This article originally appeared in Linear Audio, a book-format audio magazine published half-yearly by Jan Didden.
Impedance Balance vs. Current Balance
A confounding aspect of diff amps is that the input currents are almost never equal. Here's the situation. If I hold one input of a 10x diff amp at ground potential and drive the other, I get a factor 11 discrepancy in the input current, depending on which is the one that gets driven (Figure 15). And so, the unsuspecting engineer might naïvely reason, the input impedance is out of balance and should be put right.
Figure 15: The Imbalance Illusion.
What they do in response to this misconception is quite ghastly (figure 16).
Figure 16: Grisly outcome of cognitive illusion.
You can easily see why there is something suspect about this. If, instead of driving the circuit with one leg grounded, we drove it with a symmetrical signal, the ratio between the input currents would no longer work out as 11:1 but as 21:1. You can't scale the impedances in a way that the currents work out equal under all conditions.
What's going on here? Remember to think of a differential input as forming a Wheatstone bridge along with the source resistances. If you add source resistors to the above circuit you get something that is clearly no longer a difference amplifier as shown in figure 17.
Figure 17: Why it's grisly.
We should repair the circuit and make the two legs equal again. We once again have a fully functional diff amp in figure 18. If the input currents are different, this is no indication of imbalance.
Figure 18: Wei Wu Wei, or how balance is restored by not intervening.
We should have seen from the start that the problem was illusory. In order to contrive it we had to drag the output reference of the difference amp into the equation and falsely assume that this is the point that the common-mode input impedance refers to. The circuit is balanced, certainly, but it just so happens that the input impedance references the virtual short, not some handy point that someone calls *GND.
• Converting a circuit to differential does not require additional amplification stages.
• Each signal has its own reference.
• Making a circuit differential is not the same as building two independent copies of a ground-referenced one.
• Do not try to equalise signal currents. It doesn't work and you'll end up creating an impedance imbalance of heroic proportions.
next: In From the Cold