Amplifier. - Here are Circuit Schematic Cascode Amplifier using FET. Cascode amplifier is a two stage circuit consisting of a transconductance amplifier followed by a buffer amplifier. The word “cascode” was originated from the phrase “cascade to cathode”. This circuit have a lot of advantages over the single stage amplifier like, better input output isolation, better gain, improved bandwidth, higher input impedance, higher output impedance, better stability, higher slew rate etc. The reason behind the increase in bandwidth is the reduction of Miller effect. Cascode amplifier is generally constructed using FET ( field effect transistor) or BJT ( bipolar junction transistor). One stage will be usually wired in common source/common emitter mode and the other stage will be wired in common base/ common emitter mode.
The input stage of the circuit is an FET common source amplifier and the input voltage (Vin) is applied to its gate. The output stage is an FET common gate amplifier which is driven by the input stage. Rd is the drain resistance of the output stage. Output voltage (Vout) is taken from the drain terminal of Q2. Since the gate of Q2 is grounded, FET Q2′s source voltage and the FET Q1′s drain voltage are held almost constant. That means the upper FET Q2 offers a low input resistance to the lower FET Q1.
The Original Article : "Cascode Amplifier"
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