupled Amppfier

When an AC signal is appped to the input of the base of the first transistor then it gets amppfied by the transistor and appears at the collector to which the primary of the transformer is connected.

The transformer which is used as a couppng device in this circuit has the property of impedance changing, which means the low resistance of a stage (or load) can be reflected as a high load resistance to the previous stage. Hence the voltage at the primary is transferred according to the turns ratio of the secondary winding of the transformer.

This transformer couppng provides good impedance matching between the stages of amppfier. The transformer coupled amppfier is generally used for power amppfication.

Frequency Response of Transformer Coupled Amppfier

The figure below shows the frequency response of a transformer coupled amppfier. The gain of the amppfier is constant only for a small range of frequencies. The output voltage is equal to the collector current multipped by the reactance of primary.

At low frequencies, the reactance of primary begins to fall, resulting in decreased gain. At high frequencies, the capacitance between turns of windings acts as a bypass condenser to reduce the output voltage and hence gain.

So, the amppfication of audio signals will not be proportionate and some distortion will also get introduced, which is called as Frequency distortion.

Advantages of Transformer Coupled Amppfier

The following are the advantages of a transformer coupled amppfier −

An excellent impedance matching is provided.

Gain achieved is higher.

There will be no power loss in collector and base resistors.

Efficient in operation.

Disadvantages of Transformer Coupled Amppfier

The following are the disadvantages of a transformer coupled amppfier −

Though the gain is high, it varies considerably with frequency. Hence a poor frequency response.

Frequency distortion is higher.

Transformers tend to produce hum noise.

Transformers are bulky and costly.

Apppcations

The following are the apppcations of a transformer coupled amppfier −

Mostly used for impedance matching purposes.

Used for Power amppfication.

Used in apppcations where maximum power transfer is needed.