Forums

Optocoupler suggestions

Started by bitrex May 29, 2018
What are some optocouplers you like that would be good for designs where 
they're used in unusual ways, like that "HV Opamp" thing JL likes where 
they're used as the "output stage" of an amplifier?

Are any characteristics that I should look for in the datasheet that 
makes a particular coupler a better performer in an application where 
linearity is more important than high-speed switching as in e.g. digital 
signaling? Not all datasheets seem to provide good charts of CTR over 
all the relevant independent variables.
I don't know of any that are particularly well specified, except for the 
dual-photodiode ones, which are still a bit sloppy (enough that you need 
trimmers for a modestly precise analog coupler circuit).

CTR isn't reliable due to emitter and detector nonlinearity, and emitter 
aging, AFAIK.  The nonlinearity is modest over current (+/- 25%?), but the 
aging is... something like max to min spec over lifetime (decades)?  Unsure.

It's rare enough to find one specified for B-E resistance (for speeding up 
digital applications -- not much use for analog, AFAIK*).

*This uses the (photoactive) B-E junction as a summing node, which is 
interesting; speed (a few MHz) is about as fast as the opto can go, but the 
response still looks first-order, suggesting there is yet more speed to be 
won.
https://www.seventransistorlabs.com/Images/4N35_TIA.png

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: https://www.seventransistorlabs.com/

"bitrex" <user@example.net> wrote in message 
news:_EkPC.212388$bS4.4705@fx01.iad...
> What are some optocouplers you like that would be good for designs where > they're used in unusual ways, like that "HV Opamp" thing JL likes where > they're used as the "output stage" of an amplifier? > > Are any characteristics that I should look for in the datasheet that makes > a particular coupler a better performer in an application where linearity > is more important than high-speed switching as in e.g. digital signaling? > Not all datasheets seem to provide good charts of CTR over all the > relevant independent variables.
On 05/29/2018 11:08 PM, Tim Williams wrote:
> I don't know of any that are particularly well specified, except for the > dual-photodiode ones, which are still a bit sloppy (enough that you need > trimmers for a modestly precise analog coupler circuit). > > CTR isn't reliable due to emitter and detector nonlinearity, and emitter > aging, AFAIK.&#2013266080; The nonlinearity is modest over current (+/- 25%?), but > the aging is... something like max to min spec over lifetime (decades)? > Unsure. > > It's rare enough to find one specified for B-E resistance (for speeding > up digital applications -- not much use for analog, AFAIK*). > > *This uses the (photoactive) B-E junction as a summing node, which is > interesting; speed (a few MHz) is about as fast as the opto can go, but > the response still looks first-order, suggesting there is yet more speed > to be won. > https://www.seventransistorlabs.com/Images/4N35_TIA.png > > Tim >
I'm experimenting with using optocopulers as the gain-control element of a Wien bridge-style oscillator; the LED nonlinearity provides the corrective action for small deviations, the phototransistors feed a lil bias network/current source that controls the gain of an OTA in the main feedback loop for coarse corrective action vis a vis temperature and component variation etc. In sim at least it produces lower distortion sines than diodes alone and tracks frequency changes faster than that JFET + integrator...thing...
