I'm trying to analyze the below circuit. It's a textbook problem (Malvino's Electronic Principles) and the answer is given in the back (8966), but I am unable to reach that answer or approximation.
The goal is to figure out what is the total voltage amplification of the below circuit:
The Op-Amp part is pretty easy, since it's simply a non-inverting amplifier with a feedback configuration, so the amplification is (47k/1k + 1 = 48).
For the transistor, I'm having a difficult time. Particularly, the 1k resistor is throwing me off. I think it's in series with the voltage divider biased circuit, so I am approaching the analysis like this:
\$I_E = (15(10/33)-0.7)(1/5600) = 36.4 mV\$ (assuming 1k, 22k, 10k resistors form a voltage divider)
\$r'_e = 25 mV/I_E = 36.4 ohm \$
\$r_c = 6800\$ assuming the input impedance of the 741c is very high
\$A = r_c / r'_e = 186.8 \$
Total Amplification = 186.8 * 48 = 8967 which gets me pretty close to the textbook's answer.
I don't know if my analysis is correct though. I tried to simulate the circuit in LTSpice, but I got a larger voltage drop across the 1k resistor than my analysis would suggest so I just wanted to see if anyone would approach differently.
Thanks
Best Answer
It is not a voltage divider. For AC signals we assume the capacitors to be a short, both 10uF caps are large enough for this so assume they're shorts.
So the AC input signal is directly on the base, do the 2 base resistors then still matter? Nope
Same for the 5.6k emitter resistor, it is shorted by 10 uF. So for AC this is a common emitter circuit. Only the 6.8k collector resistor matters for the gain.
The other three resistors are only for setting the DC biasing point.