From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Prof Searle's 100MHz 30dB 1968-era 3-trannie amplifier
References: <3E2358BE.email@example.com> <3E2A0C89.7FAB@sneakemail.com>
Date: Sun, 19 Jan 2003 02:06:53 -0500
NNTP-Posting-Date: Sun, 19 Jan 2003 02:06:18 EST
Organization: Bell Sympatico
Winfield Hill wrote:
> Mike wrote...
[... save space]
> > Golly, that sure looks familiar. I can't say for sure as it was such
> > a long time ago, but let's go with it.
> > As I recall, the input was driven from the 50 ohm source impedance of
> > a network analyzer. I'm not sure, but I think the output drove 50 ohms
> > directly. That might account for the third transistor used as an
> > emitter follower, but I rather see what else you had in mind.
> Yes a good possibility, although no voltage gain results.
> > The compensation cap went across R7.
> Hmm. Normally there isn't enough loop gain in the shunt-series pair
> beasts I've designed to need such a trick.
This may have been due to the layout. There was no attempt to shorten the
leads. Instead, the circuit was wired directly over the schematic, and I
was astonished to see it work, especially at 100MHz.
> > Should there be a bypass cap from the emitter of Q2 to ground? R5 is
> > going to be quite large due to the stacked voltages, which would make
> > R4 really large to obtain much gain. Bandwidth would suffer, and the
> > feedback resistor R7 would start to dominate.
> Yes, exactly right. Ideally one would share the gain task between the
> two transistors, but I wasn't able to get the 2nd stage gain up to snuff
> and a bypass (with a series resistor?) could well solve that. Ah, with
> no degenerating series resistor for the bypass cap, perhaps a cap across
> R7 might be needed after all, to add a fortuitous zero in the loop.
> - Win
I tossed this in SPICE using three MMBR941's, the last one as an emitter
follower. After fiddling a bit with the bias, here's my results:
Parameter 1KHz 200MHz
Open Loop Gain 46.14 dB 37.3 dB
Closed Loop Gain 30.50 dB 29.99 dB
Maximum Output 9.11V p-p
No compensation cap was needed, but it definitely needed the 100uF from
Q2 emitter to ground.
Now I'd like to see how it really should be done!