# Resistors connected to op-amps

#### ericgibbs

Joined Jan 29, 2010
16,790
Hi NE,
Welcome to AAC.
E

#### Ian0

Joined Aug 7, 2020
6,705
There are four resistors which have no voltage across them, so you can ignore them.
There are four more resistors where the current through them has no effect on the operation of the circuit. You can ignore them as well.
That only leaves three that you have to deal with.

• BobTPH

#### crutschow

Joined Mar 14, 2008
31,123
Should I calculate an equivalent resistance for each group? And how do the resistors connected between the terminals of the op-amps influence the circuit?
No, you can't solve for the equivalent resistance to solve the problem.

Do you understand the characteristics of an ideal op-amp and how it works with negative feedback?
If not, then first read a tutorial on that.

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#### MrChips

Joined Oct 2, 2009
27,679
Firstly, consider the impedance of a source driving a load.
In general, you want the impedance of the source to be lower than 1/10 that of the load. In this way, the effect of the load is less than 10% on the source.

Assuming that the impedance of Vi is much lower than Ri, you can ignore Ri.
Similarly, the output impedance of an ideal op-amp is 0Ω. For this reason, ignore R4, R9, and R10.

#### Ian0

Joined Aug 7, 2020
6,705
And how do the resistors connected between the terminals of the op-amps influence the circuit?
WHat is the voltage between the inputs of the op-amp when the feedback is operating correctly?
So how much current flows in the resistors?
How will that current affect the rest of the circuit?

#### MrChips

Joined Oct 2, 2009
27,679
Two rules apply when working with ideal op-amps.

1) No current flows in the input pins.
2) The inputs must be at the same voltage otherwise the output will saturate to +infinity or -infinity.

How did I arrive at this?

Here are the properties of an ideal op-amp:
1) The input impedance is infinite.
2) The output impedance is zero.
3) The voltage gain is infinite.
4) The bandwidth is infinite.

#### WBahn

Joined Mar 31, 2012
27,880
Let's give the TS an opportunity to apply the numerous hints they've already been given and to show their best attempt to work the problem.

I wonder if the two hidden 'gotchas' in the circuit were intentional, but let's let them come out in due course.

• LvW and Kjeldgaard