What is Transformer Coupled Amplifier and Its Working

The major characteristics of the signal are voltage and frequency. If the signal has a sufficient range of voltage, then we can transmit information up to a distance and it is used for communication purposes. Here the interesting concept is “amplifier”. An amplifier amplifies the voltage or increases the voltage value. The designing of amplifiers can be done in several ways. Few of them are transistors based amplifiers; resistor & capacitors based amplifiers, transformer-based amplifiers, etc. To drive more output then multistage amplifiers are introduced. In these multistage-amplifiers, the cascading of amplifiers can be done through capacitors, transformers, inductors, etc. The issues with RC coupled amplifiers are it has the low voltage gain, power gain, low input impedance, and high output impedance. Due to these drawbacks, the transformer-coupled amplifier is used. Coupling the transformers in cascading way at one stage, the input impedance will be high and output impedance will below. By the end of this article, we can understand the terms like what is a transformer-coupled amplifier, its circuit diagram, working, applications, advantages & disadvantages.

What is Transformer Coupled Amplifier?

This amplifier comes under the category of the multi-stage amplifier. In this type of amplifier one stage of the amplifier is connected to the second stage of amplifiers by coupling the “transformer”. Because we can achieve impedance equality through the transformers. The impedances of the two stages can be equaled if any stage has low or high impedance value by transformers. So, the voltage gain and power gain also increases. These amplifiers are preferable when the load is small and used for power amplification purposes.

“The reason behind preferring the transformers in amplifiers is, they provide equal impedance (impedance matching with load can be possible) through primary, secondary windings of the two transformers which are using in the amplifier”.

P1, P2, and B1, B2 are the primary and secondary windings of the transformers. The primary coil and secondary coil impedance are related to B2 = B1*(P2/P1)^2. According to this formula, the two transformers coil impedances are related to each other.

Transformer Coupled Amplifier Circuit Diagram

The above diagram shows the circuit diagram of the transformer-coupled amplifier. In the circuit diagram, one stage output is connected as an input to the second stage amplifier through a coupling transformer. In the RC coupling amplifier, cascading of the first & second stage amplifier can be done through a coupling capacitor. The coupling transformer is T1 & it’s primary and secondary windings are P1 and P2. Similarly, secondary transformer T2 having the primary windings p1 and secondary windings are indicated by p2.

transformer-coupled amplifier
transformer-coupled amplifier
  • R1 & R2 resistors provide the biasing and stabilization for the circuit.
  • Cin isolates DC and allows only AC components from the input signal to the circuit.
  • The emitter capacitor provides a low reactance path to the signal and offers stability to the circuit.
  • The first stage of output is connected as an input to the second stage through secondary windings (p2) of the primary transformer.

Transformer Coupled Amplifier Working

The working and operation of the transformer-coupled amplifier will be discussed in this segment. Here, the input signal is applied to the base of the first transistor. If the input signal has any DC signal then the components can be eliminated by the input capacitor Cin. When the signal is applied to the transistor then it amplifies & forwards to the collector terminal. Here this amplified output connected as an input to the second stage of the transformer-coupled amplifier through secondary windings (p2) of the coupling transformer.

Then, this amplified voltage is applied to the base terminal of the second transistor of the secondary stage of the transformer-coupled amplifier. The transformer has the property of impedance matching. By this property, low resistance of one stage can be reflected as high load resistance to the previous stage. Therefore the voltage at primary windings can be forwarded according to the ratio of secondary windings of the transformer.

Frequency Response of Transformer Coupled Amplifier

The frequency response of an amplifier allows us to analyze the output gain and phase response for a particular frequency or over a wide range of frequencies. The frequency response of any electronic circuit indicates the gain i.e., how much of output we are getting for an input signal. Here, the frequency response of the transformer-coupled amplifier is shown in the following figure.

frequency response-of-transformer-coupled-amplifier
frequency response-of-transformer-coupled-amplifier

It offers low-frequency response characteristics than the RC coupled amplifier. And also transformer-coupled amplifier offers constant gain over a small range of frequencies. At low frequencies, due to the reactance of the primary transformer p1, the gain is decreased. At higher frequencies, the capacitance between the turns of the transformer will act as a condenser and this reduces the output voltage and this leads to decrement in gain.

Transformer Coupled Amplifier Applications

  • Mostly applicable in systems where to match the impedance levels.
  • Applicable in circuits for transferring the maximum power to the output devices like speakers.
  • For power amplification purposes these transfer coupled amplifiers are preferable


The advantages of a transformer-coupled amplifier are

  • It Provides a higher gain than the RC coupled amplifier. It offers 10 to 20 times higher gain value than the RC coupled amplifier.
  • The biggest advantage is it has the feature of impedance matching that can be done by the turn’s ratio of the transformer. So, one stage lower impedance can be adjusted with a high impedance of the next stage amplifier.
  • The collector resistor and base resistor don’t have any power loss.


The disadvantages of a transformer-coupled amplifier are

  • It offers poor frequency responses than the RC coupled amplifier, so gain varies according to the frequencies.
  • In this technique, the coupling can be done by using transformers. So looks bulky and expensive for audio frequencies.
  • There will be frequency distortions in the speech signal, audio signal, music, etc.

The transformer coupled amplifier gives high gain and amplifies the input signal. But to get more output than these types of amplifiers then we can use the power amplifiers. The power amplifiers are preferable to deliver more power to the load like speakers. And the power amplifier’s input amplitude range is higher than the voltage amplifiers. And also in power amplifiers, the collector current is very high (greater than 100mA).

The power amplifiers are classified as

  • Audio power amplifier
  • Class A power amplifier
  • Class B power amplifier
  • Class AB power amplifier
  • Class C power amplifier

All these different types of power amplifiers are categorized based on the mode of operation and flow status of the collector current according to the conduction angle of the input signal. Class A power is simple to design and transistor is in ON condition for the complete input cycle. So, it offers a high-frequency response. But one of the drawbacks is its poor efficiency. This can be overcome by coupling a transformer to the class A power amplifier. Then it’s called a transformer-coupled class A power amplifier. The below circuit diagram shows the transformer-coupled class A amplifier.
You can get more information about the transformer-coupled class A amplifier on.

Thus, this is all about the transformer-coupled amplifier. These are useful to increase the voltage level and power amplifiers are useful to drive more power to the load. And this can be increased by various coupling techniques like implementing the coupling capacitor, the transformer between one stage amplifier to the next stage amplifier. If the coupling can be done through the transformer then we can achieve the impedance matching between inputs to an output. And we can get more efficiency than remain coupling techniques.