4

The small signal stages

‘A beginning is the time for taking the most delicate care that the balances are correct.’ Frank Herbert, Dune.

The role of the input stage

The input stage of an amplifier performs the critical duty of subtracting the feedback signal from the input, to generate the error signal that drives the output. It is almost invariably a differential transconductance stage; a voltage-difference input results in a current output that is essentially insensitive to the voltage at the output port. Its design is also frequently neglected, as it is assumed that the signals involved must be small, and that its linearity can therefore be taken lightly compared with that of the VAS or the output stage. This is quite wrong, for a misconceived ...

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