We make friends by focusing on our common experiences, thoughts, opinions, and tastes. If our differences are too great, we typically don’t bond. The opposite is true with a difference amplifier, the heart of most electronics you listen to.
We’ve looked at balanced inputs and cables to discover how balanced audio ignores common noise and amplifies only differences. Now, let’s look at a balanced amplifier itself.
A balanced amplifier is more properly called a difference amplifier. Both terms are accurate. Differential amplifiers are balanced, and balanced amplifiers are differential. But I prefer to think of this type of circuit as a difference amplifier because it more accurately describes how it does it, rather than what it does.
Difference amplifiers are based upon this circuit, called a differential pair (diff pair for short).
It looks complicated but it’s not. Q1 and Q2 are the pair in diff pair. What’s drawn are regular transistors, though they could be FETs or vacuum tubes. What they are doesn’t matter. It’s how they are connected that does.
V+in is where the + of a balanced cable would go, V-in is where the second, out of phase, input from the cable goes (Remember? There are two signal wires in a balanced cable).
Ignore all the rest of the squiggles and notes.
Focus instead on where Q1 and Q2 are connected directly together—the two arrows are pointing at this junction. See it?
For purposes of our understanding, this connection of the two transistors is all we’re interested in.
Whatever signal is placed on either of the two inputs, an identical copy comes out where the respective arrow is (called the emitter).
If the two emitters have the same signal (because the inputs have the same signal), there is no difference between the two, thus, no current flows. Like a teeter totter with two equal weight people, or closer to home, our example of the two wires of a lightbulb placed on the + terminal of a battery. Nothing flows, because there are no differences.
However—and of course you saw this coming—place opposite signals (out of phase: one rising as the other is falling) in the diff pair’s inputs, and current flows.