OPAMP & BJT circuits

Thread Starter


Joined Jul 22, 2007
Hi I have 3 problems on one of my question sheets, which i am not able to solve partially.

I have uploaded the question sheet & and also what I have worked out so far.
Q62 shows the problem sheet. the three problems are 7,8,9(a).

Regarding 7: I have uploaded what i have done so far in Q623 & Q624 and as you can see they are not correct according to the answers on the problem sheet. Rm should be -100kohm and Rin 275ohm but i get -100250ohm for Rm and -250ohm for Rin. :(

Regarding 8: I worked out the Current Gain correctly in the box, but I get 0.9999 for the voltage gain and according to the answer on the Problem Sheet it should be 0.9523, I dont know what went wrong. :(

Regarding 9(a): I have absolutely no clue how to work out the voltage gain for this circuit. I know that the two top resistors form a negative loop but i dont know how to include the bottom resistor and capacitor. Could somebody please tell me how to do this one?? i tried really hard looking up so many books and notes. :confused:

thank you very much for your help in advance :)


for 9a, are you supposed to assume an ideal op amp? Have you dealt with opamp circuits before? I am a bit confused about the second half of the question actually... it appears that Vi is DC, but then it asks for filter response and bode plot which are frequency dependent... That is, they need an AC input to be meaningful.

Anyways, i will assume you havent dealt with opamps before so stop me if you have:

Opamp Golden rules (for ideal opamps) :

1) Current at the input terminals = 0
2) Voltage drop across the input terminals = 0 (that is, the voltage at the +ve input is the same as the voltage at the -ve input)
3) Don't do KCL at the output of the opamp when analyzing the circuit. This is because you don't know the output current of the output.

Use those rules, and apply nodal analysis and see where you get from there

Thread Starter


Joined Jul 22, 2007
ok yeh u r rite our lecturer uses a bad notation for voltage sources; it is an ac source.

yeh i have done opamps bfore so i know about these norators and nullators and we should assume that it is ideal.

but how am i going to find the gain function, where do i start??
please give me a hint
Well i'm sure you know that gain = Output/input. The voltage gain is Vout/Vin. Well basically, for the first problem, what i would do is apply nodal analysis to find the two unknown node voltages (at the inputs of the opamp). Keep in mind that these voltages will be equal. If you do this and some back substitution, you will end up with an expression in terms of only Vout and Vin. Than from there it's just algebra to turn the function into the form Vout/Vin.

I hope this helps. I haven't actually worked the problem, but that is what i would do. If you're still having trouble, i'll work it out and try to help you out a little more specifically. Let me know how it goes.