Input impedance

[Mark Hennessy]

It's easy to look at this circuit and think how simple it appears - more simple than the one I presented to NM in the other thread. However, nothing could be further from the truth. This version is popular because it uses one less resistor, but it's a lot harder to analyse. IIRC, you need to revert to simultaneous equations to determine the collector voltage. The collector voltage depends on hfe, so Vc will vary in production. It's OK for small-signal stuff, but where you definitely want Vc in the middle for maximum output swing, you'd pick something else.

Thank goodness for op-amps