• If the voltage or power gain obtained from the single stage small amplifier is not sufficient for practical application, then we have to use more than one stage of amplification to achieve necessary power or voltage gain.Such an amplifier is known as multistage amplifier.

• In multistage amplifier the output of first stage fed as input to next stage as shown in the figure such a connection is commonly referred to cascading as shown in the figure..

• In amplifiers, cascading is also done to achieve correct input and output impedance for specific applications.Depending upon the type of amplifier used in individual stages, multistage amplifiers can be classified into several types.

1. A multistage amplifier using two or more single stage common emitter emitter amplifier is called as cascaded amplifier.

2. A multistage amplifier with common emitter as the first stage and common base as second stage is called cascode amplifier.

• Such cascade and cascode connection is also possible with FET amplifiers.

• When the amplifiers are cascaded, it is necessary to use a coupling network between output of one amplifier and input of following amplifier.This type of coupling is called as inter stage coupling.

• In multistage amplifier there are three coupling techniques are used.

1. Resistance-capacitance (RC) coupling: It is most commonly used and least expensive technique which has satisfactory frequency response.In this method the signal developed across the collector resistor RC of each stage i.e coupled through output coupling capacitor Cc to the base of next stage.Cc isolates the DC conditions of first stage to the following stages.

2. Transformer coupling:In this method the signal develops across the primary winding of the transformer which is act as an load .Secondary winding transfers the ac output signal directly to the base of next stage.Such technique increases the overall gain and level impedance matching.however transformer with broad frequency response is very expensive.

3. Direct coupled: In this method the AC output signal is fed directly to the next stage, no reactance is used in coupling network.It is used when amplification of low frequency signal is to be done.

• Frequency response of multistage amplifiers: The voltage gain of the amplifier and phase shift of the gain depend on the frequency range over which the amplifer operate.In general the enitire frequency range can be divided into three type. 1. Mid frequency range

1. Low frequency range

2. High frequency range

• In general analysis of multistage amplifiers we need to find out different parameters.

1. Voltage Gain: Voltage of mulitstage amplifier is equal to the product of voltage gain of individual stages.

2. Current Gain: Current gain of multistage amplifiers is equal to the product of current gain of individual stages

3. Input impedance : it is the impedance of first stage

4. Output impedance : Its is theimpedance of last stage.