What is a good procedure to use to analyze op amp circuits?

Question

Many students have asked me for the best procedure to use when trying to solve operational amplifier problems. I recommend this procedure.

Answer 1

Analyzing “Negative-Feedback” Operational Amplifier Circuits

If you can identify an OP AMP circuit as one of the “3” simple configurations discussed in class, just use the gain equations as shown in the previous examples to find the output voltage:

1. Buffer Amplifier:                      A_V = 1
2. Inverting Amplifier:                  A_V = - (R_f / R₁)
3. Non-inverting Amplifier:          A_V = 1 + R_f / R₁

If the circuit is more complex, the procedure given below is the key to analyzing most circuits:

Procedure to analyze most “Negative-Feedback Operational Amplifier Circuits is:

1^st Study the circuit to determine which input voltage to the op amp (v+ or v-) you know or can calculate. Most of the time you will either know an input voltage directly, can calculate the voltage via the voltage divider rule or can make it a function of v_out!

2^nd Then project this voltage to the other input’s node. If you know the v+ input voltage, project it to the v- input. If you know the v- input voltage, project it to the v+ input. 

3^rd Then use KCL to write a node equation at the “projected” node and solve the equation(s).

If you need help, please contact me.

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