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The cascode amplifier is the two stage amplifier in which common emitter stage is connected to common base stage. The CE-CB cascode. ESE Introduction to Microelectronics. 1. Kenneth R. Laker, update 08Oct12 KRL. High Frequency BJT Model. Cascode BJT Amplifier. ESE Introduction to Microelectronics. 1. Kenneth R. Laker, update 01Oct14 KRL. High Frequency BJT Model &. Cascode BJT Amplifier.

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Cascode amplifier is a two stage circuit consisting of a transconductance amplifier followed by a buffer amplifier.

Miller effect is actually the multiplication of the drain to source stray capacitance by the voltage gain. The drain to source stray capacitance always reduces the bandwidth and when it gets multiplied by the voltage gain the situation is made further worse.

The Cascode Amplifier | Bipolar Junction Transistors | Electronics Textbook

Mulitiplication of stray capacitance increases the effective input capacitance and as we know, for an amplifier, the increase in input capacitance increases the lower cut of frequency and that means reduced bandwidth.


Miller effect can be reduced by adding a current buffer stage at the output of the amplifier or by adding a voltage buffer stage before the input. The circuit diagram of a typical Cascode amplifier using FET is shown above. The input stage of the circuit is an FET common source amplifier and the input voltage Vin is applied to its gate.

The Cascode Amplifier

The output stage is an FET common gate ampliier which is driven by the input stage. Rd is the drain resistance of the output stage.

This reduces the gain of lower FET Q1 and as a result the Miller effect also gets reduced which results in increased bandwidth. In Cascode configuration, the output is well isolated from the input. Q1 has almost constant voltage at the drain and source terminals while Q2 has almost constant voltage at its source and gate terminals and practically there is nothing to feed back from the output to input. The only points with importance in terms of voltage are the input and output terminals and they are well isolated by a central connection of constant voltage.


A practical Cascode amplifier circuit based on FET is shown above. R3 is the drain resistor for Q2 and it limits the drain current.

Cascode amplifier. Theory and working. Cascode amplifier using FET, BJT, Miller effect

R2 is the source resistor of Q1 and C1 is its by-pass capacitor. R1 ensures zero voltage at the gate of Q1 during zero signal condition. Amplifier Circuits Previous post Next post You may also like: Your email address will not be published. Cascode amplifier admin March 13, 1 Comment.

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