Yet another Class AB autobias

On 15/12/2023 2:16 pm, Mike Monett VE3BTI wrote:
> Mike Monett VE3BTI <spamme@not.com> wrote:
> 
> We don't need no stinking automatic bias supply for class B operation.
> 
> 

Spoil sport! You have deprived me a nice intellectual challenge :>

piglet

On Fri, 15 Dec 2023 08:36:42 +0100, Arie de Muijnck
<eternal.september@ademu.com> wrote:

>On 2023-12-15 04:47, John Larkin wrote:
>> On Thu, 14 Dec 2023 19:25:00 -0800, John Larkin <jl@997PotHill.com>
>> wrote:
>> 
>>> On Fri, 15 Dec 2023 00:45:53 -0000 (UTC), Mike Monett VE3BTI
>>> <spamme@not.com> wrote:
>>>
>>>> piglet <erichpwagner@hotmail.com> wrote:
>>>>
>>>>> I expect over the decades many clever ways have been devised at
>>>>> eliminating crossover distortion and autobiasing class A/B so I have no
>>>>> shame in adding another. Does this look interesting or has it been done
>>>>> already?
>>>>
>>>> Your circuit has quite visible distortion throughout the range, including
>>>> at crossover.
>>>>
>>>> Here is a version that is simpler and has no visible distortion throughout
>>>> the range. The transistors are off at zero voltage output, so there is no
>>>> need to fool with bias.
>>>>
>>>> I believe the original was posted by Larkin, but this was a long time ago
>>>> so I might be mistaken.
>>>
>>> Probably me. I invented that way back in my youth.
>>>
>>> It doesn't work as well with mosfets, because it loses the gate turnon
>>> voltage, but that's easily fixed.
>>>
>>> The other fun topology uses the opamp power pins as outputs, to drive
>>> booster fets. That one can swing to the rails. The power pins can be
>>> used to make an ideal polarity splitter.
>> 
>> Here's another one.
>> 
>> https://www.dropbox.com/scl/fi/lduajoo7dyxy6h97tipes/Another_AB_amp.jpg?rlkey=lcyq6dsw4wpl64vr15gi0cdj2&raw=1
>> 
>> The transfer function is S-shaped but smooth. Just arrange for a few
>> tenths of a volt across each diode with no signal.
>> 
>
>Where those diodes could also be emitter followers for more output current... Nice!
>
>Arie

Yes, a sort of darlington where the first transistor has some standing
current and the second one kicks in at some point. The actual transfer
function would be interesting, as the secondary path kicks in but the
opposite-polarity side turns off.

Applying more design should make things simpler.

We mostly use n-channel mosfets these days. The best mosfet circuits
are different.

On Fri, 15 Dec 2023 02:45:19 -0800 (PST), John Walliker
<jrwalliker@gmail.com> wrote:

>On Friday 15 December 2023 at 03:25:57 UTC, John Larkin wrote:
>> On Fri, 15 Dec 2023 00:45:53 -0000 (UTC), Mike Monett VE3BTI 
>> <spa...@not.com> wrote: 
>> 
>> >piglet <erichp...@hotmail.com> wrote: 
>> > 
>> >> I expect over the decades many clever ways have been devised at 
>> >> eliminating crossover distortion and autobiasing class A/B so I have no 
>> >> shame in adding another. Does this look interesting or has it been done 
>> >> already? 
>> > 
>> >Your circuit has quite visible distortion throughout the range, including 
>> >at crossover. 
>> > 
>> >Here is a version that is simpler and has no visible distortion throughout 
>> >the range. The transistors are off at zero voltage output, so there is no 
>> >need to fool with bias. 
>> > 
>> >I believe the original was posted by Larkin, but this was a long time ago 
>> >so I might be mistaken.
>> Probably me. I invented that way back in my youth. 
>> 
>> It doesn't work as well with mosfets, because it loses the gate turnon 
>> voltage, but that's easily fixed. 
>> 
>> The other fun topology uses the opamp power pins as outputs, to drive 
>> booster fets. That one can swing to the rails. The power pins can be 
>> used to make an ideal polarity splitter.
>
>That topology was used by Burr Brown a very long time ago for some of their linear
>opto-isolators.  There was an LED in each supply rail and a resistor from the
>output to ground.  Feedback from a photodiode to the input and the other
>matched photodiode to the output amplifier.  There was no hint in the datasheet
>that it was done this way.  I opened one up.
>John

Here's a cheap 400-volt p-p amp, basically the same idea:

https://www.dropbox.com/s/pbdcg3fgrc0xyfi/HVamp.JPG?raw=1

I have some Spice models if anyone is interested.

