BJT collector-base feedback impedance
Hello.
I'm getting trouble on getting (using PSPICE-like software) the input and output impedance of a common-emitter BJT amplifier with collector-base resistor, to try to see how it compares with a more common emitter degeneration feedback (or both). I attached schematic pictures of my set-up. I hope someone can help me see where am I missing something.
As far as I understand, this is a paralell-parallel negative feedback (voltage sampling, current mixing), so, in theory, both impedances should decrease in a factor of (1+AB), being A: amplifier gain, B: feedback circuit gain. Thus, this amplifier should have lower input and output impedance than and emitter-degeneration one (generally speaking, of course).
The first picture shows the circuit and simulation for input impedance, by doing an AC sweep and plotting:
Zi = (voltage at Vi) / (current through R4).
I get 186 ohms. This example is taken out of a book, where the author works out a very close 165 ohms Zi, so I think it's OK...
But, the second picture shows the simulation for output impedance, by placing a 1A test signal at output and shorting amplifier input to ground.
The book states that Zo = 495 ohm.
I get Zo = 4,27 kohms (which is just the parallel of collector and feedback resistor, which I find to be something logic. I don't know how to get the close-to 500 ohm result...
The values are significant. In fact, (1+AB) = 8,63 for this circuit.
and the value 495 ohms is got from:
Zo = 4,27 kohms / 8,63 = 495 ohms.
4,27kohms is what I find.
Any light?
Thank you so much.
Hello.
I'm getting trouble on getting (using PSPICE-like software) the input and output impedance of a common-emitter BJT amplifier with collector-base resistor, to try to see how it compares with a more common emitter degeneration feedback (or both). I attached schematic pictures of my set-up. I hope someone can help me see where am I missing something.
As far as I understand, this is a paralell-parallel negative feedback (voltage sampling, current mixing), so, in theory, both impedances should decrease in a factor of (1+AB), being A: amplifier gain, B: feedback circuit gain. Thus, this amplifier should have lower input and output impedance than and emitter-degeneration one (generally speaking, of course).
The first picture shows the circuit and simulation for input impedance, by doing an AC sweep and plotting:
Zi = (voltage at Vi) / (current through R4).
I get 186 ohms. This example is taken out of a book, where the author works out a very close 165 ohms Zi, so I think it's OK...
But, the second picture shows the simulation for output impedance, by placing a 1A test signal at output and shorting amplifier input to ground.
The book states that Zo = 495 ohm.
I get Zo = 4,27 kohms (which is just the parallel of collector and feedback resistor, which I find to be something logic. I don't know how to get the close-to 500 ohm result...
The values are significant. In fact, (1+AB) = 8,63 for this circuit.
and the value 495 ohms is got from:
Zo = 4,27 kohms / 8,63 = 495 ohms.
4,27kohms is what I find.
Any light?
Thank you so much.