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Class B distortion

Started by Unknown December 20, 2014
On Sat, 20 Dec 2014 16:02:25 -0800, meow2222 wrote:

> On Saturday, December 20, 2014 12:11:02 PM UTC, Phil Hobbs wrote: >> Yup. And to get anywhere near it, they have to run a huge input error >> voltage--as much as a volt for FET types or bipolars with lots of input >> stage degeneration. >> >> That's not a huge problem when the amp itself is reasonably linear, but >> it sur is in this case. >> >> Cheers >> >> Phil Hobbs > > Does that mean if I ran the opamp at gain=1 it would at least reduce the > problem to some degree? The bad news is it will be any old opamp, eg > LM358, LM324 etc.
Probably not. The slew rate limitation is a nonlinear phenomenon, changing the op-amp (or, presumably the whole-circuit) gain is a mostly- linear fix. Biasing your output to class AB, and living with the extra dissipation, may be the road you'll have to take. -- www.wescottdesign.com
On Sat, 20 Dec 2014 20:01:49 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Sat, 20 Dec 2014 16:02:25 -0800, meow2222 wrote: > >> On Saturday, December 20, 2014 12:11:02 PM UTC, Phil Hobbs wrote: >>> Yup. And to get anywhere near it, they have to run a huge input error >>> voltage--as much as a volt for FET types or bipolars with lots of input >>> stage degeneration. >>> >>> That's not a huge problem when the amp itself is reasonably linear, but >>> it sur is in this case. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Does that mean if I ran the opamp at gain=1 it would at least reduce the >> problem to some degree? The bad news is it will be any old opamp, eg >> LM358, LM324 etc. > >Probably not. The slew rate limitation is a nonlinear phenomenon, >changing the op-amp (or, presumably the whole-circuit) gain is a mostly- >linear fix. > >Biasing your output to class AB, and living with the extra dissipation, >may be the road you'll have to take.
Yep. There's no OpAmp with a fast enough slew-rate (that you can afford ;-) ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Sat, 20 Dec 2014 16:22:50 -0800 (PST), meow2222@care2.com wrote:

