# Analyzing a dual op-amp circuit

Discussion in 'Homework Help' started by d_695, Mar 7, 2011.

1. ### d_695 Thread Starter New Member

Feb 27, 2011
5
0
Hello!

I'm studying for a test, and I wanted to make sure I'm trying to do this right, but I also don't know how to calculate a couple things.

This is what I get by changing the capacitors to open circuits:​

Pins 2 and 3 of U2 are at 7.5 volts. The output voltage of U2 is also 7.5 volts, since pin 2 of U2 is connected to pin 6 of U2. This is connected to pins 2 and 3 of U1, which are both going to be 7.5 volts. Since both those pins are also connected to pin 6 of U1, the bias voltage is going to be 7.5 volts. PSPICE gives approximately the same answer, when I simulate the DC model of this circuit.​

Am I correct in assuming that it's 1/(2*(Pi)*1000*0.000010) = 15.9155 Hz?​

Now here, I don't know how to calculate this. Simulating the circuit in PSPICE gives me a gain of 1.05 volts at 1 KHz, using the following values for V3:​

Code ( (Unknown Language)):
1. AC=1
2. [LEFT]VOFF=0
3. VAMPL=1 [/LEFT]
4. FREQ=1KHz

Power is voltage times current, but given the circuit, I'm kind of lost, even though it's probably something simple.​

Thanks in advance for any help!​

2. ### mjhilger Member

Feb 28, 2011
119
16
I think a) and b) look fine;
c) You just took the output into C2 & R3; I'd have to do some math, but are you sure you can ignore the integrator of C3 & R1? I think they might come into play. I have not written the eqn's so maybe not.
d)I don't know what you are doing in your class, but when he asks for gain - is it just the pure peak to peak gain? Or does he want the vector gain (amplitude and phase)?
e)look at you schematic, you have already realized that U2 just provides a 7.5V point (theoretically 0 ohms) what about the input into the U1 op amp, what is its impedance (if this is ideal op amp, then infinity) so that leaves what? Large value C1 its bias value won't change - for ac value it is 0, so what's left?

I think you are mainly on the right track, but just for completeness you should calculate the cutoff of C1 & R4 input into U1. Then it is just the analysis of the U1 gain. I think you are going to have to include C3, R1 and R2 into the mix.