# Circuit with Voltage Amplifier

Discussion in 'Homework Help' started by epheterson, Feb 5, 2009.

1. ### epheterson Thread Starter New Member

Feb 5, 2009
1
0
Here's the question, you don't need to read it all to help me out. I need to find R4.

I'm given R1 = R3 = 10kOhms and Gain = 2.
Using this, I find that R2 = 20 kOhms and using the forumula G=Vc/(V2-V1) with V2=[-5,5], I find that when V2 = 5V, V1 = 2.5V.

So knowing all this, which is pretty much the entire circuit, how can I find R4? I think I have to convert the voltage amplifier into its voltage-source equivalent with a voltage of A.(V+-V_) where A. = infinity. It's this step that confuses me. Is it even necessary to do this?

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2. ### MasterSnow Member

Jan 18, 2009
22
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It looks to me like it's as simple as solving for an expression that equates that output gain of the amplifier to the resistances of the circuit using node analysis. I think you can solve this in two equations. For this, just assume the op amp is ideal - no need to break it down into an equivalent circuit model.

-MasterSnow