On Tue, 4 Sep 2007 20:36:46 -0500, "Tim Williams"
wrote:

I think I've got the key bits in there... like the common mode feedback the
right way around.

It's a diff amp, but most scopes have a SE front end. So, like most scopes,
the position knob goes to -in and signal to +in. A preamp might be a good
idea for the more sensitive ranges.

The 2N5179 pair should be fast enough that I get the ~20 gain I want up to
50MHz or so. '3904's are kind of pokey for this, but as emitter followers,
who cares. That just leaves the hope that the C1569's are as fast as
claimed.

And if not, I could replace the *******s with a 6CL6. That'll show 'em.

Tim

The other trick is to add peaking inductors. The simplest is to add an
inductor in series with the final 1K collector loads. The ballpark
value is

L = K * R^2 * C

where C is the net capacitance (transistor + deflection plate +
wiring), R is your 1K, and K=0.5 to 1 maybe. Figure a 40% bandwidth
increase.

Next step is another inductor, in series with the outputs. Another 20%
maybe. Finally, an optimun t-coil can increase bandwidth by 2.8x over
no peaking at all.

As JP mentions, it helps to cascode the output stage, which reduces
feedback (Miller) C and allows you to use something really fast on the
bottom, like a gaasfet or a phemt maybe.

Hey, how about a fast opamp driving a phemt-C1569 cascode, with
peaking inductors?

John