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EE201-SEMICONDUCTOR DEVICESCHAPTER 4 : MULTISTAGES AMPLIFIER

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Introduction∗ Amplifiers containing one active device have limited gain, input impedance, output impedance and power handling capabilities.∗ Multistage amplifier is used to increase the gain or modify the characteristics of the amplifier to satisfy a particular specification.

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Working principlesa) Cascode connection- Places one transistor on top of anotherb) Cascade connection- Provides stages in seriesc) Coupling- Circuitry used to connect the output of one stage of a multistage amplifier to the input of next stage.

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Block diagram

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The Gain∗ The overall gain of a multistage amplifier is the product of the gain of the individual stage Gain (A) = A1A2A3A4….AN∗ If the gain of each amplifier stage is expressed in decibels (dB) the total gain is the sum of the gain of individual stage Gain in dB (A) = A1+A2+A3+A4+…AN

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Type of multistage∗ Depending on the manner which the different amplifier stages are connected, one of the following amplifiers may result :a) RC coupling @ capacitor couplingb) Direct coupling (CE-CC, Darlington pair)c) Transformer coupling

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RC COUPLING

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Cont’∗ One way to connect various stage is via capacitor∗ Coupling capacitor is used to block the flow of dc current between the output of one amplifier stage and the input of the next stage

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Advantages∗ Make dc bias at the output of one stage is different from dc bias voltage at the input to next stage∗ Prevent the flow of dc signal current between stage∗ Provides enough high frequency to keep capacitive reactance small

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Disadvantage∗ Affects the lower frequency response of amplifier

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DIRECT COUPLING

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Cont’∗ In this type of amplifier, the output of one stage of amplifier is connected to the input of the next stage directly without using any reactive component like capacitor, inductor.∗ As a result, the frequency response of this amplifier is quite flat all the way down to dc (0Hz)

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Advantages∗ Simple circuit∗ Can be used to amplify zero and low frequency signal∗ Used in differential and op-amp

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Disadvantages∗ At high frequency, gain decrease∗ Transistor parameter change with temperature, hence output is affected by temperature variation∗ Any noise appearing at the input also present at the output in amplified form due to high gain

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DARLINGTON PAIR

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Cont’∗ Collector of two BJT are tied together and emitter of one is direct coupled to the base of the other∗ The combination is used in amplifier circuit as if were a single transistor having three terminal – B,C,E∗ Is important and useful configuration because has high B BD = B1B2

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TRANSFORMER COUPLING

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Cont’∗ Transformer coupling is where the primary winding is in the output of one stage and secondary winding is in the input circuit of the following stage∗ The ac signal is passed from one stage to the next stage without the possibility of dc current flowing between the two windings

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Advantages∗ Low power dissipation∗ Capability of designing a turn ratio that result in maximum power transfer between stage∗ Often used in narrow band application (RF amplifier)

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Disadvantages∗ Poor frequency response due to inductance and inter- winding capacitance tend to reduce the usable bandwidth of these amplifier

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Frequency response curve of multistage

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Cont’∗ The greater the number of identical stages, the lower cut-off frequency is increasing and the upper cut-off frequency is decreasing∗ Therefore, this will reduce the bandwidth of the multistage amplifier