From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Prof Searle's 100MHz 30dB 1968-era 3-trannie amplifier
Date: Sat, 18 Jan 2003 21:25:13 -0500
NNTP-Posting-Date: Sat, 18 Jan 2003 21:24:39 EST
Organization: Bell Sympatico
Winfield Hill wrote:
> Mike, let's not drop this subject. We'll use a new title for the rest
> of this thread, Prof Searle's 100MHz 30dB 1968-era 3-trannie amplifier
> OK, I'll start by posting a simple back-of-the-envelope design. The
> drawing below is taken from our book, figure 13.13, where we describe
> the operating principle of the amplifier. It has two local emitter-
> degeneration-feedback gain stages (R2/R3 and R4/R5), plus two more
> nested two-transistor feedback loops (R6/R1 and R7/R3). Clever, huh?
> . V+
> . shunt-series pair |
> . Art of Electronics R4
> . page 872 V+ |
> . | +----+---- out
> . R2 Q2 | |
> . | C |
> . +------- B |
> . | E |
> . | | |
> . ,--- | --- R6 --+ |
> . | | | |
> . | C R5 |
> . -||-- R1 -+- B Q1 | |
> . E gnd |
> . | |
> . +---||--- R7 ---'
> . |
> . R3
> . |
> . gnd
> I first learned of this configuration in the late 60s, it was popular
> in some circles. Mike, does this look familiar?
> Of course it has only two transistors, but with simple calculations
> using 2n918 parts, I fell just a bit short of the specs. There are
> several good ways to add a third transistor to the shunt-series pair
> to enhance its performance. Which is best depends on the answer to
> an important question, "What's the source impedance, and what's the
> load?" I'd prefer a low-capacitance scope probe for the latter. :>)
> Give it a quick lookover and we'll pick up the thread again.
> - Win
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.
The compensation cap went across R7.
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.