Envelope Sensor Ossilates

[Alpha579]

Sorry if i spelled ossilates wrong  :? ...

I built a really simple envelope sensor, which is just a high gain bjt stage, followed by a diode to ground (anode to ground), then a diode in series with the signal (anode pointing to the collecter), followed by some caps to ground to smooth it out. Its working, coz im getting about -4v out of it(which is what i wanted), but when i use it with a circuit, it ossilates. Why?

Thanx for any help,
Alex

[Travis]

RF noise?  Is it on a breadboard?  Are the attached circuits noisy?  Do you get oscillation with the gain stage only?

The oscillation could be caused by a lot of things, but might be situation dependent.  You may want to try shielding the circuit, lowering the gain, perhaps even dropping in a pot to adjust the level of the input signal.

Travis

[Mark Hammer]

Depends on what you mean by "oscillate".  If you mean that it produces a high-pitched sound like feedback squeal, then the gain may be a little too high or you may simply need to stick a cap in the feedback loop to reduce the gain for high frequencies.

If the oscillation is a kind of bubbly gurgle as the strum decays, then you are experiencing what is called "envelope ripple".  This is a common property of half-wave rectifiers such as you made.  The simplest way to eliminate it is to stick a large value cap to ground after the second diode (the one in series with the envelope signal) to extend the decay time.  This has the property of averaging small variations in the envelope signal as the input  decays.