On Thu, 17 Dec 2015 19:44:57 -0500, M Philbrook wrote:> In article <Bv2dnZfYpKJK1u7LnZ2dnUVZ5vKdnZ2d@giganews.com>, > seemywebsite@myfooter.really says... >> >> On Thu, 17 Dec 2015 12:06:01 -0800, Joerg wrote: >> >> > On 2015-12-17 11:28, Fibo wrote: >> >> Hello, >> >> >> >> I'm trying to make a voltage controlled amplifier. I only need a >> >> gain of about 11 max. So I thought I would use an opto as a variable >> >> resistor on an inverting amplifier. I went with the H11F3M >> >> >> >> https://www.fairchildsemi.com/datasheets/H1/H11F3M.pdf >> >> >> >> I built up the Automatic Gain Control circuit in the datasheet. I >> >> used a 50k resistor as my feedback resistor. >> >> >> >> When I fired it up, my output looked kinda distorted. I swapped out >> >> the opto with a regular resistor and it cleaned up. >> >> >> >> Any idea why my output looks like that? Can I not use the opto this >> >> way? >> >> >> >> I tried a bunch of different opamps, but the scope shot is from an >> >> LF411 with +/- 5V rails. Scope shots are on the link below. >> >> >> >> >> >> http://s555.photobucket.com/user/panfilero/media/OptoAmp.jpg.html >> >> >> >> Much thanks! >> >> >> >> >> > FET couplers work as controllable resistors only for very small >> > voltages across the FET, plus minus very few tens if millivolts. >> > After that it leaves the ohmic region and distorts. Figure 2 shows >> > it, past +/-50mV it all flattens out. >> > >> > You might be better off with an LDR and a light source. Unless it's >> > for a product in a RoHS country, then no dice because they contain >> > Cadmium and some <nasty outburst suppressed> armchair bureaucrats >> > outlawed that. >> >> Ditto what Joerg and Phil have said. I assume these things act like >> regular JFETs. My experience matches what's been said: the key to >> making a decent circuit out of them is to keep the voltage swing across >> them small. > > Or, use a DUAL opto, one to send and the other as the feed back for the > input stage. > > You'll have isolation, if that is what's being sought?The OP just states that he wants a voltage-controlled amplifier. -- www.wescottdesign.com
Opto in amplifier screws up output
Started by ●December 17, 2015
Reply by ●December 17, 20152015-12-17
Reply by ●December 17, 20152015-12-17
On 18/12/2015 11:44, M Philbrook wrote:> In article <Bv2dnZfYpKJK1u7LnZ2dnUVZ5vKdnZ2d@giganews.com>, > seemywebsite@myfooter.really says... >> >> On Thu, 17 Dec 2015 12:06:01 -0800, Joerg wrote: >> >>> On 2015-12-17 11:28, Fibo wrote: >>>> Hello, >>>> >>>> I'm trying to make a voltage controlled amplifier. I only need a gain >>>> of about 11 max. So I thought I would use an opto as a variable >>>> resistor on an inverting amplifier. I went with the H11F3M >>>> >>>> https://www.fairchildsemi.com/datasheets/H1/H11F3M.pdf >>>> >>>> I built up the Automatic Gain Control circuit in the datasheet. I used >>>> a 50k resistor as my feedback resistor. >>>> >>>> When I fired it up, my output looked kinda distorted. I swapped out the >>>> opto with a regular resistor and it cleaned up. >>>> >>>> Any idea why my output looks like that? Can I not use the opto this >>>> way? >>>> >>>> I tried a bunch of different opamps, but the scope shot is from an >>>> LF411 with +/- 5V rails. Scope shots are on the link below. >>>> >>>> >>>> http://s555.photobucket.com/user/panfilero/media/OptoAmp.jpg.html >>>> >>>> Much thanks! >>>> >>>> >>> FET couplers work as controllable resistors only for very small voltages >>> across the FET, plus minus very few tens if millivolts. After that it >>> leaves the ohmic region and distorts. Figure 2 shows it, past +/-50mV it >>> all flattens out. >>> >>> You might be better off with an LDR and a light source. Unless it's for >>> a product in a RoHS country, then no dice because they contain Cadmium >>> and some <nasty outburst suppressed> armchair bureaucrats outlawed that. >> >> Ditto what Joerg and Phil have said. I assume these things act like >> regular JFETs. My experience matches what's been said: the key to making >> a decent circuit out of them is to keep the voltage swing across them >> small. > > Or, use a DUAL opto, one to send and the other as the feed back for the > input stage.No, that doesn't help for this problem. What you suggest will help keep the conductivity (or resistance, depending on what you choose) linearly proportional to the control input, but this is a different problem: For a fixed control input, if the device is a FET there will be a non-linear relationship between Id and Vds. To fix this one needs to keep Vds very small (which might lead to poor signal-to-noise ratio), or change to a Cadmium Sulfide LDR etc., or re-design it to use a "digital potentiometer", multiplying DAC, VGA, etc. Chris Chris
Reply by ●December 18, 20152015-12-18
On 12/17/2015 06:08 PM, Tim Williams wrote:> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message > news:00b67bhrjceg2fuegpqaqgc90iko8qp8ll@4ax.com... >> You might consider using an analog multiplier, or buying an official >> VGA. > > Or the poor man's VGA, an OTA. LM13700 is still in plentiful supply > (for the moment), and is more stable than the opto, anyway (which is > probably saying something). > > > The signal level isn't any more (~60mV with predistortion diodes in > use), but you get a lot more voltage at the output for that swing. > > Tim >If you use the input diodes, the input is a current as well. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●December 18, 20152015-12-18
