J
Jon Kirwan
I've seen R3 used in that position too, but never gave it much
thought until you brought it up. Offhand I still can't see a
reason for it either. Maybe for stability against a local
oscillation? Perhaps taking some time to think about it will
bring some revelation. Or someone else can save us the trouble
and enlighten us.
<snip>
I had earlier said I thought you might be right about this R3
value. Now, I don't. I think it deals with something else
-- unregulated rail voltage variations.
In this thread, you've posted circuits with a resistor on one
side and the VAS on the other of this structure. The VAS
yanks one end around while the other side mostly follows it
around. However, the current through the resistor varies, of
course. Even if one of those BJTs+2 diodes thingies is used
for a current source instead of the resistor, which does
improve things, it still isn't very constant. Using the 2
BJT structure doesn't change that fact, though it does impact
variations. No matter how you arrange it, resistor or
current source, the fact is that the current into the Vbe
multiplier device changes around as the VAS yanks around one
side of it.
This variation means that Q1's Ic varies. To accomodate that
variation, Vbe varies. Since Vbe varies, so does the
multiplied value. And for no _other_ reason than variations
in signal. That changes the bias. Changing the bias changes
the quiescent current. Etc.
(Also, I suppose, the Early effect will add yet another
slight modification, since the Vce is slightly changed so is
the Ic for the same Vbe. The higher required Ic (assuming
the current source or resistor is supplying more current,
instead of less) requires a higher Vbe, as stated. So the
multiplied voltage at Vce is higher. But that multipled
voltage also slides over on the Vce axis for whatever Vbe
that has become and that suggests still more Ic due to Early
effect, so it is a positive feedback contributing to the
already existing problem, I think. I haven't tried to work
out just what percent it contributes, though.)
So a cludge fix for this is to insert a resistor in the
collector, which will act in the opposite direction to some
degreee. I'd imagine this would create a second degree poly
curve, with a maximum somewhere but gentle 'arms' outward,
which means less variation of the Vbe-multiplied value with a
tweakable peak point based upon a nominal Ic.
I imagine this is NOT nearly as important for class-A
operation, though, since it is already "biased up" and
variation at that point of operation probably isn't so
important. It _would_ matter, I think, in other classes of
operation.
Thinking as I am that I don't want to go with class-A, I am
trying to think of still better ways of replacing R3 with (or
adding) an active device to further improve it. Anyone have
a suggestion there?
Jon