The one op amp difference circuit has four resistors. In theory, the CMRR is inf

Question

The one op amp difference circuit has four resistors. In theory, the CMRR is infinite if the resistors are perfectly matched. However, you use resistors that have a plusminus1% tolerance (i.e., can have a value of 99% to 101% of the stated resistance). What is the worst possible CMRR? (You'll have to approximate the difference mode gain as equaling 1 for this problem). (b) You add two more op amps to make a differential amplifier (configuration shown in lecture and lab). Assuming that you can perfectly match the R_2 resistors, what is the ratio of R_2 to R_1 for the CMRR of the whole three op amp circuit to be 100 dB (use the CMRR calculated above for the difference portion of the circuit)? (c) Describe in one or two sentences, what is the point of this problem?

Explanation / Answer

a)resistors with tolerance value of 1% will have worst case CMRR of 34.0 db.

b)the ratio for r1/r2 is 2.074 for cmrr value 100 by using formula cmrr=20log(1+r2/r1/4t/100).

c)limit mismatch in the source and input impedance of amplifier. Limit the manufacturing tolerance value. Finite cmrr for individual opamp.

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