On Wednesday, December 6, 2023 at 12:27:44 PM UTC-5, John Larkin wrote:> On Wed, 6 Dec 2023 09:04:53 -0800 (PST), Fred Bloggs > <bloggs.fred...@gmail.com> wrote: > >On Wednesday, December 6, 2023 at 11:45:32?AM UTC-5, Clive Arthur wrote: > >> On 06/12/2023 16:19, John Larkin wrote: > >> > On Wed, 6 Dec 2023 15:26:00 +0000, Clive Arthur > >> > <cl...@nowaytoday.co.uk> wrote: > >> > > >> >> I'm not an analog design expert, but needs must. > >> >> > >> >> I recently adapted a Doug Self audio amplifier design for use on a 60V > >> >> single supply, and to go up to 100kHz with a working ambient temperature > >> >> of -20'C to 180'C. > >> >> > >> >> I can't show the circuit, but it was based on Fig 1a here... > >> >> http://www.douglas-self.com/ampins/dipa/dipa.htm > >> > > >> > That's really ancient. And what is fig 1b all about? > >> It's old, but discrete and can take some pain. Too many IC's have > >> thermal limiting. Fig 1b? dunno, I just Googled the circuit diagram as > >> an example of the architecture. > > > >Fig 1b is Class A emitter follower, > Where is the follower? The pullup pair, TR5 and its pal, can source > about 12 mA max. TR4 and its friend are certainly not followers.The output PNP is the e. follower.> > There was one Motorola power opamp that drew the output stage all > wrong. National or someone 2nd sourced it and drew theirs the same > wrong way. > >Fig 1a is a Class B, also emitter follower pair. > without current limiting.It has those 0.33R emitter resistors, but they're mainly used to attenuate the bias current imbalance due to VBE offset between the transistors. Nothing in either of the circuits protects it against shorts.> > > >The TR8/9 and TR6/7 are well known composite emitter follower configurations. They should be wideband and not significantly affect loop phase at 100KHz. Dunno how you get crossover distortion with 1A, unless your 'VBIAS' is be too slight. > > > >What exactly does '100KHz" refer to? Is this a bandwidth or CW frequency? > > > >Do you have that right about 180oC? Seems extreme. > > > > > >> > What are your requirements? Current, distortion, protections? What's > >> > your load? > >> It's a few watts, coupled capacitively to various very lossy lines, > >> maybe 30R. I just want to know if there are any bright ideas for > >> improving it within the constraints. > >> > It won't work if you call the transistors TR; they feel insulted and > >> > oscillate. They want to be called Q. > >> No, PNP's are pink and NPN's are blue. I'll have none of this LGBTQ > >> nonsense. > >> > >> <snip> > >> > >> -- > >> Cheers > >> Clive
Power Amplifier for 100kHz.
Started by ●December 6, 2023
Reply by ●December 6, 20232023-12-06
Reply by ●December 6, 20232023-12-06
On Wednesday, December 6, 2023 at 12:16:40 PM UTC-5, John Walliker wrote:> On Wednesday 6 December 2023 at 17:08:28 UTC, Fred Bloggs wrote: > > On Wednesday, December 6, 2023 at 11:49:17 AM UTC-5, Clive Arthur wrote: > > > On 06/12/2023 15:59, Jan Panteltje wrote: > > > > On a sunny day (Wed, 6 Dec 2023 15:26:00 +0000) it happened Clive Arthur > > > > <cl...@nowaytoday.co.uk> wrote in <ukq3qb$qjik$1...@dont-email.me>: > > > > > > > >> I'm not an analog design expert, but needs must. > > > >> > > > <snip> > > > >> Any ideas for improving crossover distortion, bearing in mind the > > > >> temperature range? The signal is OFDM, so pretty much a load of > > > >> 'random' steps, some of which may be small and at a crossover point. > > > > > > > > My experience with this sort of transistor amplifiers is that it depends a lot on what transistors and what manufacturer > > > > you use. > > > > Hard to tell this way > > > > In the 3055 days one make transistor oscillated, the other was OK. > > > > The temperatiure range you mention is extreme, not much power left at 180C! > > > > Huge heatsink? > > > > How much power output do you need? > > > A few watts, the heatsink isn't huge, there's no room. Tj on the output > > > pair is probably around 210'C at 180'C ambient. It doesn't come with a > > > lifetime guarantee. > > There are metal packages rated for Tj,max of 200oC continuous. > I guess that this will be driving a few km of cable from a very deep hole in the ground.It's using OFDM to transmit multiple channel frequency encoded readouts, with everything tightly spaced in frequency. So you can see how high linearity is paramount.> > John > > > > > > > -- > > > Cheers > > > Clive
Reply by ●December 6, 20232023-12-06