On Tue, 29 May 2018 22:08:08 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>I don't know of any that are particularly well specified, except for the >dual-photodiode ones, which are still a bit sloppy (enough that you need >trimmers for a modestly precise analog coupler circuit). > >CTR isn't reliable due to emitter and detector nonlinearity, and emitter >aging, AFAIK. The nonlinearity is modest over current (+/- 25%?), but the >aging is... something like max to min spec over lifetime (decades)? Unsure. > >It's rare enough to find one specified for B-E resistance (for speeding up >digital applications -- not much use for analog, AFAIK*). > >*This uses the (photoactive) B-E junction as a summing node, which is >interesting; speed (a few MHz) is about as fast as the opto can go, but the >response still looks first-order, suggesting there is yet more speed to be >won. >https://www.seventransistorlabs.com/Images/4N35_TIA.png > >Tim
You can buy 40 GHz photodiodes. Phototransistor optocouplers are slow because they use large-area transistors. Phototransistor couplers are a lot faster when used in photodiode mode, in a proper Hobbsonian circuit. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Tuesday, May 29, 2018 at 11:43:54 PM UTC-4, bitrex wrote:
> On 05/29/2018 11:08 PM, Tim Williams wrote: > > I don't know of any that are particularly well specified, except for the > > dual-photodiode ones, which are still a bit sloppy (enough that you need > > trimmers for a modestly precise analog coupler circuit). > > > > CTR isn't reliable due to emitter and detector nonlinearity, and emitter > > aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > > the aging is... something like max to min spec over lifetime (decades)? > > Unsure. > > > > It's rare enough to find one specified for B-E resistance (for speeding > > up digital applications -- not much use for analog, AFAIK*). > > > > *This uses the (photoactive) B-E junction as a summing node, which is > > interesting; speed (a few MHz) is about as fast as the opto can go, but > > the response still looks first-order, suggesting there is yet more speed > > to be won. > > https://www.seventransistorlabs.com/Images/4N35_TIA.png > > > > Tim > > > > I'm experimenting with using optocopulers as the gain-control element of > a Wien bridge-style oscillator; the LED nonlinearity provides the > corrective action for small deviations, the phototransistors feed a lil > bias network/current source that controls the gain of an OTA in the main > feedback loop for coarse corrective action vis a vis temperature and > component variation etc. > > In sim at least it produces lower distortion sines than diodes alone and > tracks frequency changes faster than that JFET + integrator...thing...
How low a distortion do you need? A gain stage like this, just clips the tops off. +->|---+ +-|<---+ Gnd---R1---+---R2--+--R3--+ | | + inv out-+ input (sorry too lazy to put in the opamp symbol.) R1 1k, R2 1.5k, R3 549 ohm. You can screw around with R2 and R3 ratio. George H.
On 05/30/2018 08:47 AM, George Herold wrote:
> On Tuesday, May 29, 2018 at 11:43:54 PM UTC-4, bitrex wrote: >> On 05/29/2018 11:08 PM, Tim Williams wrote: >>> I don't know of any that are particularly well specified, except for the >>> dual-photodiode ones, which are still a bit sloppy (enough that you need >>> trimmers for a modestly precise analog coupler circuit). >>> >>> CTR isn't reliable due to emitter and detector nonlinearity, and emitter >>> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but >>> the aging is... something like max to min spec over lifetime (decades)? >>> Unsure. >>> >>> It's rare enough to find one specified for B-E resistance (for speeding >>> up digital applications -- not much use for analog, AFAIK*). >>> >>> *This uses the (photoactive) B-E junction as a summing node, which is >>> interesting; speed (a few MHz) is about as fast as the opto can go, but >>> the response still looks first-order, suggesting there is yet more speed >>> to be won. >>> https://www.seventransistorlabs.com/Images/4N35_TIA.png >>> >>> Tim >>> >> >> I'm experimenting with using optocopulers as the gain-control element of >> a Wien bridge-style oscillator; the LED nonlinearity provides the >> corrective action for small deviations, the phototransistors feed a lil >> bias network/current source that controls the gain of an OTA in the main >> feedback loop for coarse corrective action vis a vis temperature and >> component variation etc. >> >> In sim at least it produces lower distortion sines than diodes alone and >> tracks frequency changes faster than that JFET + integrator...thing... > > How low a distortion do you need? > A gain stage like this, just clips the tops off. > > +->|---+ > +-|<---+ > Gnd---R1---+---R2--+--R3--+ > | | > + inv out-+ > input > (sorry too lazy to put in the opamp symbol.) > > R1 1k, R2 1.5k, R3 549 ohm. > You can screw around with R2 and R3 ratio. > > George H. >
Eehhh...i've used it before, it's a very fiddly circuit. Usually shown with trimpots in the feedback loop which is ugly and problematic. My goal with this thing is to have the positive feedback loop gain auto-level itself into a slightly over-unity state where the diode clipper in the negative loop can work most effectively, without grossly murdering the peaks due to an out-of-whack trimpot or out-of-tolerance resistor. It's not too hard to set up if the diodes are in optocouplers driving photodiodes/phototransistors. If the phototransistor/photodiode is pushing large currents on the peaks that means the positive feedback loop gain is too high, filter and mirror it around into an OTA in the positive loop and turn down the gain. Vice versa if they're not putting out any current at all. It should respond faster than the JFET + integrator gain control thing because the OTA gain is being controlled by charge balancing the current coming out of the photodiodes into a small cap with a current source pulling current out to ground, you don't have to wait for a DC voltage integrator with a large cap in the integrator feedback loop to wind up and wind down.
