Input Impedance of a Basic Common-Emitter Amplifier

[beeboopbeep]

Hi everyone,

I was reviewing analog circuits and was studying common-emitter amplifier from Art of Electronics (3rd edition) but I got confused analyzing a circuit. The book states that when looking at the input impedance of a common-emitter amplifier as shown below, it would be R1 || R2 || (B*R_E) where B is the current gain of the NPN.



However, why wouldn't input see the R_C resistance? The "Ohm model" of the NPN transistor is as shown above, so it makes sense to me that at the input node (i.e., base of the NPN), it would see R_C resistance as part of the input resistance. Is it not included because you don't include any impedance at the output node of the circuit?

[Ratch]

In the active mode of operation shown, the collector-base (CB) is reversed biased.  This causes the the CB junction to present a very high resistance to the base input circuit.  Since this resistance will not appreciably affect the input impedance, it is ignored.

Ratch

[beeboopbeep]

In the active mode of operation shown, the collector-base (CB) is reversed biased.  This causes the the CB junction to present a very high resistance to the base input circuit.  Since this resistance will not appreciably affect the input impedance, it is ignored.

Ratch

That makes sense! Thank you for the answer.

[danadak]

Here is a decent treatment of the question -


http://www.electronics-tutorials.ws/amplifier/input-impedance-of-an-amplifier.html


Regards, Dana.

[b_force]

Maybe a more general answer would be helpful to give.

In general, if looking at AC signals, all DC/GND nodes can be seen as the same (shorted).
This can be really a trap for people when biasing opamps or other circuits on Vcc/2 (or any other bias voltage for that matter)