On Wednesday, December 6, 2023 at 10:27:07 AM UTC-5, Clive Arthur wrote:> I'm not an analog design expert, but needs must. > > I recently adapted a Doug Self audio amplifier design for use on a 60V > single supply, and to go up to 100kHz with a working ambient temperature > of -20'C to 180'C. > > I can't show the circuit, but it was based on Fig 1a here... > http://www.douglas-self.com/ampins/dipa/dipa.htm > > Cdom needs to come down for this higher frequency application, but too > far and you have oscillation. Also, problems occur with slew rate > limiting due to Cdom, the TR5 constant current, and the increase in hFE > of TR4 with temperature. > > So I added an emitter degeneration resistor to TR4 to tame the hFE > variation, removed Cdom and put a smaller C across Rf1 instead. Seems > stable and not slew rate limiting. Took a lot longer to do than that > sentence might imply. > > However. > > The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably > get hotter and their Vbe would drop by more than the diodes, so it's > class B. As it runs at a high temperature, I obviously want to reduce > dissipation, and if I'd used say a rubber diode to get some quiescent > current, I think it would be very difficult to control Iq well enough > over the temperature range. > > So I have a circuit which works well enough, but could be better with > regard to crossover distortion (though it's lower than I would have > thought). In this application, the better the signal quality, the > higher the data rate. > > Any ideas for improving crossover distortion, bearing in mind the > temperature range? The signal is OFDM, so pretty much a load of > 'random' steps, some of which may be small and at a crossover point.For one thing you need to isolate the output with a series resistor in the output, and take your feedback from the circuit side of that resistor. Then you don't need to worry about unknown cable capacitance pulling your loop gain into instability. And it won't interfere with signal purity.> > -- > Cheers > Clive
Reply by ●December 6, 20232023-12-06
On 06/12/2023 15:26, Clive Arthur wrote:> I'm not an analog design expert, but needs must. > > I recently adapted a Doug Self audio amplifier design for use on a 60V > single supply, and to go up to 100kHz with a working ambient temperature > of -20'C to 180'C. > > I can't show the circuit, but it was based on Fig 1a here... > http://www.douglas-self.com/ampins/dipa/dipa.htm > > Cdom needs to come down for this higher frequency application, but too > far and you have oscillation. Also, problems occur with slew rate > limiting due to Cdom, the TR5 constant current, and the increase in hFE > of TR4 with temperature. > > So I added an emitter degeneration resistor to TR4 to tame the hFE > variation, removed Cdom and put a smaller C across Rf1 instead. Seems > stable and not slew rate limiting. Took a lot longer to do than that > sentence might imply. > > However. > > The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably > get hotter and their Vbe would drop by more than the diodes, so it's > class B. As it runs at a high temperature, I obviously want to reduce > dissipation, and if I'd used say a rubber diode to get some quiescent > current, I think it would be very difficult to control Iq well enough > over the temperature range. > > So I have a circuit which works well enough, but could be better with > regard to crossover distortion (though it's lower than I would have > thought). In this application, the better the signal quality, the > higher the data rate. > > Any ideas for improving crossover distortion, bearing in mind the > temperature range? The signal is OFDM, so pretty much a load of > 'random' steps, some of which may be small and at a crossover point. >Maintaining class AB bias over that temperature range is going to be difficult. I'd look instead at having no Vbias so the power stage operates pure class B and then have a fast small class A stage fill in the cross-over distortion. In other words the Quad feed-forward aka current dumping idea of the 1970s. piglet
Reply by ●December 6, 20232023-12-06
On Wed, 6 Dec 2023 18:54:05 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 06/12/2023 15:26, Clive Arthur wrote: >> I'm not an analog design expert, but needs must. >> >> I recently adapted a Doug Self audio amplifier design for use on a 60V >> single supply, and to go up to 100kHz with a working ambient temperature >> of -20'C to 180'C. >> >> I can't show the circuit, but it was based on Fig 1a here... >> http://www.douglas-self.com/ampins/dipa/dipa.htm >> >> Cdom needs to come down for this higher frequency application, but too >> far and you have oscillation.� Also, problems occur with slew rate >> limiting due to Cdom, the TR5 constant current, and the increase in hFE >> of TR4 with temperature. >> >> So I added an emitter degeneration resistor to TR4 to tame the hFE >> variation, removed Cdom and put a smaller C across Rf1 instead.