I'm looking at putting together something similar to a Class AB/B audio amp , but it will be driven outside its linear range into saturation a lot of t he time. That's all well & good but for one thing: wrapping nfb round satur ating outputs doesn't work too well as it takes time for output devices to unsaturate, and the nfb effectively overreacts, adding distortion. Keeping distortion low matters here. What tips would you recommend to keep unwanted distortion minimised?
thanks, NT
Lots of rail-rail output opamps come off the rails clean and fast. One architecture has a lot of wideband gain inside the amp, and the major loop compensation is a cap from the output pin back into a late gain stage. So there's no internal, buried compensation pole to wind up when the output stage clips.
-- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Don't see why it would be that much different than crossover distortion; it takes time for the output to pass thru the dead band as one device stops conducting and the other device begins, negative feedback reduces crossover distortion just fine at low frequency when the open loop gain of the rest of the amp is high.
How are you driving the complementary emitter followers of a class AB/B amp into saturation, anyway? Wouldn't the base drive voltages have to swing above and below the rails?
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** So are guitar amps, some have little NFB and some have lots.** Getting the basics right - global NFB acting on its own does not pull an output stage out of saturation. Fact it does the direct opposite by fixing the closed loop gain. Long as the instantaneous input signal level, times gain, exceeds the DC supply, output devices are gonna be hammered. ** So NT wants output device saturation but don't want much waveform distor tion ? While keeping all nature of his project secret?
NT can go f*ck himself.
.... Phil
On Tuesday, February 20, 2018 at 10:01:46 PM UTC-5, snipped-for-privacy@gmail.com wrote :
mp, but it will be driven outside its linear range into saturation a lot of the time. That's all well & good but for one thing: wrapping nfb round sat urating outputs doesn't work too well as it takes time for output devices t o unsaturate, and the nfb effectively overreacts, adding distortion. Keepin g distortion low matters here. What tips would you recommend to keep unwant ed distortion minimised?
I believe the effect you described is worse when there is more saturation, usually. Just some mild output clipping does not upset it so badly, so mayb e you should consider pre-clipping the input signal. If the maximum output is fairly constant, and gain gain is of course with the negative feedback, seems like at the right point in the circuit something as simple as a coupl e of diodes might do the trick. I wouldn't tske that to the bank, but I'm s ure you know how to clip a waveform at any given level.
If you do it just right you shouldn't lose any output level. The output sti ll clips, but is not driven to umpteen times what it can reproduce.
Worth a try ?
Well it does pique the curiosity a bit, and evoke an invective from you.
Translation : Things are the same as always.
Just what might be the mystery application ? Is there a prize for figuring it out maybe ? :-)
If it's going to be a guitar amp, forget distortion, it's a part of the steel-wire music ...
It seems that you have re-invented the transient intermodulation distortion (TIM). Google for articles of prof. Matti Otala about the TIM.
-- -TV
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** If you guessed it - he would deny it.
A though bubble looking for a clever idea for him to plagiarise.
.... Phil
amp, but it will be driven outside its linear range into saturation a lot of the time. That's all well & good but for one thing: wrapping nfb round s aturating outputs doesn't work too well as it takes time for output devices to unsaturate, and the nfb effectively overreacts, adding distortion. Keep ing distortion low matters here. What tips would you recommend to keep unwa nted distortion minimised?
It was very late, and I think saturation is maybe the wrong term. The ampli fier output is railed frequently, I need it to come off the rails cleanly. Producing a circuit is the next step.
NT
amp, but it will be driven outside its linear range into saturation a lot of the time. That's all well & good but for one thing: wrapping nfb round s aturating outputs doesn't work too well as it takes time for output devices to unsaturate, and the nfb effectively overreacts, adding distortion. Keep ing distortion low matters here. What tips would you recommend to keep unwa nted distortion minimised?
If what you're saying is correct, why would class AB amps exist? AIUI It's because class B's glitches aren't fully solved by nfb.
