How to use full swing of audio signal to drive resistive load?

Question

Hello all,I am working on a circuit to detect the presence of an audio signal(from a pro sound card) and, from it, drive a resistive load. Thisisn't really an amp; I just need to sense a signal, then drive a powertransistor into saturation as long as the audio is present.I have a working prototype that uses an op amp, open-loopconfiguration, driving a power MOSFET. My load is on the drain side,grounded source. Audio is decoupled through a cap into the non-inverting input; the inverting side is biased to about 0.1 volt tokeep the output low when no signal is present.This all works fine, except that I only get output when the audiosignal is in positive polarity, kind of like a class B amp, I guess.Is there any (simple) way I can get my op amp to output high when theaudio swings low?Thanks!

Jan Panteltje · Accepted Answer

On a sunny day (Sat, 20 Sep 2008 06:51:52 -0700 (PDT)) it happened "Doug B." wrote in:google 'opamp full wave rectifier'.

John Fields · Answer

---View in Courier:                                        +V                                           |                                       [LOAD]                                         |AUDIO>--[C]--+--|+\                      D             |  |  >--[1N4148>]---+----G0.1V>--------|--|-/               |      S             |                    |      |            [R1]                 [R2]    |              |                    |      |GND>---------+--------------------+------+ Even simpler; the MOSFET will have some gate capacitance, so select R2to discharge it in, say, a few cycles of the lowest frequency out of thesound card.                            JF

Jon Slaughter · Answer

Why not just use a resistor, crap and diode?

The diode converts to dc as you don't care about the actual integrity of the signal, it charges a cap. The cap is what "drives" the fet. When enough cycles pass to charge up the cap then it will turn on the mosfet independent of any local variations in the audio signal.

The only problem is that the cap will stay charged(excluding leaking) but you can discharge through a large resistor. (it has to be large enough so the cap won't discharge too quickly)

This is a quick way but isn't perfect(it isn't accurate but you don't need accuracy)

John Fields · Answer

---That's a really shitty way of doing it, Jon. ;)JF

Martin Griffith · Answer

(from memory, in the 70's), just d/l the lm3914/5/6 datasheets,fromnational, full of circuits. They have what you need for signalconditioning, you might only need 2 op ampsmartin

Doug B. · Answer

=A0 =A0 +V =A0=A0 =A0 =A0|=A0 =A0[LOAD]=A0 =A0 =A0| =A0 =A0|| =A0 =A0|Thanks for the quick reply, but there's something I don't get: I don'tsee why adding the diode will prop up the op-amp's output voltage whenthe input swings low. Is it basically intended to force the MOSFET todischarge its gate Q into R2 slowly enough to carry through a coupleof input cycles? I tried modeling this in LTSpice and it didn't seemto do anything different from my circuit, sans 4148. Am I missingsomething here?

Doug B. · Answer

B."Right, thanks. A few more parts than I was hoping to get away with,but I'll give it a try.

Jon Slaughter · Answer

hehe, why? because it doesn't involve an op amp?

ehsjr · Answer

You are missing the play on words.

Jon Slaughter · Answer

hehe... I didn't see it. lol.It's not only a shitty way but a nasty way too ;/

miso · Answer

What you want is the old analog modem "energy detect". Unfortunately, I can't get any good google hits, so you may need a trip to the library unless you have access to the IEEE online. The energy detect circuit was basically a full wave rectifier (of the audio), a comparator to set the level, and a deglitch (maybe one-shot is a better word) to make sure the signal was present enough so that what was detected was audio and not some random event.

It is slightly more complicated than what you are doing. That is, my descriptions sounds worse than it really is.

How to use full swing of audio signal to drive resistive load?

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