So I wanted to take another look at the digit display selection from the Virtual Whammy Bar. After it initially worked I laid it out in LTSpice, and simulated it. I noticed that the current thru D1 was almost 300 times (the β of the 2N3904 model) larger than the current thru D2. Swapping transistor models and measuring the current gain provided consistently similar results for each respective β. At that point I decided that I recognized what was going on and moved to the next thing. But wait... Driving an LED is typically a switching (and thus in the saturation region for a transistor) application where the current gain is typically much smaller than β and what we have here seems to be operating in the linear active region....
Admittedly, I didn't put it all together right away. I observed as I was simulating the circuit in LTSpice that if I used certain LED's the current gain would drop dramatically. From nearly 300, to say, 10. Right there should have been the clue about saturation. This circuit is a bit challenging to analyze because of how the components all seem to interact. D1 and D2 provide "active" loads on both the base and collector of the transistor Q1. But the harder you drive the base of Q1, the more current it tries to draw into the emitter, which increases the voltage drop across R, which reduces the ability to drive the base, which makes... wait...hmm.
We'll take a look at this over the next few posts.
