[AMPS] high isolation amplifiers

[Peter Chadwick]

The simple mini circuits devices don't have that good a performance.

If you're designing for really high reverse isolation in a wideband amp,
there are a number of techniques. Firstly, only use shunt nfb over one
stage, because you don't want feedback through the nfb network right the way
back from output to input. So unbypassed emitter resistors are preferable to
shunt feedback. Cascodes are pretty good, provided you can really ground the
base of the upper stage, and you have enough volts of supply. Emitter
follower ouputs are some help. Watch supply rejection, and use long tailed
pairs to improve it.

I've found that as oscillator buffers, changes in input impedance as a
result of load changes is the biggest problem, and have used, with some
success, grounded base input stages with low input impedance, preceded by a
series resistor which stabilises the input impedance. I guess you could do
the same with a grounded emitter or source stage, but you still get the
problems of Miller effect on the input impedance, so I think it tends to be
worse.

A good simulator helps!

73  Peter G3RZP

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