FET based VCAs can work very well. Set up the JFET as an voltage divider with the JFET as the R to ground. Keep the audio voltage across the DS very small. Add feedback of 1/2 of the audio on the Drain to the gate control signal to cancel the second order distortion. Google this, it is a well known technique that works very well. Apply the DC VCA control to the gate along with the distortion cancelation signal. Mark
Reply by ●December 19, 20152015-12-19
<makolber@yahoo.com> wrote in message news:f75264a3-e972-4380-abc9-70cfd61831eb@googlegroups.com...> FET based VCAs can work very well.Across a narrow range of gains (say 2:1), not bad. They're terrible if you need cutoff, though (more than 1:10, say). This rather limits their use -- e.g., small range calibration*, low modulation AM, audio effects (vibrato), etc. (*But for calibration, the residual nonlinearity would probably be as big a problem as the drift..?!)> Set up the JFET as an voltage divider with the JFET as the R to ground. > > Keep the audio voltage across the DS very small. > > Add feedback of 1/2 of the audio on the Drain to the gate control signal > to cancel the second order distortion. > > Google this, it is a well known technique that works very well. > > Apply the DC VCA control to the gate along with the distortion cancelation > signal. >What you get is, when the gain is set low, it's operating at very low [negative] Vgs: near pinchoff. It takes only a little drain voltage to fully pinch off part of the channel, and therefore enter the linear (read: constant current) region. The feedback resistors help a little, but only by a factor, not by orders of magnitude. Hence, it's no good if you need orders of variable range. I suppose you could try a limited range like 1:2, and subtract the input from it, so it's variable down to zero. But now you're subtracting harmonics that weren't there before, so you'll have a residual that's inevitably distorted, no matter the signal level (asymptotically speaking, that is). There's also no good way to null it; it has to be calibrated live, otherwise when you wanted "1%" you might end up with "-5%" instead! BJTs (e.g., OTAs, internal mult. and VGA cores) are fantastic for wide range (up to, what, 7 decades or so?), drift about as much (due to Is and Vbe), and their distortion and input range don't vary with bias (because the derivative of an exponent is an exponent is an exponent! :) ). Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
Reply by ●December 20, 20152015-12-20
On Thursday, December 17, 2015 at 2:28:51 PM UTC-5, Fibo wrote:> Hello, > > I'm trying to make a voltage controlled amplifier. I only need a gain of about 11 max. So I thought I would use an opto as a variable resistor on an inverting amplifier. I went with the H11F3M > > https://www.fairchildsemi.com/datasheets/H1/H11F3M.pdf > > I built up the Automatic Gain Control circuit in the datasheet. I used a 50k resistor as my feedback resistor. > > When I fired it up, my output looked kinda distorted. I swapped out the opto with a regular resistor and it cleaned up. > > Any idea why my output looks like that? Can I not use the opto this way? > > I tried a bunch of different opamps, but the scope shot is from an LF411 with +/- 5V rails. Scope shots are on the link below. > > > http://s555.photobucket.com/user/panfilero/media/OptoAmp.jpg.html > > Much thanks!That's a bad design. It needs a series-R before the photoFET. Ground the (+) input of the op-amp; do not connect to an input or output node.
Reply by ●December 20, 20152015-12-20
On Thu, 17 Dec 2015 11:28:46 -0800 (PST), Fibo <panfilero@gmail.com> wrote:>Hello, > >I'm trying to make a voltage controlled amplifier. I only need a gain of about 11 max. So I thought I would use an opto as a variable resistor on an inverting amplifier. I went with the H11F3M > >https://www.fairchildsemi.com/datasheets/H1/H11F3M.pdf > >I built up the Automatic Gain Control circuit in the datasheet. I used a 50k resistor as my feedback resistor. > >When I fired it up, my output looked kinda distorted. I swapped out the opto with a regular resistor and it cleaned up. > >Any idea why my output looks like that? Can I not use the opto this way? > >I tried a bunch of different opamps, but the scope shot is from an LF411 with +/- 5V rails. Scope shots are on the link below. > > >http://s555.photobucket.com/user/panfilero/media/OptoAmp.jpg.html > >Much thanks!Have you figured this one out yet? http://s555.photobucket.com/user/panfilero/media/crap.jpg.html
Reply by ●December 20, 20152015-12-20
On Sunday, December 20, 2015 at 7:12:35 PM UTC-6, John Larkin wrote:> Have you figured this one out yet? > > http://s555.photobucket.com/user/panfilero/media/crap.jpg.htmlI never did.
Reply by ●December 20, 20152015-12-20
On Sun, 20 Dec 2015 18:59:54 -0800 (PST), Fibo <panfilero@gmail.com> wrote:>On Sunday, December 20, 2015 at 7:12:35 PM UTC-6, John Larkin wrote: > >> Have you figured this one out yet? >> >> http://s555.photobucket.com/user/panfilero/media/crap.jpg.html > >I never did.The wrong-direction spikes are from gate-to-source capacitance. The big long slope is drain capacitance discharging into the 5K resistor. I designed a cool circuit recently, wrecked by mosfet g-d capacitance.
Reply by ●December 21, 20152015-12-21
On Sun, 20 Dec 2015 19:43:33 -0800, John Larkin <jjlarkin@highlandtechnology.com> wrote:>On Sun, 20 Dec 2015 18:59:54 -0800 (PST), Fibo <panfilero@gmail.com> >wrote: > >>On Sunday, December 20, 2015 at 7:12:35 PM UTC-6, John Larkin wrote: >> >>> Have you figured this one out yet? >>> >>> http://s555.photobucket.com/user/panfilero/media/crap.jpg.html >> >>I never did. > >The wrong-direction spikes are from gate-to-source capacitance.Oops, make that gate-to-drain capacitance.
