Bootstrapped Op Amp Articles

[plupo]

Thank JR for the reply.
I was referring to one of the designs that had a non inverting input and a usual opamp feedback configuration (with gain)
That was misleading, as well as the first one, with a grounded positive input: clearly shifting the PS will do the job, but then everything is bootstrapped.
I've also realized the trick of the first is in the 45 V: nothing really hurts then, but you cannot go further.
Anyway , I've got your point and is valuable for some application.
So far i'm more prone to use current mirrors driven by opamp ps currents, but on my actual design (70-100 MHz BW gain 10 ac < 10% dc component) i'm still looking for the holy grail...not easy for sure
All the best

Paolo

[mediatechnology]

Thanks for joining us! I'm glad I was able to get self-registration working on the forum.

The Analog Devices Grayson King article addresses the input over voltage considerations as well as any of the articles: viewtopic.php?f=12&t=699#p10218

The bootstrap circuit I've been using here has been bullet-proof: viewtopic.php?f=6&t=598
It is a DC-follower however.

I recently built a bootstrap line driver again and the primary limitation I found was output current into loads <=2KΩ. To get full advantage of the rail voltage bootstrapping most op amps need current-boosted outputs requiring four transistors total. FWIW the ZTX851/951 work really well in the MCI circuit.

So far i'm more prone to use current mirrors driven by opamp ps currents

I've seen that one used quite a bit as well. SSM did a power amp using that trick. Burr-Brown documents the circuit pretty well in one of the INA134 ap notes IIRC.

[JR.]

Thank JR for the reply.
I was referring to one of the designs that had a non inverting input and a usual opamp feedback configuration (with gain)
That was misleading, as well as the first one, with a grounded positive input: clearly shifting the PS will do the job, but then everything is bootstrapped.
I've also realized the trick of the first is in the 45 V: nothing really hurts then, but you cannot go further.
Anyway , I've got your point and is valuable for some application.
So far i'm more prone to use current mirrors driven by opamp ps currents, but on my actual design (70-100 MHz BW gain 10 ac < 10% dc component) i'm still looking for the holy grail...not easy for sure
All the best

Paolo

I dabbled with this topology back in the 70's for DIY. Yes there are limits to how much voltage gain you can use.
-------
Now for something completely different but not really... In the late 80's while at Peavey I designed a 35W continuous power amp with cap doublers in each PS rail, so that momentarily it could deliver 2x voltage on peaks, for 4x power (theoretically), in practice it managed 100W for 20 mSec. Not bad for a 35W sized heatsink and power transformer (PMA70+).
---------
FWIW the vast majority of Peavey's general purpose mono power amps (probably millions of channels) used a standard voltage op amp front end and a driven supply or driven rail topology. The op amp ran from +/- 15V and the output power stage drove into ground causing the power rails to swing up and down with the audio. The speaker pulled power from the center tap of the PS rails.

This topology was extremely cost effective and delivered decent performance. The economics suffers for stereo amps because you need separate transformer windings for each channel.

Peavey made lots of conventional design amps too.


JR

[mediatechnology]

Now for something completely different but not really... In the late 80's while at Peavey I designed a 35W continuous power amp with cap doublers in each PS rail, so that momentarily it could deliver 2x voltage on peaks, for 4x power (theoretically), in practice it managed 100W for 20 mSec. Not bad for a 35W sized heatsink and power transformer (PMA70+).

You reminded me of the Philips TDA1560Q which was an auto "40W" (10% THD using bursts) using a voltage-doubler and combination of class-B class-H. The 0.5% numbers were a more realistic 30W but still pretty good for a 14.4V supply.

[JR.]

Now for something completely different but not really... In the late 80's while at Peavey I designed a 35W continuous power amp with cap doublers in each PS rail, so that momentarily it could deliver 2x voltage on peaks, for 4x power (theoretically), in practice it managed 100W for 20 mSec. Not bad for a 35W sized heatsink and power transformer (PMA70+).

You reminded me of the Philips TDA1560Q which was an auto "40W" (10% THD using bursts) using a voltage-doubler and combination of class-B class-H. The 0.5% numbers were a more realistic 30W but still pretty good for a 14.4V supply.

Need to tell them the ancients (me) stole their idea.

JR

[plupo]

well... what can I say if not "thank to this forum !" couple of days i'm in and you all went up with plenty of tricks.
FYI the 20 mSec 100W amp was probably copied after you by a HiFi power amp on a magazine quite popular in the mid '80 in Italy: today there' plenty of caps' pumping for Cmos gate drivers, but nobody goes for linear: I start thinking applications, but my field is not driven by cost effectiveness so people just throw in big PSU making labs warm at evening...
I'm working now on a +/-100 V compliance, but then you need to limit the bandwidth due the different slew rates you can have on the positive and negative slope that can cause instant overload, this , on audio, should not be too much an issue for some applications.
we will see....

[mediatechnology]

So far i'm more prone to use current mirrors driven by opamp ps currents

"SSM2131 Current Feedback Audio Power Amp," Mark Alexander, PMI Audio Handbook, Volume 1, 1990, p 82-83.


SSM2131 "Current Feedback Audio Power Amp," Mark Alexander, PMI Audio Handbook, Volume 1, 1990, p 82-83.

SSM2131 Current Feedback Audio Power Amp from the PMI Audio Handbook Volume 1: https://www.proaudiodesignforum.com/ima ... 1_1990.pdf

See also US Patent 5097223 for the Alexander Current-Feedback Audio Power Amplifier: https://www.proaudiodesignforum.com/ima ... lifier.pdf

[JR.]

Those look like vertical output power FETs, designed more for switching than linear use.

JR

[mediatechnology]

Two things in combination about the SSM "Current Feedback" audio power amplifier that I have always found interesting.

1) The VAS formed by the 2131's supply current, common base stage and current mirror combined with 2) the servo which injects correction by operating in parallel with the SSM2131's supply connections.

There is a chronology of using the op amp supply pins as outputs dating back to 1970 and the RCA uA777: http://www.diyaudio.com/forums/the-loun ... ost4790921

Edit it was actually the Fairchild uA777.

[JR.]

Two things in combination about the SSM "Current Feedback" audio power amplifier that I have always found interesting.

1) The VAS formed by the 2131's supply current, common base stage and current mirror combined with 2) the servo which injects correction by operating in parallel with the SSM2131's supply connections.

There is a chronology of using the op amp supply pins as outputs dating back to 1970 and the RCA uA777: http://www.diyaudio.com/forums/the-loun ... ost4790921

I don't recall all the details but that hifi amp I built for my sister had the op amp output pin driving a resistor to ground, and power supply pins fed through zener diodes into common emitter NPN and PNP (darlingtons probably).

I'm sure I was inspired by one of the sundry design ideas in the trade magazines that I read back then.

JR