NT
Sorry, can't reveal what the end use is on this one. I know it'd be easier if I could.
NT
amp, but it will be driven outside its linear range into saturation a lot of the time. That's all well & good but for one thing: wrapping nfb round s aturating outputs doesn't work too well as it takes time for output devices to unsaturate, and the nfb effectively overreacts, adding distortion. Keep ing distortion low matters here. What tips would you recommend to keep unwa nted distortion minimised?
thanks, got it
NT
Phil does go into stupid mode quickly.
Gotcha, but traditional class AB/B audio power amps in the way I think of them with complimentary emitter followers can't get very close to the rails, particularly the positive supply rail where there's usually a current source of some type feeding the voltage amplification stage, which needs some headroom to do its job.
There are some power amps designed to run directly off say 12 volt supply in a car, they sometimes use a bootstrap cap from the output back to the current source of the VAS to boost its supply voltage above the rail so the base drive for the NPN output follower can swing higher than usual to get more power output
They're pretty much "fully solved" at very low frequency, but a pure class B amp intrinsically has a shitton of THD even for relatively small signals, and so as frequency increases and open-loop gain drops off from the dominant pole compensation the ability of the NFB loop to compensate starts going t*ts-up fairly quickly.
It's not that NFB has _no_ ability to correct for crossover distortion intrinsically, or distortion caused by any other source internal to the amplifier design, it can do all of that stuff, but it's entirely dependent on the amp having sufficient gain-bandwidth available to do it effectively for signals in the range of frequencies of interest
The large signal sine-wave output of a class B LM324 op amp with unity gain looks lovely at 20 Hz, materially indistinguishable from any class AB op amp to my eyes, but like garbage once you start getting into even the low kHz range
JL has mentioned the issue of feedback integrator windup. I'd say two aspects can be significant to success. 1) Create a circuit that's linear without feedback, use minimum feedback, without internal integrators. 2) Make the circuit very fast, faster than needed.
My AMP-70 power amplifier design is an example. The configuration is intrinsically both linear and fast.
My circuit was inspired by the Tektronix PG-508 50MHz function-generator output stage, read my AoE writeup.
It was meant to be used all the way to the high-power rails, and it recovers instantly. The version in my drawings is complicated, but that's because it works to 10MHz with a blisteringly-high slew rate, which required using many fragile video-output transistors. A bit slower version could actually be quite simple.
--
Thanks,
- Win
On Tuesday, February 20, 2018 at 10:01:46 PM UTC-5, snipped-for-privacy@gmail.com wrote :
mp, but it will be driven outside its linear range into saturation a lot of the time. That's all well & good but for one thing: wrapping nfb round sat urating outputs doesn't work too well as it takes time for output devices t o unsaturate, and the nfb effectively overreacts, adding distortion. Keepin g distortion low matters here. What tips would you recommend to keep unwant ed distortion minimised?
Sounds a little like capacitor windup in a control loop. For which I just like to make sure all the amps hit the rail at about the same time. That might not apply in your case.
George H.
Those ones (e.g. the AD8605, a fave) also have much lower open-loop output impedance, because of course the local feedback means they aren't really running open loop. I use those ones for driving single-ended ADCs a fair amount, and they work great.
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 https://hobbs-eo.com
In the usual opamp, or the usual 3T regulator, adding a capacitive load adds a pole to the closed loop and makes the loop higher order, less stable. In the kind I mentioned, adding a load cap just makes the dominant pole slower, more stable.
Those types make good comparators too, in the 1 us sort of range.
-- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
io amp, but it will be driven outside its linear range into saturation a lo t of the time. That's all well & good but for one thing: wrapping nfb round saturating outputs doesn't work too well as it takes time for output devic es to unsaturate, and the nfb effectively overreacts, adding distortion. Ke eping distortion low matters here. What tips would you recommend to keep un wanted distortion minimised?
n;
s n Bt's because class B's glitches aren't fully solved by nfb.
nodding plenty here :)
NT
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