� Seems >> stable and not slew rate limiting.� Took a lot longer to do than that >> sentence might imply. >> >> However. >> >> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably >> get hotter and their Vbe would drop by more than the diodes, so it's >> class B.� As it runs at a high temperature, I obviously want to reduce >> dissipation, and if I'd used say a rubber diode to get some quiescent >> current, I think it would be very difficult to control Iq well enough >> over the temperature range. >> >> So I have a circuit which works well enough, but could be better with >> regard to crossover distortion (though it's lower than I would have >> thought).� In this application, the better the signal quality, the >> higher the data rate. >> >> Any ideas for improving crossover distortion, bearing in mind the >> temperature range?� The signal is OFDM, so pretty much a load of >> 'random' steps, some of which may be small and at a crossover point. >> > >Maintaining class AB bias over that temperature range is going to be >difficult. I'd look instead at having no Vbias so the power stage >operates pure class B and then have a fast small class A stage fill in >the cross-over distortion. In other words the Quad feed-forward aka >current dumping idea of the 1970s. > >pigletOne trick is to have complementary class-B followers and add one resistor from the bases/gates to the output. That makes the output transfer curve continuous, granted nonlinear but the feedback mostly fixes that. No quiescent bias current, no thermal runaway hazard. Or use a TCA0372 power opamp. The thermal limit doesn't work!
Reply by ●December 6, 20232023-12-06
On 2023-12-06 10:26, Clive Arthur wrote:> I'm not an analog design expert, but needs must. > > I recently adapted a Doug Self audio amplifier design for use on a 60V > single supply, and to go up to 100kHz with a working ambient temperature > of -20'C to 180'C. > > I can't show the circuit, but it was based on Fig 1a here... > http://www.douglas-self.com/ampins/dipa/dipa.htm > > Cdom needs to come down for this higher frequency application, but too > far and you have oscillation. Also, problems occur with slew rate > limiting due to Cdom, the TR5 constant current, and the increase in hFE > of TR4 with temperature. > > So I added an emitter degeneration resistor to TR4 to tame the hFE > variation, removed Cdom and put a smaller C across Rf1 instead. Seems > stable and not slew rate limiting. Took a lot longer to do than that > sentence might imply. > > However. > > The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably > get hotter and their Vbe would drop by more than the diodes, so it's > class B. As it runs at a high temperature, I obviously want to reduce > dissipation, and if I'd used say a rubber diode to get some quiescent > current, I think it would be very difficult to control Iq well enough > over the temperature range. > > So I have a circuit which works well enough, but could be better with > regard to crossover distortion (though it's lower than I would have > thought). In this application, the better the signal quality, the > higher the data rate. > > Any ideas for improving crossover distortion, bearing in mind the > temperature range? The signal is OFDM, so pretty much a load of > 'random' steps, some of which may be small and at a crossover point. >Some local feedback around the output stage would get my vote. The Sziklai pairs have their own local feedback, but that doesn't fix the crossover problem. Another approach would be to turn TR4 into a diff pair. TR3's collector swing is going to waste, and that would let you keep the open-loop gain the same, while stabilizing the tail current. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●December 6, 20232023-12-06
On 06/12/2023 21:19, Phil Hobbs wrote:> On 2023-12-06 10:26, Clive Arthur wrote: >> I'm not an analog design expert, but needs must. >> >> I recently adapted a Doug Self audio amplifier design for use on a 60V >> single supply, and to go up to 100kHz with a working ambient >> temperature of -20'C to 180'C. >> >> I can't show the circuit, but it was based on Fig 1a here... >> http://www.douglas-self.com/ampins/dipa/dipa.htm >> >> Cdom needs to come down for this higher frequency application, but too >> far and you have oscillation. Also, problems occur with slew rate >> limiting due to Cdom, the TR5 constant current, and the increase in >> hFE of TR4 with temperature. >> >> So I added an emitter degeneration resistor to TR4 to tame the hFE >> variation, removed Cdom and put a smaller C across