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Op Amps

puck123

Sep 3, 2019
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Been struggling with this for a while now.

Trying to figure out Rf and Ra on a op amp circuit but unsure what equation to use, I have values for v1 v2 v3 and vout any help would be appreciated
 

Harald Kapp

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Welcome to EP.
A schematic diagram with the components, nets and values properly labeled could be of some help in understanding and answering your question. Don't you think so, too?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Since this is homework, we aren't going to solve your problems for you, but here are a few tips to get you started...
First, assume the op-amp has infinite gain. This means that for a finite output Vo = 5V that Vp-Vn = 0; that there is no current flowing between the inverting (-) and non-inverting (+) op-amp inputs; and the same current flows through Rf as flows through Ra.
 

Harald Kapp

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One more tip: calculate VP from V1, V2 and V3 assuming ideal voltage sources (Ri=0 and an ideal OpAmp (input resistance = infinite equiv. to no input current as stated by Hop).
 

puck123

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Thankyou, I was under the impression I could work out the gain = Vout/ Vin
 

hevans1944

Hop - AC8NS
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Thankyou, I was under the impression I could work out the gain = Vout/ Vin
If this were the usual op-amp circuit, with non-inverting gain for a single input applied to the (+) input, then you could say that the "gain" for that input is 1 + Rf / Ra. You should derive this equation yourself to prove you know what you are doing.

However, it becomes a little more complicated when the output voltage and the input voltages are all specified but the resistor values are not. Do what @Harald Kapp said: calculate the voltage at Vp. This will be the same as the voltage at Vn. From this information you should be able to calculate resistor values for R, Rf and Ra to "make it so" to produce 5V output with the three specified input voltages. Note that the resistor values are arbitrary in their absolute values.
 
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