AW: Ideas for a new circuit

[Haible Juergen]

	>In an idea for a new synth module, I need a circuit that peaks at a
certain 
	>input voltage and then reduces it's output for higher and lower
voltages.
	>The output shold be a voltage, say 5 volts, at 2 volts and then
gradually 
	>decrease in amplitude to 0 volts when the input reaches 1 and 3
volts. Above 
	>and below that, it should have no output. 
	>It should be fast enough to follow audio input signals.
	>Any circuit ideas?

Been there, done that. (;->)

There are many rather complicated solutions, but when I needed 8 of these
circuits for my Interpolating scanner, I chose to use the good old
one-transistor
saw-to-triangle converter. Build a phase splitter with one transistor and
identical
resistors in the collector and emitter branch. Drive this circuit with an
opamp
(for low impedance, and possibly for a level shift to the appropriate input
voltage
range). The emitter will follow the input, but the collector will perform
the desired
function due to current limitation of the 2 resistors and due to transistor
saturation.
The absolute best solution if you want to go audi rate, IMO. (I have not
invented
it, so I can praise it ...)

For me it was fine, because i could directly use the collector current to
drive
an OTA (what I built was continuous crossfading - "scanning" thru 8 VCAs),
but if you need a low impedance output, this probably means another opamp
buffer. In that case (or maybe in any case ...) you might build the
LM3900-based
rectifier from the Serge patent. I think this is an even more elegant
solution.
(look for US patent # 4306480).

May I ask what application you have in mind ?

JH.