Rf1 instead. Seems >> stable and not slew rate limiting. Took a lot longer to do than that >> sentence might imply. >> >> However. >> >> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably >> get hotter and their Vbe would drop by more than the diodes, so it's >> class B. As it runs at a high temperature, I obviously want to reduce >> dissipation, and if I'd used say a rubber diode to get some quiescent >> current, I think it would be very difficult to control Iq well enough >> over the temperature range. >> >> So I have a circuit which works well enough, but could be better with >> regard to crossover distortion (though it's lower than I would have >> thought). In this application, the better the signal quality, the >> higher the data rate. >> >> Any ideas for improving crossover distortion, bearing in mind the >> temperature range? The signal is OFDM, so pretty much a load of >> 'random' steps, some of which may be small and at a crossover point. >> > Some local feedback around the output stage would get my vote. The > Sziklai pairs have their own local feedback, but that doesn't fix the > crossover problem. > > Another approach would be to turn TR4 into a diff pair. TR3's collector > swing is going to waste, and that would let you keep the open-loop gain > the same, while stabilizing the tail current. > > Cheers > > Phil HobbsThanks, that second one particularly sounds like a good idea. I'll see what the sim says. Local feedback on the output stage sounds trickier, I'll have to think. One thing I thought of was to use multiple smaller output pairs in parallel, and have a DC offset for each one. Imagine replacing the Vbias diode with a string of a few series diodes, and connecting the bases of one output pair across D1, the next pair across D2 etc. Would need resistors from each pair of emitters to the output. That should give lower crossover in more places. Maybe. -- Cheers Clive
Reply by ●December 6, 20232023-12-06
On 2023-12-06 16:19, Phil Hobbs wrote:> On 2023-12-06 10:26, Clive Arthur wrote: >> I'm not an analog design expert, but needs must. >> >> I recently adapted a Doug Self audio amplifier design for use on a 60V >> single supply, and to go up to 100kHz with a working ambient >> temperature of -20'C to 180'C. >> >> I can't show the circuit, but it was based on Fig 1a here... >> http://www.douglas-self.com/ampins/dipa/dipa.htm >> >> Cdom needs to come down for this higher frequency application, but too >> far and you have oscillation. Also, problems occur with slew rate >> limiting due to Cdom, the TR5 constant current, and the increase in >> hFE of TR4 with temperature. >> >> So I added an emitter degeneration resistor to TR4 to tame the hFE >> variation, removed Cdom and put a smaller C across Rf1 instead. Seems >> stable and not slew rate limiting. Took a lot longer to do than that >> sentence might imply. >> >> However. >> >> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably >> get hotter and their Vbe would drop by more than the diodes, so it's >> class B. As it runs at a high temperature, I obviously want to reduce >> dissipation, and if I'd used say a rubber diode to get some quiescent >> current, I think it would be very difficult to control Iq well enough >> over the temperature range. >> >> So I have a circuit which works well enough, but could be better with >> regard to crossover distortion (though it's lower than I would have >> thought). In this application, the better the signal quality, the >> higher the data rate. >> >> Any ideas for improving crossover distortion, bearing in mind the >> temperature range? The signal is OFDM, so pretty much a load of >> 'random' steps, some of which may be small and at a crossover point. >> > Some local feedback around the output stage would get my vote. The > Sziklai pairs have their own local feedback, but that doesn't fix the > crossover problem. > > Another approach would be to turn TR4 into a diff pair. TR3's collector > swing is going to waste, and that would let you keep the open-loop gain > the same, while stabilizing the tail current.To reduce the tempco of the first stage's tail source, one approach would be to use a LED or a SiC diode instead of two silicon ones in series. I've sometimes used a LED with an emitter follower to make a temperature-compendated low noise 1-V reference. You have to futz around with choosing LEDs to make that work well, but some orange Avago LEDs got down to a couple of hundred microvolts/K. I don't know whether there are LEDs that survive long enough in your conditions. Alternatively, the GB01SLT06-214 SiC rectifier seems to have about the same -2 mV dV/dT as a BJT, while dropping more voltage, so that might work. Cheers Phil Hobbs (Who still wants to make a thermoacoustic fridge to make downhole instruments' lives easier.) -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●December 6, 20232023-12-06