On 05/30/2018 08:47 AM, George Herold wrote:
> On Tuesday, May 29, 2018 at 11:43:54 PM UTC-4, bitrex wrote: >> On 05/29/2018 11:08 PM, Tim Williams wrote: >>> I don't know of any that are particularly well specified, except for the >>> dual-photodiode ones, which are still a bit sloppy (enough that you need >>> trimmers for a modestly precise analog coupler circuit). >>> >>> CTR isn't reliable due to emitter and detector nonlinearity, and emitter >>> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but >>> the aging is... something like max to min spec over lifetime (decades)? >>> Unsure. >>> >>> It's rare enough to find one specified for B-E resistance (for speeding >>> up digital applications -- not much use for analog, AFAIK*). >>> >>> *This uses the (photoactive) B-E junction as a summing node, which is >>> interesting; speed (a few MHz) is about as fast as the opto can go, but >>> the response still looks first-order, suggesting there is yet more speed >>> to be won. >>> https://www.seventransistorlabs.com/Images/4N35_TIA.png >>> >>> Tim >>> >> >> I'm experimenting with using optocopulers as the gain-control element of >> a Wien bridge-style oscillator; the LED nonlinearity provides the >> corrective action for small deviations, the phototransistors feed a lil >> bias network/current source that controls the gain of an OTA in the main >> feedback loop for coarse corrective action vis a vis temperature and >> component variation etc. >> >> In sim at least it produces lower distortion sines than diodes alone and >> tracks frequency changes faster than that JFET + integrator...thing... > > How low a distortion do you need? > A gain stage like this, just clips the tops off. > > +->|---+ > +-|<---+ > Gnd---R1---+---R2--+--R3--+ > | | > + inv out-+ > input > (sorry too lazy to put in the opamp symbol.) > > R1 1k, R2 1.5k, R3 549 ohm. > You can screw around with R2 and R3 ratio. > > George H. >
Here's the general idea: <https://www.dropbox.com/s/f8k21wg3wpz14w6/Photo%20May%2030%2C%209%2057%2023%20AM.jpg?dl=0>
On Wednesday, May 30, 2018 at 9:42:48 AM UTC-4, bitrex wrote:
> On 05/30/2018 08:47 AM, George Herold wrote: > > On Tuesday, May 29, 2018 at 11:43:54 PM UTC-4, bitrex wrote: > >> On 05/29/2018 11:08 PM, Tim Williams wrote: > >>> I don't know of any that are particularly well specified, except for the > >>> dual-photodiode ones, which are still a bit sloppy (enough that you need > >>> trimmers for a modestly precise analog coupler circuit). > >>> > >>> CTR isn't reliable due to emitter and detector nonlinearity, and emitter > >>> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > >>> the aging is... something like max to min spec over lifetime (decades)? > >>> Unsure. > >>> > >>> It's rare enough to find one specified for B-E resistance (for speeding > >>> up digital applications -- not much use for analog, AFAIK*). > >>> > >>> *This uses the (photoactive) B-E junction as a summing node, which is > >>> interesting; speed (a few MHz) is about as fast as the opto can go, but > >>> the response still looks first-order, suggesting there is yet more speed > >>> to be won. > >>> https://www.seventransistorlabs.com/Images/4N35_TIA.png > >>> > >>> Tim > >>> > >> > >> I'm experimenting with using optocopulers as the gain-control element of > >> a Wien bridge-style oscillator; the LED nonlinearity provides the > >> corrective action for small deviations, the phototransistors feed a lil > >> bias network/current source that controls the gain of an OTA in the main > >> feedback loop for coarse corrective action vis a vis temperature and > >> component variation etc. > >> > >> In sim at least it produces lower distortion sines than diodes alone and > >> tracks frequency changes faster than that JFET + integrator...thing... > > > > How low a distortion do you need? > > A gain stage like this, just clips the tops off. > > > > +->|---+ > > +-|<---+ > > Gnd---R1---+---R2--+--R3--+ > > | | > > + inv out-+ > > input > > (sorry too lazy to put in the opamp symbol.) > > > > R1 1k, R2 1.5k, R3 549 ohm. > > You can screw around with R2 and R3 ratio. > > > > George H. > > > > Eehhh...i've used it before, it's a very fiddly circuit. Usually shown > with trimpots in the feedback loop which is ugly and problematic.
