Measuring Input Impedance of Transistor with LTSpice

Question

How do you measure the input impedance of a transistor with LTSpice?

Consider the following circuit:

The above just biases a transistor so that the collector is 1/2 the supply. I used a current source with 1A AC. I then plotted the voltage at the base divided by the current at the base because we all know R = V/I.

So from this plot it looks like it's about 2.9K. Is this correct?

Although the impedance depends on the bias. Obviously with the bias high enough the impedance of the transistor is 0 - it's just a diode. Right?

Answer 1

So from this plot it looks like it's about 2.9K. Is this correct?

Yes

Obviously with the bias high enough the impedance of the transistor is 0 - it's just a diode. Right?

Probably not. The diode model probably includes parasitic resistance in the base and emitter contacts, so the input resistance will never go all the way to zero. Also, your R3 (multiplied by the transistor beta) will set a lower limit on how low a resistance you measure looking in to the base of the transistor in your circuit.