In Example 13.4 we investigated the effect of a mismatch between R1 and R2 on the input
Chapter 13, Problem 13.54(choose chapter or problem)
In Example 13.4 we investigated the effect of a mismatch between R1 and R2 on the input offset voltage of the op amp. Conversely, R1 and R2 can be deliberately mismatched (using the circuit shown in Fig. P13.54, for example) to compensate for the op-amp input offset voltage. (a) Show that an input offset voltage VOS can be compensated for (i.e., reduced to zero) by creating a relative mismatch R/R between R1 and R2, R R = VOS 2VT 1+re/R 1VOS /2VT where re is the emitter resistance of each of Q1 to Q6, and R is the nominal value of R1 and R2. (Hint: Use Eq. 13.94.) (b) Find R/R to trim a 3-mV offset to zero. (c) What is the maximum offset voltage that can be trimmed this way (corresponding to R2 completely shorted)? (Recall that each of Q5 and Q6 is biased at 9.5 A.) Figure P13.54
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