From: John Woodgate
Subject: Re: Prof Searle's 100MHz 30dB 1968-era 3-trannie amplifier
Date: Sun, 19 Jan 2003 20:41:41 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 19 Jan 2003 21:17:41 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Winfield Hill
wrote (in ) about 'Prof Searle's 100MHz 30dB
1968-era 3-trannie amplifier', on Sat, 18 Jan 2003:
>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?
>. 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 ---'
> I first learned of this configuration in the late 60s, it was popular
> in some circles. Mike, does this look familiar?
One of the circles was broadcast audio. A lot of these were used in BBC
audio equipment, AIUI, with variations around R5/R6, e.g. R5 decoupled
or not, R5 split in two with R6 going to the join, or a resistor in
series with the decoupling capacitor. IIRC, if the capacitor in series
with R7 is a big one (as it would often have to be), settling time on
switch-on can be long, so the capacitor is omitted. It then becomes an
interesting exercise to *calculate* appropriate resistor values, without
the aid of SPICE, of course; that didn't exist in the late 60s.
A little exercise was set in an examination for service techs in UK at
the time or a bit later: given the component values, determine the d.c.
voltages at all electrodes and the gain! I saw what was alleged to be a
'model answer', in which the collector current of the first transistor
was apparently deduced by divine inspiration. The circuit was alleged to
be the input stage of a household tape recorder recording amplifier,
which seemed a bit unlikely since R1 was 680 ohms! I wonder how many of
the poor B***s failed through that piece of incompetence.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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