Right.. and with the bases held at the same potential by the resistor divide, the Vbe voltages differ by the capacitor voltage. If I recall correctly, the TIS97 has beta around 500 (modern datasheet from onsemi says 250-700, but I remember seeing "typical" figures in some older datasheet around 500) so these are higher gain than a basic 2n3904 (or TIS92 with typical beta 160 used elsewhere in the schematic). This suggests that ideally (by Moog's design) the base current is negligible, but in practice with real-world parts it might not be entirely the case. The schematic does use TIS97 for a bunch of other stuff too, but I wonder if that's partially "well, it won't hurt" when they probably had a bunch of them left over after having to match the pairs in the filter.. because frankly I don't see why they'd need very high beta for the current source of the LTP in VCA when the filter uses a TIS92/2n3904 for the same purpose (probably because they want to match with complementary TIS93/2n3906, I don't think there's one for TIS97) .. but I suspect unless they already had a large stock of TIS97 lying around, the transistor was probably chosen for the ladder and precisely because of the large beta? Or perhaps they just wanted to use beta as high as possible everywhere and had to compromise where they needed complementary pairs? No idea.There are two parallel current "rails" (going through emitter-collector pairs of transistors). If we ignore the current leakage through the bases, then the current can only flow from one side to the other via a capacitor, thereby the current escaped through a capacitor from one side appears on the other side in the same amount, leaving the sum unchanged.The part that makes me uneasy in particular is the idea that we can assume the emitter current across the transistor pairs other than the first stays constant. It must certainly stay constant on average because there is only one current supply in the bottom, but with the capacitor in there, is this truly so (to a good approximation anyway) in the short-term when we acknowledge imperfect real-world components?
I don't know. I have not found the energy to actually try to figure out what finite beta does here. Perhaps it just means the capacitors higher up see less current overall so the poles aren't quite on top of each other before resonance.
In that case I have no idea what ZDF might mean. The way I've always approached "ZDF" is that we use trapezoidal integration and then we (typically) prewarp the "control voltage" going into the model so that we can get the cutoff where we want (even if technically we are simulating a filter with a higher cutoff and errors from the trapezoidal integration just pull it down to where we want).There is even a difference between trapezoidal and ZDF, but it shows up only under extreme conditions.
Statistics: Posted by mystran — Wed Feb 14, 2024 11:32 am