Transfer Function for Op-Amp Circuit

Thread Starter

tquiva

Joined Oct 19, 2010
176
Could someone please assist me w/ 1a of this problem?
I'm not sure what method to use.
I know I would first convert it to frequency domain, but what's next?

 

t_n_k

Joined Mar 6, 2009
5,455
You could start by considering all (ideal) op-amp input terminals being at Vo/2. This is constrained by the 1Ω/1Ω voltage divider at the positive input of the lower amp.

This looks to be a tuned circuit with the 1Ω & C values giving the resonant frequency [f0=1/2∏C] and R determining the circuit 'Q' factor.
 
Last edited:

t_n_k

Joined Mar 6, 2009
5,455
Yes - otherwise the circuit isn't operating in the linear region. The amplifiers would be considered to be ideal, for the purposes of deriving the transfer function.
 

t_n_k

Joined Mar 6, 2009
5,455
Had a bit of a closer look at the circuit response using simulation - it probably isn't a true tuned circuit. Overall, it's rather more like a highpass filter with a curious tunable peak near the corner frequency. Perhaps it has some audio function - judging by the remainder of the question.
 

Thread Starter

tquiva

Joined Oct 19, 2010
176
Thank you. So for each + terminal of each op amp, the voltage is Vo/2, and for each negative terminal, the voltage will be a 0? Is that correct?
 

Thread Starter

tquiva

Joined Oct 19, 2010
176
Alright, but there's one part I'm a bit confused about. Say I want to take the nodal voltage at the very first node at the very top left corner. If I were to name it Va, with the input at Vs, then:

(Vs - Va)(sC) + (0 - Va)/R = Va - ?????

I'm not sure what to put in for the last current that is flowing out of that node. There's no element present besides another node.
What would the equation be for this case?
 

t_n_k

Joined Mar 6, 2009
5,455
Well, call the lower opamp output node Vb, and you have

(Vs - Va)(sC) + (0 - Va)/R = (Va - Vb)/1

You just need to work out the relationship between Vo and Vb.

As already discussed, Va=Vo/2
 

Thread Starter

tquiva

Joined Oct 19, 2010
176
So basically... I have 4 node voltages already known which are the input terminals which is Vo/2. This leaves 2 more voltages that I need to solve for, and from then, I am able to solve for Vo.

Is my method correct?
 

t_n_k

Joined Mar 6, 2009
5,455
Also, remember I gave you a lead in suggesting you find the relationship between the output Vo and the feedback integrator's output - the lower opamp circuit being the integrator stage.
 

t_n_k

Joined Mar 6, 2009
5,455
Everywhere along the "conductor" line, from the right-hand side of the input capacitor to the +ve input terminal of the top opamp, will be at the same potential of Vo/2.
 
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