Re: Transistors

On a sunny day (Mon, 19 Nov 2007 09:54:43 -0800 (PST)) it happened Winfield
Hill <hill@xxxxxxxxxxx> wrote in
<644eb595-720e-43ce-a98e-454ee446b122@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:

Looking at your classic output stage, with the
class-AB bias voltage, driving output transistors:
Imagine instead a design where you have such an
output stage running at high class-AB currents,
for high fT and slew rates, but with relatively
small BJTs to keep the capacitance down, imagine
two sets of those, one for the high side and one
for the low side of the class-AB bias voltage.
Now imaging using these two fast medium-power
stages to drive two final output transistors.

OK, get the idea.

This way you are providing serious turn-on and
turn-off currents separately to the two bases,
out of phase with each other. That's the type
of circuit, with extra internal stuff to drive
each output component, that has possibilities
to work to higher frequencies and slew rates.

Interesting.
My original idea was using BFY90 in the pre stages, but could
not find a spice model for it (it is very old, but 1.3GHz ft).
In some Dutch magazine a 300MHz wide scope Y amp was published
by somebody at Tek, many years ago, it used the BFY90.
I do not remember what they used in the final stage, driving
the CRT defection plates, though.
BFY90 has a Vcb max of 30V, but only a Vce of 15 V.
Could be used in common base perhaps :-)
Maybe I am a bit stuck in old times here,
I am sure there are better ones now.



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