Forums

Opto in amplifier screws up output

Started by Fibo December 17, 2015
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!
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!
Looks like too much gain. (slew rate limit on the opamp.) Try less current in the AGC part of the circuit. George H.
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. -- Regards, Joerg http://www.analogconsultants.com/
On 12/17/2015 02:28 PM, 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
Way too much javascript and tracking crudware on that site.
> > Much thanks! >
Was the resistor the same value as the resistance of the photoFET at your bias point? Those suckers are pretty nonlinear at high resistance. 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
On Thursday, December 17, 2015 at 2:26:40 PM UTC-6, Phil Hobbs wrote:
> On 12/17/2015 02:28 PM, 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 > > Way too much javascript and tracking crudware on that site. > > > > > Much thanks! > > > > Was the resistor the same value as the resistance of the photoFET at > your bias point? Those suckers are pretty nonlinear at high resistance. > > 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
Yes, I had them both around 60k, and your right way in the non-linear region. I've dropped the value down to 4k, and it's not perfect but looks a whole lot better. thanks!
On Thu, 17 Dec 2015 12:41:32 -0800 (PST), Fibo <panfilero@gmail.com>
wrote:

>On Thursday, December 17, 2015 at 2:26:40 PM UTC-6, Phil Hobbs wrote: >> On 12/17/2015 02:28 PM, 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 >> >> Way too much javascript and tracking crudware on that site. >> >> > >> > Much thanks! >> > >> >> Was the resistor the same value as the resistance of the photoFET at >> your bias point? Those suckers are pretty nonlinear at high resistance. >> >> 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 > >Yes, I had them both around 60k, and your right way in the non-linear region. I've dropped the value down to 4k, and it's not perfect but looks a whole lot better. > >thanks!
What's the time scale on those scope shots? That photofet has 50 pF of capacitance at 15 volts, probably lots more for small signals. You might consider using an analog multiplier, or buying an official VGA. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
"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 -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
On Thursday, December 17, 2015 at 1:45:20 PM UTC-8, John Larkin wrote:
> On Thu, 17 Dec 2015 12:41:32 -0800 (PST), Fibo <panfilero@gmail.com> > wrote: > > >On Thursday, December 17, 2015 at 2:26:40 PM UTC-6, Phil Hobbs wrote: > >> On 12/17/2015 02:28 PM, 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
> >> Was the resistor the same value as the resistance of the photoFET at > >> your bias point? Those suckers are pretty nonlinear at high resistance.
> >Yes, I had them both around 60k, and your right way in the non-linear region. I've dropped the value down to 4k, and it's not perfect but looks a whole lot better.
> What's the time scale on those scope shots? That photofet has 50 pF of > capacitance at 15 volts, probably lots more for small signals. > > You might consider using an analog multiplier, or buying an official > VGA.
OTA chips (LM13700, NE5517 etc.) are well-behaved variable gain devices, as long as you keep the differential Vin under 20-50 mV. You get a current-source output, so voltage shift and Zoutput control are freebies.
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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
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? Jamie