Electronic Design

# Bob's Mailbox

• Louis E. Frenzel, Communications/Test Editor

Pease: I tend to agree. Often, a good technician must use and understand op amps—and measuring equipment, DVMs, spectrum analyzers, and automated test equipment. Circuit analysis is a specialty, and even for engineers, this is challenging. Of course, they should be aware of circuit analysis and bias setup. Even 20 and 40 years ago, we did not demand them to be experts at that. We’re now asking our senior techs to do more and more analysis of data and to tell us when things look right— and when things look suspicious or “funny.” They’re usually quite good. Of course, we’ve had the luxury of several excellent techs out of the College of San Mateo, and with just a few years of mentoring, many of them have become circuit engineers.

Dear Bob: Your article “What’s All This Input Impedance Stuff, Anyhow?” (Sept. 7, 2004, ED Online 8576) describes a single op-amp differential amplifier circuit with a gain of 100; resistor pairs of R1 = 1k and R2 = 100k; and front-end buffers ignored. I do not argue about circuit gains of voltages. They seem to be okay. But the impedances are not. Various sources (like NS’s Linear Application Handbook: AN-20, AN-29, etc.) state that the input impedance of the circuit at the inverting input is equal to R1. I do not accept this unconditional statement! When inputs are equal and in opposite phase, as in an ideal case, the input impedance is actually 500 Ω. (Ah, but that is a special condition! /rap) In your article, the impedance is claimed to be R1 = 1k (according to the handbook statement), which is incorrect. When inputs are driven to 10 V dc each, the impedances are 101k on both sides, just as in your article. I agree. But this 101k impedance is not equal to R1 = 1k. If gain = 1 (all four resistors equal), then impedance is 2/3 * R. I argued about this with Dr. Michael Ellis a few years ago, and after some calculations and simulations, we agreed. (First, I was wrong when believing the unconditional statement above.) But when the input voltages are not equal, the situation changes further. For example, if positive input is 100 times negative input and in opposite phase (gain = 1 circuit), then the impedance is only 1/50 * R. You may simulate various conditions as I did and find out that the impedance varies vastly. (Well, if you pick the right “special conditions,” anything can happen! /rap) Obviously, we must come to the conclusion that the impedance at the inverting input node is not equal to 1, neither constant, but depends on magnitudes and phases of input voltages and is therefore largely variable. In general, when inputs are not correlated with amplitude or phase (random input or noise), one can not guess the impedance. Do you agree?

• Eero-Pekka Mand

Pease: I tend to agree. But (a) there is nothing simple about this, and (b) you have waited three years to comment! So (c), shall we consider the case where the input is a transformer winding, not center-tapped? Or (d) cap-coupled? If the caps are big enough, it may still work. But you may need 100X bigger capacitors on the negative input. Let’s discuss. This might lead to “What’s All This Z In Stuff, Anyhow? (Revisited).”

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BOB PEASE obtained a BSEE from MIT in 1961 and is Staff Scientist at National Semiconductor Corp., Santa Clara, California.