# Coupling in amplifiers

## What are the types of coupling used in amplifiers?

The term Coupling occurs in multi stage amplifiers and refers to the way in which the output of one stage is connected to the input of next stage. There are mainly three types of coupling which are

• Transformer coupling
•  Capacitive coupling
•  DC coupling
• Opto coupling

## What is direct coupling?

A direct coupling is one in which the output of one stage of amplifier is connected to the input of second stage in such a way to allow even DC signals from one stage to another. The frequency response of the direct coupled amplifier is similar to low pass filter and is characterized by rise time.

The main disadvantage with DC coupling is, since there is no DC isolation the first stage acts as a biasing circuitry for the second stage, the AC signal current is superimposed on DC quiescent currents and this effect propagates.

## What is capacitive coupling?

A capacitive coupling is one in which the output of one stage of amplifier is connected to the input of second stage through a capacitor. This is the most frequently used coupling. The coupling capacitor blocks DC signal propagation across the amplifier and allows only AC signals. This makes the circuit analysis and design simplified and each stage can be considered as isolated as far as DC signals are considered. Also it avoids the amplifiers going into DC saturation.

## What is transformer coupling amplifier?

A Transformer coupling is one in which the output stage of one amplifier is coupled to the input stage of next amplifier through a transformer. By choosing the appropriate turns ratio a transformer can be used to provide impedance matching with the load. If N1, N2 and Z1,Z2 are the turns ration and impedance of primary and secondary coil of the transformer then the primary coil impedance and secondary coil impedance are related as Z2 = Z1*(N2/N1)^2. Such coupling is used in high frequency amplification. Introduction of Transformer makes the amplifier bulkier and costly.

## What is miller’s theorem?

The Miller’s theorem establishes that in a linear circuit, if there exists a branch with impedance Z, connecting two nodes with nodal voltages V1 and V2, we can replace this branch by two branches connecting corresponding nodes to ground by impedances Z/(1-K) and K*Z/(K-1) where K = V2/V1. It is shown in the figure below

millers theorem

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