On Wed, 6 Dec 2023 22:49:40 +0000, Clive Arthur wrote:> On 06/12/2023 21:19, Phil Hobbs wrote: >> On 2023-12-06 10:26, Clive Arthur wrote: >>> I'm not an analog design expert, but needs must. >>> >>> I recently adapted a Doug Self audio amplifier design for use on a 60V >>> single supply, and to go up to 100kHz with a working ambient >>> temperature of -20'C to 180'C. >>> >>> I can't show the circuit, but it was based on Fig 1a here... >>> http://www.douglas-self.com/ampins/dipa/dipa.htm >>> >>> Cdom needs to come down for this higher frequency application, but too >>> far and you have oscillation. Also, problems occur with slew rate >>> limiting due to Cdom, the TR5 constant current, and the increase in >>> hFE of TR4 with temperature. >>> >>> So I added an emitter degeneration resistor to TR4 to tame the hFE >>> variation, removed Cdom and put a smaller C across Rf1 instead. Seems >>> stable and not slew rate limiting. Took a lot longer to do than that >>> sentence might imply. >>> >>> However. >>> >>> The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably >>> get hotter and their Vbe would drop by more than the diodes, so it's >>> class B. As it runs at a high temperature, I obviously want to reduce >>> dissipation, and if I'd used say a rubber diode to get some quiescent >>> current, I think it would be very difficult to control Iq well enough >>> over the temperature range. >>> >>> So I have a circuit which works well enough, but could be better with >>> regard to crossover distortion (though it's lower than I would have >>> thought). In this application, the better the signal quality, the >>> higher the data rate. >>> >>> Any ideas for improving crossover distortion, bearing in mind the >>> temperature range? The signal is OFDM, so pretty much a load of >>> 'random' steps, some of which may be small and at a crossover point. >>> >> Some local feedback around the output stage would get my vote. The >> Sziklai pairs have their own local feedback, but that doesn't fix the >> crossover problem. >> >> Another approach would be to turn TR4 into a diff pair. TR3's >> collector swing is going to waste, and that would let you keep the >> open-loop gain the same, while stabilizing the tail current. >> >> Cheers >> >> Phil Hobbs > > Thanks, that second one particularly sounds like a good idea. I'll see > what the sim says. Local feedback on the output stage sounds trickier, > I'll have to think. > > One thing I thought of was to use multiple smaller output pairs in > parallel, and have a DC offset for each one. Imagine replacing the > Vbias diode with a string of a few series diodes, and connecting the > bases of one output pair across D1, the next pair across D2 etc. Would > need resistors from each pair of emitters to the output. That should > give lower crossover in more places. Maybe.Some years ago Jim Thompson posted an audio amplifier design which used current mirrors to provide bias to the output transistors for the express purpose of keeping crossover distortion low over a large temperature range. He claimed it was the bees knees, but a quick search failed to turn it up. Perhaps someone else saved it or remembers the thread? Glen
Reply by ●December 6, 20232023-12-06
On 06/12/2023 22:58, Phil Hobbs wrote: <snip>> > To reduce the tempco of the first stage's tail source, one approach > would be to use a LED or a SiC diode instead of two silicon ones in series. > > I've sometimes used a LED with an emitter follower to make a > temperature-compendated low noise 1-V reference. You have to futz > around with choosing LEDs to make that work well, but some orange Avago > LEDs got down to a couple of hundred microvolts/K.Yeah, the circuit linked is the basis of my amplifier rather than the actual thing, and I do use a red LED for this - one used elsewhere which works at 180'C (and very few do). It actually does very well at tracking the transistor Vbe plus about 1.2V, I think more by luck than judgement. Upshot is that Iconst changes from 13mA to 14mA from 20'C to 180'C as measured. And it glows nicely.> I don't know whether there are LEDs that survive long enough in your > conditions. Alternatively, the GB01SLT06-214 SiC rectifier seems to > have about the same -2 mV dV/dT as a BJT, while dropping more voltage, > so that might work. > > Cheers > > Phil Hobbs > > (Who still wants to make a thermoacoustic fridge to make downhole > instruments' lives easier.)-- Cheers Clive