I use the above in production, 1% R's.. no trimming. Hey I also made one with a light bulb..the bulb costs ~$1. (I think) Way back when I had issues with it motorboating sometimes. Turned out I needed better caps in the R/C part of the circuit. (The light bulb transient response is the pits. :^)
> > My goal with this thing is to have the positive feedback loop gain > auto-level itself into a slightly over-unity state where the diode > clipper in the negative loop can work most effectively, without grossly > murdering the peaks due to an out-of-whack trimpot or out-of-tolerance > resistor. It's not too hard to set up if the diodes are in optocouplers > driving photodiodes/phototransistors. If the phototransistor/photodiode > is pushing large currents on the peaks that means the positive feedback > loop gain is too high, filter and mirror it around into an OTA in the > positive loop and turn down the gain. Vice versa if they're not putting > out any current at all. > > It should respond faster than the JFET + integrator gain control thing > because the OTA gain is being controlled by charge balancing the current > coming out of the photodiodes into a small cap with a current source > pulling current out to ground, you don't have to wait for a DC voltage > integrator with a large cap in the integrator feedback loop to wind up > and wind down.
I assume you read J. William's long piece on the subject. I tried the Jfet thing, but could never get it to balance nicely. George H.
On Wednesday, May 30, 2018 at 10:04:38 AM UTC-4, bitrex wrote:
> On 05/30/2018 08:47 AM, George Herold wrote: > > On Tuesday, May 29, 2018 at 11:43:54 PM UTC-4, bitrex wrote: > >> On 05/29/2018 11:08 PM, Tim Williams wrote: > >>> I don't know of any that are particularly well specified, except for the > >>> dual-photodiode ones, which are still a bit sloppy (enough that you need > >>> trimmers for a modestly precise analog coupler circuit). > >>> > >>> CTR isn't reliable due to emitter and detector nonlinearity, and emitter > >>> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but > >>> the aging is... something like max to min spec over lifetime (decades)? > >>> Unsure. > >>> > >>> It's rare enough to find one specified for B-E resistance (for speeding > >>> up digital applications -- not much use for analog, AFAIK*). > >>> > >>> *This uses the (photoactive) B-E junction as a summing node, which is > >>> interesting; speed (a few MHz) is about as fast as the opto can go, but > >>> the response still looks first-order, suggesting there is yet more speed > >>> to be won. > >>> https://www.seventransistorlabs.com/Images/4N35_TIA.png > >>> > >>> Tim > >>> > >> > >> I'm experimenting with using optocopulers as the gain-control element of > >> a Wien bridge-style oscillator; the LED nonlinearity provides the > >> corrective action for small deviations, the phototransistors feed a lil > >> bias network/current source that controls the gain of an OTA in the main > >> feedback loop for coarse corrective action vis a vis temperature and > >> component variation etc. > >> > >> In sim at least it produces lower distortion sines than diodes alone and > >> tracks frequency changes faster than that JFET + integrator...thing... > > > > How low a distortion do you need? > > A gain stage like this, just clips the tops off. > > > > +->|---+ > > +-|<---+ > > Gnd---R1---+---R2--+--R3--+ > > | | > > + inv out-+ > > input > > (sorry too lazy to put in the opamp symbol.) > > > > R1 1k, R2 1.5k, R3 549 ohm. > > You can screw around with R2 and R3 ratio. > > > > George H. > > > > Here's the general idea: > > <https://www.dropbox.com/s/f8k21wg3wpz14w6/Photo%20May%2030%2C%209%2057%2023%20AM.jpg?dl=0>
Hmm OK, I'm mostly confused. I've never used an OTA... George H.
