Tuesday, October 8, 2019

Another look inside a transistor voltage and current relationship

I set the generator to produce a 10mv signal riding a 1700mv bias. The 1k emitter resistor will 'self adjust' to about 1 volt producing the bias required to allow a 1 ma collector current. The emitter resistor would drop AC signal and produce degeneration C1 provides a path for AC. C2 provides a path for AC to the load (R3). The .55ua pk-pk signal provided by V2 when set to 10 mv produces 10mv across R3. It appears the amp produces unity gain? Yes 10 mv in produces 10 mv output but look at the currents. I(V2) is .5ua pk-pk, I(C1) is 100ua pk-pk, and I(C2) is 100 ua pk-pk. So the current gain is 200. Next let's increase the input and see what happens.
I simply increases the AC to 100mv. I(V2) is about 5ua pk-pk. The increase created an increase in base current as would be expected. V(out) is 100mv with 100mv input. We have unity voltage gain again. The current gain is more than 200? Why?
Hint: When a semiconductor current increases its resistance decreases. re is 26 ohms at 1ma. re decreases as current increases. gain is the ratio of Rc to re.
It is a good time for a simulation using this circuit. You could try removing C1, increasing and decreasing C1 and C2 and changing the frequency.