I'm trying to solve for \$R_{x}\$, \$R_{y}\$, and \$R_{z}\$, given a know \$R_{1}\$, \$R_{2}\$, \$V_{in}\$, \$V_{out}\$, \$I_{in}\$, \$I_{sc}\$, \$V_{CC}\$, and \V_{EE}\$,

\$V_{CC}\$ , \$V_{EE}\$, after being measured with a DMM should be \$+10 V\$ and \$-10V\$ respectively

If i measure \$I_{SC}\$,\$V_{OC}\$, \$V_{IN}\$, \$I_{IN}\$. I should have enough information - but I'm not sure of my calculations.

There are some formulas I have found: \$\frac{V_{IN}}{I_{SC}}=r_{1}\$; \$\frac{V_{IN}}{V_{OC}}=\frac{-r_2}{r_1}\$; and \$\frac{V_{IN}}{V_{OC}}=r_3\$

Any help would be greatly appreciated. Even just helping me setup the simulator too. [circuitlab]82wj9z[/circuitlab]


simulate this circuit – Schematic created using CircuitLab

EDIT: I corrected the schematic - I meant for VEE to be on the blue wire. The goal here is to be able to determine what \$r_1\$, \$r_2\$, and \$r_3\$ are just by measuring the voltage and current in/out. Then, I should be able to calculate what \$r_x\$, \$r_y\$, and \$r_z\$ are from \$r_1\$, and \$r_r\$. The second part I really am not sure about, but I haven't been able to test it.

I guess \$r_2\$ is my \$r_{feedback}\$, and \$r_{z}\$ is my \$r_{load}\$?


First, VEE should be your blue line, not the + input to the amplifier, and you should show separate supplies, both referenced to ground.

As to resistor values, the output resistor is easy - zero ohms. Any ouput resistor will probably interact with the next stage and reduce the effective gain, so don't use one unless you have to. An example of when you have to is driving a terminated transmission line such as 50 ohms, and in that case you would select a resistor equal to the load resistor.

As for the input and feedback resistors, let's take it one at a time. I'm assuming you want to produce an inverting amplifier. You have to realize that \$R_1\$ and \$R_2\$ will act as a voltage divider, and will produce a Thevenin equivalent voltage of $$V_T = V_{\text{IN}}\frac{R_1}{R_1+R_2}$$ with an equivalent resistance $$R_T = \frac{R_1\times R_2}{R_1+R_2}$$

So pick a handy \$R_{\text{IN}}\$. It doesn't matter too much at this point. The input to the op amp now looks like \$V_T\$ followed by \$R_T + R_{\text{IN}}\$. So the output voltage will be $$V_{\text{OUT}} = V_T \frac{R_{\text{feedback}}}{R_T + R{_{\text{IN}}}}$$ and $$R_{\text{feedback}} = \frac{V_{\text{OUT}}\times{(R_T + R_{\text{IN}})}}{V_T}$$

If \$R_{\text{feedback}}\$ is inconveniently high or low, adjust \$R_{\text{IN}}\$ as needed.

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  • \$\begingroup\$ What vales of \$R_{x}\$,\$R_{y}\$, or \$R_{z}\$ can I choose to test my calculations? I have tried just arbitrary values but I can't always get them. \$\endgroup\$ – LetThisNightExplode Oct 13 '15 at 15:32
  • \$\begingroup\$ @LetThisNightExplode - How much precision do you need? And why can't you, for instance, order resistors through Digikey or Allied? They are perfectly happy to sell you 1% resistors for a few pennies apiece. \$\endgroup\$ – WhatRoughBeast Oct 13 '15 at 15:36
  • \$\begingroup\$ Precision is irrelevant to the question, nor are ordering practices. The questions states the resisters concerned are provided. \$\endgroup\$ – LetThisNightExplode Oct 13 '15 at 21:26
  • \$\begingroup\$ Sorry, @WhatRoughBeast - I'm not sure which resister \$R_{in}\$ is. Is this the resister I have labelled \$R_{1}\$?. I'm trying to solve for \$R_{x}\$, \$R_{y}\$, and \$R_{z}\$, given a know \$R_{1}\$, \$R_{2}\$, \$V_{in}\$, \$V_{out}\$, \$I_{in}\$, and \$I_{sc}\$. \$\endgroup\$ – LetThisNightExplode Oct 13 '15 at 21:29

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