Transimpedance Amplifier: Silly question that’s killing me

operational-amplifier

I'm having huge trouble with this problem, I've probably done it 10 times already and there must be something that's slipping my attention.

The givens are:
\$I_{PMT}\$ = 150.0 pA, R1 = 73.0 MΩ, R2 = 2.0 kΩ and R3 = 200.0 kΩ.
What my calculations have been:

V+ = 0 so V- (at U1) is also zero. The equation at the node entering V- :

\$\frac{(0-V_O)}{R_1} + I_{PMT} = 0\$

which gives:
\$V_o = -I_{PMT} * R_1 = -73*150*10^{(-12+6)}\$
in the other input node entering V- of U2:

\$\frac{(0- V_O)}{R_2} + (0 – V_{out})/R3\$

which gives:
\$V_{out} = 73*-150*10^{(-4)} = -1095 mV\$

Someone help!!!

Thank you 🙂

enter image description here

edit1: by V_o I mean the voltage at node after R1 and before R2- the node output for the first op-amp

Best Answer

Your math seems to work out for me, though you're missing the inversion in the second section.

Try breaking the schematic up into two complete sections, and solve them independently.

\$V_o = I_{PMT} * R_1 = 73,000,000 * 0.000,000,000,15 = 73 * 10^{6} * 150 * 10^{-12} = 0.01095 V\$
at the point you're calling \$V_O\$ (e.g. the output node of U1).

Your second-stage gain is: \$V_{gain} = -\frac{R3}{R2} = -\frac{200k}{2k} = -100\$

your output voltage will be \$0.01095 * -100 = -1.095V\$

Your problem seems to be that you forgot that the inverting amplifier, well, inverts. You need to multiply your output voltage by -1.

Silliness: Wolfram Alpha page for the calculation.


Further verification - I stuck the circuit in the falstad circuit simulator. It's an ideal-circuit simulator, so it's not great for real-world analysis, but you're evaluating this circuit with ideal op-amps anyways, so that's not a problem.

enter image description here

At this point, if you're still getting an answer that is "incorrect" (I assume this is homework), you either have a typo somewhere, or the "correct" answer is actually not correct.