Difference between bjt and fet pdf

As the lower FET is conducting, by providing a gate voltage, the upper FET conducts due to the potential difference now appearing between its gate and source. This loss of voltage gain is recovered by the upper FET. Difference between bjt and fet pdf, its low input impedance would limit its usefulness to very low-impedance voltage drivers.

Adding the lower FET results in a high input impedance, allowing the cascode stage to be driven by a high-impedance source. CS configuration would offer the same input impedance as the cascode, but the cascode configuration would offer a potentially greater gain and much greater bandwidth. The cascode arrangement is also very stable. Its output is effectively isolated from the input both electrically and physically. The lower transistor has nearly constant voltage at both drain and source, and thus there is essentially “nothing” to feed back into its gate.

The upper transistor has nearly constant voltage at its gate and source. Thus, the only nodes with significant voltage on them are the input and output, and these are separated by the central connection of nearly constant voltage and by the physical distance of two transistors. Thus in practice there is little feedback from the output to the input. Metal shielding is both effective and easy to provide between the two transistors for even greater isolation when required. Ensurance of this condition for FETs requires careful selection for the pair or special biasing of the upper FET gate, increasing cost.