Transformer-Coupled Amplifier

Engineer Simple

Transformer-Coupled Amplifier

The main reason for low voltage and power gain of RC coupled amplifier is that the effective load (R_AC) of each stage is *decrease due to the low resistance present by the input of each stage to the
preceding stage. If the effective load resistance of each stage could be increased, the voltage and
power gain could be increased. This can be achieve by transformer coupling. By the use of **im-

The input impedance of an amplifier is low while its output impedance is very high. When they are
coupled to make a multistage amplifier, the high output impedance of one stage comes in parallel with the
low input impedance of next state. Hence effective load (R_AC) is decrease.

** The resistance on the secondary side of a transformer reflected on the primary depends upon the turn ratio of the transformer.

pedance-changing properties of transformer, the low resistance of a stage (or load) can be reflected as
a high load resistance to the previous stage.

Transformer coupling is generally employ when the load is small. It is mostly use for power
amplification. Fig. 1 shows two stages of transformer coupled amplifier. A coupling transformer
is used to feed the output of one stage to the input of the next stage. The primary P of this transformer
is make the collector load and its secondary S gives input to the next stage.

Operation

When an a.c. signal is apply to the base of first transistor, it appears in the amplified form across primary P of the coupling transformer. The voltage developed across primary is
transferred to the input of the next stage by the transformer secondary as shown in Fig.1. The
second stage renders amplification in an exactly similar manner.

Frequency response

The frequency response of a transformer coupled amplifier is show in Fig.2.

It is clear that frequency response is rather poor i.e. gain is
constant only over a small range of frequency. The output voltage is equal to the collector current multiply by 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. It follows, therefore, that there will be disproportionate amplification of frequencies in a complete signal such as music, speech etc.
Hence, transformer-coupled amplifier introduces frequency distortion.
It may be added here that in a properly designed transformer, it is possible to achieve a fairly constant gain over the audio frequency range. But a transformer that achieves a frequency response comparable to RC coupling may cost 10 to 20 times as much as the inexpensive RC coupled amplifier.