Electronics > Beginners
Common emitter amplifier output question
Lucky-Luka:
Hi all
Can somebody explain to me why I get increased NPN transistor collector voltage when I decrease the resistor value in the current mirror?
When I decrease that value the current mirror can give more current to the NPN transistor but I cannot understand why my OP voltage increases.
Do I have to see the current mirror as its ouput equivalent resistor? Doesn't that equivalent ouput resistor value decrease when the output current increases? I'm confused.
Cheers
ferdieCX:
When you increase the transistor collector current, you also increase the gm parameter of the hybrid pi equivalent circuit.
Lucky-Luka:
--- Quote from: ferdieCX on December 15, 2019, 05:55:31 pm ---When you increase the transistor collector current, you also increase the gm parameter of the hybrid pi equivalent circuit.
--- End quote ---
ok, but how does this effect affects Vo?
Zero999:
Well if you increase the collector current, then you're effectively pulling the collector up harder, so it intuitively makes sense the collector voltage will rise.
Obviously, this is not a practical circuit and won't work in real life, even though a lot can be learned by playing with it in SPICE.
magic:
Output impedance of the mirror decreases, but this only means that it takes less variation in Q8 collector current to produce larger variation in Q4 collector voltage.
However, your Q8 collector current is constant, determined by R1 and R2, as far as operating point analysis is concerned. Therefore the additional Q4 collector current is not sunk by Q8 until Q8 collector voltage rises a bit, which allows the additional current to flow into Q8 due Q8's output impedance.
Q4 and Q8 are both set up as current sources and they are fighting each other trying to drive different currents through the same wire between them. Output voltage will be determined by the difference between their collector currents multiplied by paralleled output impedances of the two transistors.
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