From: Winfield Hill
Subject: Prof Searle's 100MHz 30dB 1968-era 3-trannie amplifier
Date: 18 Jan 2003 12:07:38 -0800
Organization: Rowland Institute
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Mike Monett wrote...
> Winfield Hill wrote:
>> Mike Monett wrote...
>>> Do you remember Cam Searle at MIT? He would walk into the
>>> lecture and pull a ball of circuitry from his coat pocket.
>>> He would flatten it out and show everyone it was a 3
>>> transistor amplifier with feedback, built directly over
>>> the schematic. I forget the gain, perhaps 30 dB or so.
>>> The amazing thing it was flat to 100 MHz!
>>> This was in 1968. Transistors were not very good in those days:)
>> Hey you two wags, transistors _were_ pretty good by that time!
>> For example, the 2n2222 suggested in this thread as an improvement
>> over the BC337 had already been available for what, 5 years?
>> (One of my friends was involved in its design.) And many other
>> better transistors were available for RF designs. For example,
>> remember the 2n918, 1300MHz typ at 4mA - 10V, and 1.25pF at 10V?
>> I used the 2n918 in 1965 and it was an old part then, IIRC.
> This brings back memories. Of course you are right. The 2N918 was
> a pretty hot transistor.
>> I'm sure we can come up with a design matching Dr Searle's circuit
>> using parts from that era. Let's see, three transistors, 30dB and
>> 100MHz? Hmm, probably a enhanced variant on a shunt-series pair...
>> - Win
> I was kind of hoping you would say that. I'd like to see what a
> PRB941 could do, and figure out how to get a healthy signal into
> 50 ohms without burning up.
> IIRC, the feedback resistor required a small cap in parallel for
> compensation. This might get tricky at the higher bandwidths.
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?
. 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?
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.