Yesterday I was writing about building an intuition.
Yes… it’s a single NPN transistor amplifier. Can hardly get much simpler without getting into passives, right?
This is nothing more than a common emitter amplifier. This, effectively, is a voltage (*) amplifier. All the resistors are nothing more than the biasing for it. The transistor is as jellybean as you can get — the classic 2N2222.
Here’s the thing: I hooked it up the way it was in the book and it didn’t work right. I fed in a simple triangle wave and out came something that looked, well, not triangular. It looked like a rounded lump coming up from a flat valley.
And this is where I started to get a little intuition.
What would cause the output to flatline?
If the transistor was saturated it would pull Vout as low as it could. Only on the peaks (or, actually, the valleys of the input) would the transistor not be turned on completely.
What would cause it to saturate?
The biasing network (R1 and R2) pulling the base of the transistor too high.
I started off with R1 = 100KΩ and R2 = 10KΩ. V+ = 20V. This got the base bias to 1.8V. Apparently this was turning on the transistor hard. I swapped in a 220KΩ and that got the base bias down to 0.87V and all was well.
In hindsight I think the original schematic has V+ at 10V. The original biasing would have put the base at 0.91V.
That said, this is exactly what I was wanting. Fuck something up then learn to debug and fix it. You don’t learn when things work right. Well, you do, but it doesn’t stick as much.