Negative resistance amplification
We once looked at how conducted emissions testing could be affected by the negative input impedance of a switch-mode power supply. Please see: “Conducted Emissions testing.”
Digital data signals that a client’s electric power company was putting on the power lines were being amplified by the negative input impedance of the power supply being tested, which made it look like the power supply itself was generating conducted emissions, which, in fact, it was not.
I have since been asked by someone, “How can a negative impedance result in amplification?” The sketch below will illustrate how that can come about.
Figure 1 Negative resistance amplification.
Let our “impedance” in question be a resistance. In our sketch, voltages E2 and E4 are derived by voltage dividers from identical “Esig” sources for which standard voltage division equations apply. What is NOT standard here is that we are going to set R4 to negative numerical values.
My SPICE simulator will not let me assign a negative number to any resistance value (I think of that as picky, picky, picky!), but given that as the case, the voltage divider equations can be set up in GWBASIC. Line 150 of that code is where that happens.
With R1 and R3 arbitrarily set to 1K each and held there, we vary R2 and R4 together as shown to look at the effects on outputs E2 and E4, where we find the following.
E2 is always a lesser voltage than Esig. E2 varies versus the choices of value of R2, but it is always smaller than Esig.
On the other hand, E4 is always a greater voltage than Esig. E4 varies versus the negative value of R4, but it is always larger than Esig.
This effect on E4 is the amplification effect referred to in the earlier essay.
John Dunn is an electronics consultant and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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