On 05/30/2018 10:33 AM, George Herold wrote:
> On Wednesday, May 30, 2018 at 10:04:38 AM UTC-4, bitrex wrote: >> On 05/30/2018 08:47 AM, George Herold wrote: >>> On Tuesday, May 29, 2018 at 11:43:54 PM UTC-4, bitrex wrote: >>>> On 05/29/2018 11:08 PM, Tim Williams wrote: >>>>> I don't know of any that are particularly well specified, except for the >>>>> dual-photodiode ones, which are still a bit sloppy (enough that you need >>>>> trimmers for a modestly precise analog coupler circuit). >>>>> >>>>> CTR isn't reliable due to emitter and detector nonlinearity, and emitter >>>>> aging, AFAIK.&nbsp; The nonlinearity is modest over current (+/- 25%?), but >>>>> the aging is... something like max to min spec over lifetime (decades)? >>>>> Unsure. >>>>> >>>>> It's rare enough to find one specified for B-E resistance (for speeding >>>>> up digital applications -- not much use for analog, AFAIK*). >>>>> >>>>> *This uses the (photoactive) B-E junction as a summing node, which is >>>>> interesting; speed (a few MHz) is about as fast as the opto can go, but >>>>> the response still looks first-order, suggesting there is yet more speed >>>>> to be won. >>>>> https://www.seventransistorlabs.com/Images/4N35_TIA.png >>>>> >>>>> Tim >>>>> >>>> >>>> I'm experimenting with using optocopulers as the gain-control element of >>>> a Wien bridge-style oscillator; the LED nonlinearity provides the >>>> corrective action for small deviations, the phototransistors feed a lil >>>> bias network/current source that controls the gain of an OTA in the main >>>> feedback loop for coarse corrective action vis a vis temperature and >>>> component variation etc. >>>> >>>> In sim at least it produces lower distortion sines than diodes alone and >>>> tracks frequency changes faster than that JFET + integrator...thing... >>> >>> How low a distortion do you need? >>> A gain stage like this, just clips the tops off. >>> >>> +->|---+ >>> +-|<---+ >>> Gnd---R1---+---R2--+--R3--+ >>> | | >>> + inv out-+ >>> input >>> (sorry too lazy to put in the opamp symbol.) >>> >>> R1 1k, R2 1.5k, R3 549 ohm. >>> You can screw around with R2 and R3 ratio. >>> >>> George H. >>> >> >> Here's the general idea: >> >> <https://www.dropbox.com/s/f8k21wg3wpz14w6/Photo%20May%2030%2C%209%2057%2023%20AM.jpg?dl=0> > > Hmm OK, I'm mostly confused. I've never used an OTA... > > George H. >
On the input side OTAs like e.g. the LM13700 are just a differential pair with the "other side" of the current mirror feeding the tail brought out to a pin so you can put a varying current in to control the gain of the pair. The collectors are connected to a structure of current mirrors arranged such that when one side of the diff pair is drawing more current than the other the IC output acts as a current source that "blows" the difference current out, vice versa the output acts as a current sink that "sucks" it in. It's an V-I converter with an adjustable transconductance basically. The V-I curve is only approximately linear in the area where the tanh function is approximately linear so if you want a linear current response to control voltage you need to divide large signals down before putting them onto the inputs. The response of the tanh function itself is a pretty good way to massage a trinagle wave into a sine, too, though that's not what I'm doing here