31 May
2008
31 May
'08
8:39 p.m.
Tony Duell wrote:
Actually, this reminds me of another application...
the audio oscillator.
As is well-known, it's very difficult to make an adjustable LC oscillator
at audio frequencies. An RC oscillaotr is possible, but stabilising it
(so you get a reasonably non-distorted sine wave output) is the problem.
Now many yeats ago, a guy working for Prof Terman (you do know who I
mean, right...) solved this problem. He used a Wien bridge circuit as the
'resonant element', in the feedback loop of a *3 amplifier. The gain of
the latter was stabilised by a light bulb as a non-linera resistor with
just about the right time constant.
Anyway, I read somewhere (possibly in one of Bob Pease's columns) that
somebody had tried to improve upon this circuit using a more modern
approach, an FET to cotnrol the gain, a carefully-designed control loop,
etc. The result was a an oscillator with a more distorted output than the
simple light-ulb-stailised one...
This particular subject is near and dear to my heart.
The incandescent light bulb linearization technique is attributed to
Bill Hewlett, and was presented in his Master's thesis at Stanford
University in 1938. The design was commercialized into
Hewlett-Packard's first product, the model 200A audio oscillator, in
1939. (was Hewlett studying under Terman at the time?)
Later, well-known analog electronics god Jim Williams (staff
scientist at Linear Technology) developed a modernized equivalent of
that circuit which actually does manage to outperform it by a
considerable degree. It uses several very high-tech op-amps, along with
an analog optoisolator (an LED optically coupled to a CdS photocell), in
a fairly elaborate circuit. It is far, far more complex than Hewlett's
original oscillator, but it does work amazingly well.
Williams presents this circuit (and many pages of very interesting
reading, as he describes the process by which he arrived at his final
result...ideas, testing, failures, successes, optimization, etc) in the
absolutely fantastic book entitled "Analog Circuit Design: Art, Science,
and Personalities". I strongly recommend that book.
Bob Pease may have done something like this, but I suspect you're
thinking of Jim Williams' work.
-Dave
--
Dave McGuire
Port Charlotte, FL