>On Saturday, December 20, 2014 4:06:02 PM UTC, John Larkin wrote: >> On Sat, 20 Dec 2014 00:55:39 -0800 (PST), meow2222@care2.com wrote: > >> >Something I thought I knew, but am left puzzling... >> > >> >Take a simple class B output pair driven directly by an opamp. Result is nasty crossover distortion. Now, the nfb is taken from the circuit's output terminal, not from the opamp's output, so surely an ideal opamp should correct for the 0.6v Vbe drops. Question is why doesn't it? >> > >> >The other question is what strategies exist to tackle it. >> >- bias the output trs to class AB >> >- helper resistor from opamp output to circuit output, so the opamp powers the output directly from -0.6 to +0.6v >> >- low Vbe transistors, ie germanium >> >- a diode drop in the feedback path seems to reduce it to a degree >> > >> >Any techniques I've missed? > >> This is class B, I guess. >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/Opamp_Boosted.JPG > >I like the upsides, minimal part count and ok on linearity. The need to tailor the Rs to the opamp is a big problem for people that have no testgear, and wouldnt know how to use it anyway.
Then let them use data sheets. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
John Larkin <jlarkin@highlandtechnology.com> Wrote in message:
> On Sat, 20 Dec 2014 16:22:50 -0800 (PST), meow2222@care2.com wrote: > >>On Saturday, December 20, 2014 4:06:02 PM UTC, John Larkin wrote: >>> On Sat, 20 Dec 2014 00:55:39 -0800 (PST), meow2222@care2.com wrote: >> >>> >Something I thought I knew, but am left puzzling... >>> > >>> >Take a simple class B output pair driven directly by an opamp. Result is nasty crossover distortion. Now, the nfb is taken from the circuit's output terminal, not from the opamp's output, so surely an ideal opamp should correct for the 0.6v Vbe drops. Question is why doesn't it? >>> > >>> >The other question is what strategies exist to tackle it. >>> >- bias the output trs to class AB >>> >- helper resistor from opamp output to circuit output, so the opamp powers the output directly from -0.6 to +0.6v >>> >- low Vbe transistors, ie germanium >>> >- a diode drop in the feedback path seems to reduce it to a degree >>> > >>> >Any techniques I've missed? >> >>> This is class B, I guess. >>> >>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/Opamp_Boosted.JPG >> >>I like the upsides, minimal part count and ok on linearity. The need to tailor the Rs to the opamp is a big problem for people that have no testgear, and wouldnt know how to use it anyway. > > Then let them use data sheets. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing laser drivers and controllers > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com > >
Could you use a large-ish resistor in the op amp power supply lead, with a small current sense emitter resistor on the power transistor and another transistor connected to pull base current? Bypass the emitter resistor with a capacitor. Then I think it would self-regulate and you wouldn't need to tailor the opamp sense resistor so much. -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
On Sunday, December 21, 2014 3:02:34 AM UTC, John Larkin wrote:
> On Sat, 20 Dec 2014 16:22:50 -0800 (PST), meow2222@care2.com wrote: >=20 > >On Saturday, December 20, 2014 4:06:02 PM UTC, John Larkin wrote: > >> On Sat, 20 Dec 2014 00:55:39 -0800 (PST), meow2222@care2.com wrote: > > > >> >Something I thought I knew, but am left puzzling... > >> > > >> >Take a simple class B output pair driven directly by an opamp. Result=
is nasty crossover distortion. Now, the nfb is taken from the circuit's ou= tput terminal, not from the opamp's output, so surely an ideal opamp should= correct for the 0.6v Vbe drops. Question is why doesn't it?
> >> > > >> >The other question is what strategies exist to tackle it. > >> >- bias the output trs to class AB > >> >- helper resistor from opamp output to circuit output, so the opamp p=
owers the output directly from -0.6 to +0.6v
> >> >- low Vbe transistors, ie germanium > >> >- a diode drop in the feedback path seems to reduce it to a degree > >> > > >> >Any techniques I've missed? > > > >> This is class B, I guess. > >>=20 > >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/Opamp_Boost=
ed.JPG
> > > >I like the upsides, minimal part count and ok on linearity. The need to =
tailor the Rs to the opamp is a big problem for people that have no testgea= r, and wouldnt know how to use it anyway.
>=20 > Then let them use data sheets.
Its an option. It would fit the plan better if possible to have one small d= ocument that contains everything they need - I only want the moon on a stic= k. Putting a table of R values in there might well work. Its certainly a si= mple circuit, I like that. NT
On Sunday, December 21, 2014 3:30:45 AM UTC, bitrex wrote:

> Could you use a large-ish resistor in the op amp power supply > lead, with a small current sense emitter resistor on the power > transistor and another transistor connected to pull base current? > Bypass the emitter resistor with a capacitor. Then I think it > would self-regulate and you wouldn't need to tailor the opamp > sense resistor so much.
That sounds like a current limit. Then if the opamp lead R is too high, the op trs will sit drawing lots of current, just being limited to i_max. It doesnt sound good. If one left out the 2nd tr that steals base current from the main one, I think I'll sim it, see if it does what I think, sounds interesting. NT
On Sunday, December 21, 2014 2:34:47 AM UTC, Jim Thompson wrote:
> On Sat, 20 Dec 2014 20:01:49 -0600, Tim Wescott <tim@seemywebsite.com> > wrote: > >On Sat, 20 Dec 2014 16:02:25 -0800, meow2222 wrote: > >> On Saturday, December 20, 2014 12:11:02 PM UTC, Phil Hobbs wrote:
> >>> Yup. And to get anywhere near it, they have to run a huge input error > >>> voltage--as much as a volt for FET types or bipolars with lots of input > >>> stage degeneration. > >>> > >>> That's not a huge problem when the amp itself is reasonably linear, but > >>> it sur is in this case. > >>> > >>> Cheers > >>> > >>> Phil Hobbs > >> > >> Does that mean if I ran the opamp at gain=1 it would at least reduce the > >> problem to some degree? The bad news is it will be any old opamp, eg > >> LM358, LM324 etc. > > > >Probably not. The slew rate limitation is a nonlinear phenomenon, > >changing the op-amp (or, presumably the whole-circuit) gain is a mostly- > >linear fix. > > > >Biasing your output to class AB, and living with the extra dissipation, > >may be the road you'll have to take.
I may do. But I would like to find a better option if it exists.
> Yep. There's no OpAmp with a fast enough slew-rate (that you can > afford ;-) > > ...Jim Thompson
Total budget zero on this one. Its not like I'm tight or anything :) NT
On Sat, 20 Dec 2014 20:20:32 -0800 (PST), meow2222@care2.com wrote:

