I have an opamp with a 10 volt positive supply and need the output to swing to about 16 volts. I was thinking I could add a paralleled 6.8 volt zener, a cap, and a floating power supply at the opamp output. The power supply could be a PV optocoupler like this: https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 My load is obviously light, a varicap in fact.
opamp level shifter
Started by ●November 3, 2023
Reply by ●November 3, 20232023-11-03
On Friday, 3 November 2023 at 15:02:35 UTC, John Larkin wrote:> I have an opamp with a 10 volt positive supply and need the output to > swing to about 16 volts. I was thinking I could add a paralleled 6.8 > volt zener, a cap, and a floating power supply at the opamp output. > > The power supply could be a PV optocoupler like this: > > https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 > > My load is obviously light, a varicap in fact.Why use a zener at all? Its not likely to add much voltage stability at the very low current available from the optocoupler. Alternatively, with a low power op-amp, put the optocoupler in series with the positive power supply and its voltage instability will vanish from the final output. John
Reply by ●November 3, 20232023-11-03
On Fri, 3 Nov 2023 08:20:56 -0700 (PDT), John Walliker <jrwalliker@gmail.com> wrote:>On Friday, 3 November 2023 at 15:02:35 UTC, John Larkin wrote: >> I have an opamp with a 10 volt positive supply and need the output to >> swing to about 16 volts. I was thinking I could add a paralleled 6.8 >> volt zener, a cap, and a floating power supply at the opamp output. >> >> The power supply could be a PV optocoupler like this: >> >> https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 >> >> My load is obviously light, a varicap in fact. > >Why use a zener at all? Its not likely to add much voltage stability at the very >low current available from the optocoupler. >Alternatively, with a low power op-amp, put the optocoupler in series with >the positive power supply and its voltage instability will vanish from the >final output. >JohnDumping the zener is a possibility, if the PV voltage were predictable and stable, which it may not be. PVs have a big negative voltage tempco. A PV opto can only supply 10s of uA, and the opamp needs many mA of supply voltage. I'll test the pv+zener combo and see how stable it is. Zeners can oscillate at low currents, but a hefty bypass cap should fix that.
Reply by ●November 3, 20232023-11-03
John Larkin <jl@997PotHill.com> wrote:> I have an opamp with a 10 volt positive supply and need the output to > swing to about 16 volts. I was thinking I could add a paralleled 6.8 > volt zener, a cap, and a floating power supply at the opamp output. > > The power supply could be a PV optocoupler like this: > > https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 > > My load is obviously light, a varicap in fact. > >Looks nice. You’ll want to filter the LED drive carefully, because with a bang-bang phase detector your loop bandwidth will be limited. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
Reply by ●November 3, 20232023-11-03
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> John Larkin <jl@997PotHill.com> wrote: >> I have an opamp with a 10 volt positive supply and need the output to >> swing to about 16 volts. I was thinking I could add a paralleled 6.8 >> volt zener, a cap, and a floating power supply at the opamp output. >> >> The power supply could be a PV optocoupler like this: >> >> https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 >> >> My load is obviously light, a varicap in fact. >> >> > > Looks nice. You’ll want to filter the LED drive carefully, because with a > bang-bang phase detector your loop bandwidth will be limited. > > Cheers > > Phil Hobbs >Oh, and at 10 uA, the zener noise will probably be a worry. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
Reply by ●November 3, 20232023-11-03
On Fri, 3 Nov 2023 15:47:12 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> John Larkin <jl@997PotHill.com> wrote: >>> I have an opamp with a 10 volt positive supply and need the output to >>> swing to about 16 volts. I was thinking I could add a paralleled 6.8 >>> volt zener, a cap, and a floating power supply at the opamp output. >>> >>> The power supply could be a PV optocoupler like this: >>> >>> https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 >>> >>> My load is obviously light, a varicap in fact. >>> >>> >> >> Looks nice. You�ll want to filter the LED drive carefully, because with a >> bang-bang phase detector your loop bandwidth will be limited. >> >> Cheers >> >> Phil Hobbs >> > >Oh, and at 10 uA, the zener noise will probably be a worry.Hence the beefy bypass cap, 10 uF maybe. Gotta try it. I'm dealing with a bunch of unspecified behavior. The current design has a 24 volt power supply whose only function is to bias that zener. A PV coupler is a small, cheap, quiet, wimpy isolated DC/DC converter. The PV LED will just be driven from 5 volts through a resistor. The PV current keeps the zener alive, so a little noise on the +5 shouldn't get into the loop. The loop isn't bang-bang, it's based on a fast 10-bit ADC observing the waveform from a 50 MHz LC oscillator, and a lot of DSP.
