OP amp question. Calculate the gain v0/vi

Thread Starter

Marc0P0l0

Joined Aug 20, 2019
7
Hi I'm currently stuck on current question.

Consider the following OP amp circuit in . Obtain the gain vo/vi of the amplifier.

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MOD NOTE: Following text copied from other thread on same question.

What I can see is I can find the voltage on positive and negative pin of top OP amp, which is 30/50*vi (because positive pin of top OP amp is in parallel with 30 ohm resistor, in other words, they have same voltage and inverting and non-inverting pins of OP amplifier have same voltage.
Next, I can find the voltage of bottom OP amp, the same way, 80/120*vi or 20/30*vi.
After this I'm stuck ,as I don't know what to do. Two of the OP amps are connected together, but have different voltages. Is that mean that excess voltage (ie 20/30*vi - 30/50*vi) is at the 2k ohm resistor? I see that I require to use KVL in order to solve it, I just don't know from where to start
 
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WBahn

Joined Mar 31, 2012
25,765
Saying that a specific pin (i.e., a single point) is in parallel with something is meaningless. Even if it didn't, what would make the bottom op-amp any different?

It's just an op-amp analysis problem with the same rules. As you've already identified, if the op-amp is in it's active region, the voltage difference between the input pins will be zero. Use that to determine the voltages and currents in the resistors on the right part of the circuit.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
9,875
What I can see is I can find the voltage on positive and negative pin of top OP amp, which is 30/50*vi (because positive pin of top OP amp is in parallel with 30 ohm resistor, in other words, they have same voltage and inverting and non-inverting pins of OP amplifier have same voltage.
Next, I can find the voltage of bottom OP amp, the same way, 80/120*vi or 20/30*vi.
So now you can work out the current in the 2k resistor.
How much current flows in the inverting inputs?
Therefore how much current in the two 10k resistors?
Therefore what the output voltages of the first two opamps?
 
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