>On Sunday, December 21, 2014 3:02:34 AM UTC, John Larkin wrote: >> On Sat, 20 Dec 2014 16:22:50 -0800 (PST), meow2222@care2.com wrote: >> >> >On Saturday, December 20, 2014 4:06:02 PM UTC, John Larkin wrote: >> >> On Sat, 20 Dec 2014 00:55:39 -0800 (PST), meow2222@care2.com wrote: >> > >> >> >Something I thought I knew, but am left puzzling... >> >> > >> >> >Take a simple class B output pair driven directly by an opamp. Result is nasty crossover distortion. Now, the nfb is taken from the circuit's output terminal, not from the opamp's output, so surely an ideal opamp should correct for the 0.6v Vbe drops. Question is why doesn't it? >> >> > >> >> >The other question is what strategies exist to tackle it. >> >> >- bias the output trs to class AB >> >> >- helper resistor from opamp output to circuit output, so the opamp powers the output directly from -0.6 to +0.6v >> >> >- low Vbe transistors, ie germanium >> >> >- a diode drop in the feedback path seems to reduce it to a degree >> >> > >> >> >Any techniques I've missed? >> > >> >> This is class B, I guess. >> >> >> >> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/Opamp_Boosted.JPG >> > >> >I like the upsides, minimal part count and ok on linearity. The need to tailor the Rs to the opamp is a big problem for people that have no testgear, and wouldnt know how to use it anyway. >> >> Then let them use data sheets. > >Its an option. It would fit the plan better if possible to have one small document that contains everything they need - I only want the moon on a stick. Putting a table of R values in there might well work. Its certainly a simple circuit, I like that. > > >NT
DC-wise, it's simple. Size the base resistors for, say, 0.3 volts drop at the opamp's max supply quiescent current. A 1 mA max Iq implies 300 ohm resistors. The opamp drives light loads without help but, when the load current gets up to 2 mA or so, the transistors turn on and help. The AC situation is more complex. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
meow2222@care2.com Wrote in message:
> On Sunday, December 21, 2014 3:30:45 AM UTC, bitrex wrote: > >> Could you use a large-ish resistor in the op amp power supply >> lead, with a small current sense emitter resistor on the power >> transistor and another transistor connected to pull base current? >> Bypass the emitter resistor with a capacitor. Then I think it >> would self-regulate and you wouldn't need to tailor the opamp >> sense resistor so much. > > That sounds like a current limit. Then if the opamp lead R is too high, the op trs will sit drawing lots of current, just being limited to i_max. It doesnt sound good.
My idea was that it's not a lot of current - just enough to keep the power transistors conducting a little so their bases are sitting at ~ 0.6 volts without having to match the opamp resistor to each op amp...you'd just need to select it for the minimum expected quiescent op amp current. If the current was higher the second transistor would pull away the excess. The capacitors bypass the emitter resistor on the power transistor for AC; the emitter resistors are just to set the DC operating point. Maybe this won't work and I'm missing something?
> If one left out the 2nd tr that steals base current from the main one, I think I'll sim it, see if it does what I think, sounds interesting. > > > NT >
-- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
meow2222@care2.com wrote:

> On Saturday, December 20, 2014 2:38:14 PM UTC, Tom Swift wrote:
>> Add a complimentary NPN/PNP emitter follower stage between the op amp >> and output to bias the output stage so it is barely on, and to add >> more drive > > I'm not managing to work out what you mean there
Take 2N2222 and 2N2907. Tie the bases together to the output of the op amp. Connect the collectors to the appropriate supply voltage. Works best with symmetrical supplies but can be made to work with a single supply if you keep it centered. Add 1k resistors from the emitters to the appropriate supply voltages. You now have two emitter followers with 1.2V between the emitters and centered on the output of the op amp. Tie the emitters to the base of the opposite polarity output transistors. They each now have 0.6V bias to turn them on. Add 22 ohm resistors to the output emitters to stabilize the current. The junction of the 22 ohm resistors is the output signal. Tie this to the load and to the feedback for the op amp. This would be so much simpler in LTspice but I got to go and finish my laundry.