Reply by ●November 3, 20232023-11-03
On Friday, November 3, 2023 at 11:02:35 AM UTC-4, John Larkin wrote:> I have an opamp with a 10 volt positive supply and need the output to > swing to about 16 volts. I was thinking I could add a paralleled 6.8 > volt zener, a cap, and a floating power supply at the opamp output. > > The power supply could be a PV optocoupler like this: > > https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 > > My load is obviously light, a varicap in fact.Not seeing the need for a zener... I take it mean the standard non-inverting OA with the PV in series with OA output, and a high-Z resistor feedback divider from the composite output (PV cathode) to OA (-). Loop gain, which should be very large attenuates noise and the unknowns at the composite output. Is there some reason you can't drive current through the detector? Maybe too non-linearizing... They datasheet is so fixed on detector Voc Isc with input LED drive, it's almost tempting to use the LED drive to modulate the final output into place.
Reply by ●November 3, 20232023-11-03
On 03/11/2023 16:47, Fred Bloggs wrote:> On Friday, November 3, 2023 at 11:02:35 AM UTC-4, John Larkin wrote: >> I have an opamp with a 10 volt positive supply and need the output to >> swing to about 16 volts. I was thinking I could add a paralleled 6.8 >> volt zener, a cap, and a floating power supply at the opamp output. >> >> The power supply could be a PV optocoupler like this: >> >> https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 >> >> My load is obviously light, a varicap in fact. > > Not seeing the need for a zener... > > I take it mean the standard non-inverting OA with the PV in series with OA output, and a high-Z resistor feedback divider from the composite output (PV cathode) to OA (-). Loop gain, which should be very large attenuates noise and the unknowns at the composite output. > > Is there some reason you can't drive current through the detector? Maybe too non-linearizing... > > They datasheet is so fixed on detector Voc Isc with input LED drive, it's almost tempting to use the LED drive to modulate the final output into place.Not clear to me from JL's original post if the op amp has a negative supply rail so that even after the 7V output offset the varicap can be biased below 7V? Modulating the PV LED drive could have horrible delays and upset the dynamics? piglet
Reply by ●November 3, 20232023-11-03
On Fri, 3 Nov 2023 09:47:09 -0700 (PDT), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Friday, November 3, 2023 at 11:02:35?AM UTC-4, John Larkin wrote: >> I have an opamp with a 10 volt positive supply and need the output to >> swing to about 16 volts. I was thinking I could add a paralleled 6.8 >> volt zener, a cap, and a floating power supply at the opamp output. >> >> The power supply could be a PV optocoupler like this: >> >> https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 >> >> My load is obviously light, a varicap in fact. > >Not seeing the need for a zener...The PV voltage has a bad negative tempco, and that could cause us problems. A zener helps that. An LM4040 bandgap would be great, but they only come in 5 and 10 volts, too low and too high!> >I take it mean the standard non-inverting OA with the PV in series with OA output, and a high-Z resistor feedback divider from the composite output (PV cathode) to OA (-). Loop gain, which should be very large attenuates noise and the unknowns at the composite output.If I have a pretty constant offset added to the opamp output, I don't need to feed back from the offset, just from the opamp output. If I use the PV for the offset, I don't have enough current for the opampm feedback resistor.> >Is there some reason you can't drive current through the detector? Maybe too non-linearizing... > >They datasheet is so fixed on detector Voc Isc with input LED drive, it's almost tempting to use the LED drive to modulate the final output into place.To slow. Nice puzzle.
Reply by ●November 3, 20232023-11-03
On 03/11/2023 19:19, john larkin wrote:> On Fri, 3 Nov 2023 09:47:09 -0700 (PDT), Fred Bloggs > <bloggs.fredbloggs.fred@gmail.com> wrote: > >> On Friday, November 3, 2023 at 11:02:35?AM UTC-4, John Larkin wrote: >>> I have an opamp with a 10 volt positive supply and need the output to >>> swing to about 16 volts. I was thinking I could add a paralleled 6.8 >>> volt zener, a cap, and a floating power supply at the opamp output. >>> >>> The power supply could be a PV optocoupler like this: >>> >>> https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP3905-TPR-E/7809804 >>> >>> My load is obviously light, a varicap in fact. >> >> Not seeing the need for a zener... > > The PV voltage has a bad negative tempco, and that could cause us > problems. A zener helps that. An LM4040 bandgap would be great, but > they only come in 5 and 10 volts, too low and too high! >> >> I take it mean the standard non-inverting OA with the PV in series with OA output, and a high-Z resistor feedback divider from the composite output (PV cathode) to OA (-). Loop gain, which should be very large attenuates noise and the unknowns at the composite output. > > If I have a pretty constant offset added to the opamp output, I don't > need to feed back from the offset, just from the opamp output. If I > use the PV for the offset, I don't have enough current for the opampm > feedback resistor. >> >> Is there some reason you can't drive current through the detector? Maybe too non-linearizing... >> >> They datasheet is so fixed on detector Voc Isc with input LED drive, it's almost tempting to use the LED drive to modulate the final output into place. > > To slow. > > Nice puzzle. >There is the adjustable LM4041 but the minimum operating current will be too much for the PV. Most action in a varicap is in the 1-10V range, can't you simply pad the varicap capacitance down so you avoid the need to go to 16V? piglet
