voltage contrlled resitor circuit

williB · Jul 15, 2005

audioguru said:
This circuit will work for audio if you keep the signal level low. A negative contol voltage allows it to pass signals and it attenuates more if the control voltage becomes closer to ground. :lol:

audioguru, how is he gonna hook that up to an op amp feedback loop..???

Roff · Jul 15, 2005

audioguru said:
This circuit will work for audio if you keep the signal level low. A negative contol voltage allows it to pass signals and it attenuates more if the control voltage becomes closer to ground. :lol:

You shouldn't need the capacitor if the signal is not riding on a DC voltage.

Below are the results of some sims of a gain control in a feedback loop. The top plot shows how the gain varies as Vg is varied. The bottom plot is an FFT, showing about -50dB 3rd harmonic distortion at 1kHz with Vg= -3V (about 30dB of gain). The actual numbers will vary quite a bit from device to device, even within the same part number.
You can also put a resistor in series with the drain to limit the maximum gain, and improve the linearity.
The op amp in the sim is just a simple behavioral model with a gain bandwidth product of 10MHz.
I have no idea how close these results are to reality. :roll:

williB · Jul 15, 2005

audioguru · Jul 15, 2005

Hi Wlli,
If a FET attenuator was reversed so that the FET passes the signal, and it was put in the negative feedback loop of an opamp that has gain, then the signal across it would be excessive and it would distort like mad.

A simple attenuator can be used ahead of an opamp that has gain like this:

williB · Jul 15, 2005

Thunderchild hasnt said how he wants to control the op amp gain..
i still like my maxim idea..

audioguru · Jul 15, 2005

Lots of ways to do the trick. :lol:

Roff · Jul 15, 2005

audioguru said:
Hi Wlli,
If a FET attenuator was reversed so that the FET passes the signal, and it was put in the negative feedback loop of an opamp that has gain, then the signal across it would be excessive and it would distort like mad.<snip>

Audio, I'm hoping you're not suggesting that's what my circuit does...

audioguru · Jul 15, 2005

Hi Ron,
I understand that your circuit attenuates the negative feedback to get lots of gain. :lol:

Roff · Jul 16, 2005

audioguru said:
Hi Ron,
I understand that your circuit attenuates the negative feedback to get lots of gain. :lol:

Sounds like you are equivocating.

williB · Jul 16, 2005

Ron
i'm not sure i understand how that circuit does what the origional poster wished..
as i understood he was looking for a way to vary the output of an op amp by changing the resistance by means of a voltage..
I think an analog switch or two could do the job..
ya think??

Roff · Jul 16, 2005

williB said:
Ron
i'm not sure i understand how that circuit does what the origional poster wished..
as i understood he was looking for a way to vary the output of an op amp by changing the resistance by means of a voltage..
I think an analog switch or two could do the job..
ya think??

Willi, I went back and looked at the original post. Either my circuit or Audioguru's does exactly what he asked for. The JFET is a voltage-controlled resistor. Analog switches will yield digital control of the gain - I don't think that's what he wants.

audioguru · Jul 17, 2005

Hi Willi,
The negative voltage at the gate of the FET adjusts the gain of the opamp from 1 to about 60 in Ron's circuit, and my circuit provides adjustable attenuation ahead of the opamp for about the same function. :lol:

Dr.EM · Jul 17, 2005

Hi, if you decide to use the LED-LDR idea, try this, here it is implemented in a compressor circuit, but all you need to do is detach the LED, and by applying varying current, different output levels can be achieved with low distortion. Making the LED current high, ie, bright, reduces the level it can pass. It will not go down to full mute however, but do try an ultrabright LED and increase that 2.7K near the LDR to get more level variation at the expense of some distortion.

**broken link removed**

At a long shot, you could try to modify this for your application, I managed it with the filter schematic:

**broken link removed**

Alternatively, you could try this OTA voltage controlled amp, intended for guitar use, I have no experience with this circuit though:[/url]

audioguru · Jul 17, 2005

Hi Dr. EM
It's too bad the author of the LDR attenuator had to use FETs. They aren't needed today since good quality FET-input opamps are inexpensive.
EDIT: The FETs provided some gain. The opamp will have some gain too if you add a resistor between its pins 1 and 2. Am I a poet? :lol:

The LM13700 has a 1:30 attenuator on its input to keep distortion low, so might be noisy. :lol:

Thunderchild · Jul 17, 2005

thanks for all of your efforts but i need a circuit that has 2 inputs: "Voltage control" and audio input.
i had a thought about the led ldr idea and of how to make it linear: use a normal light bulb and two ldrs one for feedback to control the lightbulb to make it linearthis way the second would be an isolated replica of the first used for auto controlthe light will increase or decrease until the value of the control LDR coresponds to the input voltage of course this is very theoretical and could cost maybe more than the more expensive voltage controlled amplifier. this method coud also be applyed maybe more simply to the motor pot system by using a secong pot as a positon detector - all theory of course

audioguru · Jul 17, 2005

Hi Thunder,
The two FET circuits do control the gain of the audio input with a "voltage control". I didn't think you would want a linear volume control, most are logarithmic to match your hearing. What is the linearity of your hand's closeness to the antenna of the theramin? Far away would make very little change in its voltage output while up close it would change a lot. I think that Ron's FET circuit would match the closeness of your hand to the theramin's antenna very well. :lol:

Dr.EM · Jul 18, 2005

Hmm, don't ask me how to wire this up, but it looks perfect for your theremin volume control:

**broken link removed**

Nigel Goodwin · Jul 18, 2005

As I've mentioned previously, EPE magazine have published a number of theremin designs in recent years, at least two of them have volume control via a second 'channel' - why reinvent the wheel?.

audioguru · Jul 18, 2005

What do the EPE theremin projects use for "a voltage controlled resistor"? :?:

Nigel Goodwin · Jul 18, 2005

audioguru said:
What do the EPE theremin projects use for "a voltage controlled resistor"? :?:

I'm not at home at the moment, so I can't check - but I 'think' at least one of them used an OTA chip?.

voltage contrlled resitor circuit

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