Bypass caps are good, like wearing garlic to keep vampires away. They
both work.
What if I put a bypass cap on the clove of garlic before wearing? Will
that keep twice as many vampires away? (Or a clove of garlic across the
bypass cap in a circuit. Hmm.)
Ideally yes, but the feedback polarity would be wrong.
If it was hooked up like I said, as the base goes more and more
negative, the transistor conducts less and less, and steals less current
from C1? That isn't what you want, yeah.
When the output finally gets close to -12, the emitter of Q1 hits
about -0.6, and Q1 starts to conduct.
It's a common-base, non-inverting amplifier.
Aha. That, and the fact that negative voltages are involved, makes it
make sense to me.
I am used to seeing either common-emitter or common-collector circuits
with a single supply, where the signal into the base tells the
transistor what to do. From that perspective, having a grounded base
seemed weird at first - "it's never gonna do anything". The fact
that it isn't one of those topologies, plus the fact that there is a
negative supply, means the emitter can get down to a voltage where the
transistor will start turning on.
I have seen common-base circuits before, but drawn with the whole
transistor symbol rotated 90 degrees counterclockwise, mostly to
emphasise that something different is happening.
As the emitter goes negative, Q1 collector current flows, pulling C1
down, reducing the oscillator duty cycle.
The current "stolen" from C1 via Q1 ends up in the -12 V out, right?
Along with the < 1 mA from the +5 V reference that comes through R4 and
R5?
It's not super accurate, but I'm just using it to power opamps.
If you needed more accuracy, and as long as the opamps need between a
few and few dozen mA or less, maybe you could follow your circuit with
a 79L12 and be happy. On the other hand, if the opamps only need a
couple of mA total, the current that the 79L12 needs to run becomes a
big part of the load. If you're running on a watch battery or
something, you're counting every mA.
Certain Engineers Who Must Not Be Named think I should use Cuk
converters or something complicated like that instead.
PIC, AVR or equal with a software PID loop.
Matt Roberds
