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'[OT] Wein Bridge (was Ceramic Resonators)'
1998\03\16@214442 by Mike Keitz

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On Tue, 17 Mar 1998 12:41:19 +1200 Steve Baldwin <spam_OUTstevebTakeThisOuTspamKCBBS.GEN.NZ>
Horowitz and Hill have an example,
>> with
>> a miniature incandescent lamp that I can't identify. Adjustable
>> would
>> be a special bonus.
>The lamp is used to provide automatic gain because it has a positive
>temperature coefficient. ie. As it heats up, the resistance goes up.

In order for any oscillator to output a low-distortion sine wave, the
overall gain needs to be just enough to keep it oscillating, i.e. exactly
unity.  If the gain is less than unity, the oscillator won't start or
keep oscillating.  Too much gain, and the amplitude increases until the
amplifier becomes nonlinear.  At this point, its effective gain is
reduced until the overall gain is unity again, but the output becomes
distorted.  Most oscillators operate in this mode, and produce distorted
outputs.  The Wein bridge design is optimized for low distortion by
adding the lamp to automatically reduce the gain before the amplifier is
overdriven and becomes nonlinear.

A lamp is ideal for this purpose because of its thermal time constant.
To a signal at the oscilator frequency (of several hundred Hz or more),
it appears nearly to be a linear resistance.  Thus it can be included in
the oscillator loop without adding (much) distortion.  However, the long
term average power changes this resistance.  When the average power
dissipated by the lamp increases, it heats up and its resistance
increases.  So, before the oscillator starts, the lamp is cold, and
overall loop gain is maximum.  The maximum gain is designed with enough
margin to ensure that oscillation starts.  Once AC current starts flowing
through the lamp, it heats up and increases in resistance, causing the
overall loop gain to decrease until it stabilizes at unity.

Other circuits other than lamps have been devised to provide the
automatic gain control.  Most depend on the filtering action of the
frequency-selective network to filter out most of the distortion they
produce.  The gain-reducing part is installed in the feedback path
between the amplifier (oscillator) output and the frequency-selective
network.  Often FETs are used.  They are driven by an amplitude detector
to produce the desired regulation effect.  A lamp is nice because it
combines the detector and control functions in one part.  The lamp will
last practically forever since it is operated at very low current, barely
enough to make it glow.

>The best idea is to build one and have a play with it.

Yes, that's the best way to deal with it.  Just about any low-voltage,
low-current lamp should work.  The circuit will also work just replacing
the lamp with an adjustable resistance, but it will either not oscillate
reliably or it will have a distorted output.

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1998\03\17@034937 by Morgan Olsson

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>A lamp is ideal for this purpose because of its thermal time constant.
A PTC (or cheaper, a NTC in another connection) can also be used.

/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

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