From: Winfield Hill
Subject: Re: Prof Searle's 100MHz 30dB 1968-era 3-trannie amplifier
Date: 19 Jan 2003 01:17:27 -0800
Organization: Rowland Institute
References: <3E2358BE.email@example.com> <3E2A0C89.7FAB@sneakemail.com> <3E2A4E8D.78BE@sneakemail.com>
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Mike Monett wrote...
>Winfield Hill wrote:
>> Hmm. Normally there isn't enough loop gain in the shunt-series pair
>> beasts I've designed [using parts like a 2n918] 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.
Actually it gets better not worse. Remember Bob Pease and his classic
rat's nest lashups to test some IC-designs on the bench? It keeps the
capacitance down and I imagine low inductance is a possibility with some
effort at places you really need it.
> 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.
Hmm, pretty nice! That was with a 50-ohm load, right? Did you find
the -3dB down point? Also, what supply voltage did you use, are you
sure each part's Pd was reasonable?
Hmm, did On Semiconductor pick up or continue the '941 after Motorola?
I see APT and Microsemi 2nd-source the part. Philips has the PBR941.
But Mike, the MMBR941 wasn't available in 1968 was it? When did it
come out? I mean, I don't remember any jelly-bean 8GHz transistors
that far back, and having only 0.3pF of capacitance is a big help!
Let's see what we can accomplish with a true vintage component, like
the 2n918, or better if it was available.
> Now I'd like to see how it really should be done!
Nah, your circuit the looks like the winner. But I'll show you my
2n918 circuit if you show me yours. :>)