It can be cascoded for higher voltages, or use pv couplers into HV
mosfets.

My NMR gradient amps used opamp power rails in the signal path too. 

On Fri, 15 Dec 2023 14:16:46 -0000 (UTC), Mike Monett VE3BTI
<spamme@not.com> wrote:

>Mike Monett VE3BTI <spamme@not.com> wrote:
>
>> The op amp needs to have 100MHz bandwidth and slew rate, be able to supply 
>> 50 ma output, and handle +/- 15V supplies. I tried searching Octopart, but 
>> had no luck.
>
>I finally figured how to search. The first hit showed exactly what is needed.
>
>The TI THS4031 has 100MHz bandwidth and slew rate, +/- 15V supplies, and can 
>drive 90 ma. It is also low noise at 1.6 nV/Hz, $4.39
>
>https://www.ti.com/lit/ds/symlink/ths4031.pdf
>
>Further measurements show the difference between the input and output signals 
>is a 10mV sinusoid at 10V output.
>
>We don't need no stinking automatic bias supply for class B operation.

Right. Apply engineering to make things simpler.

JT and Jim Williams were interesting guys, but they did some wildly
complex designs.

On a sunny day (Fri, 15 Dec 2023 14:16:46 -0000 (UTC)) it happened Mike Monett
VE3BTI <spamme@not.com> wrote in <XnsB0DB5E5D7A3C4idtokenpost@135.181.20.170>:

>Mike Monett VE3BTI <spamme@not.com> wrote:
>
>> The op amp needs to have 100MHz bandwidth and slew rate, be able to supply 
>> 50 ma output, and handle +/- 15V supplies. I tried searching Octopart, but 
>> had no luck.
>
>I finally figured how to search. The first hit showed exactly what is needed.
>
>The TI THS4031 has 100MHz bandwidth and slew rate, +/- 15V supplies, and can 
>drive 90 ma. It is also low noise at 1.6 nV/Hz, $4.39
>
>https://www.ti.com/lit/ds/symlink/ths4031.pdf
>
>Further measurements show the difference between the input and output signals 
>is a 10mV sinusoid at 10V output.
>
>We don't need no stinking automatic bias supply for class B operation.

Tj max is 150C
Highly non linear with 150 Ohm load and high voltage swing see fig 5

?

piglet <erichpwagner@hotmail.com> wrote:

> On 15/12/2023 2:16 pm, Mike Monett VE3BTI wrote:
>> Mike Monett VE3BTI <spamme@not.com> wrote:
>> 
>> We don't need no stinking automatic bias supply for class B operation.
>> 
>> 
> 
> Spoil sport! You have deprived me a nice intellectual challenge :>
> 
> piglet

LOL! Thanks. 



-- 
MRM

On 2023-12-15 09:16, Mike Monett VE3BTI wrote:
> Mike Monett VE3BTI <spamme@not.com> wrote:
> 
>> The op amp needs to have 100MHz bandwidth and slew rate, be able to supply
>> 50 ma output, and handle +/- 15V supplies. I tried searching Octopart, but
>> had no luck.
> 
> I finally figured how to search. The first hit showed exactly what is needed.
> 
> The TI THS4031 has 100MHz bandwidth and slew rate, +/- 15V supplies, and can
> drive 90 ma. It is also low noise at 1.6 nV/Hz, $4.39
> 
> https://www.ti.com/lit/ds/symlink/ths4031.pdf
> 
> Further measurements show the difference between the input and output signals
> is a 10mV sinusoid at 10V output.
> 
> We don't need no stinking automatic bias supply for class B operation.
> 
> 
> 
We like the THS4361, which is a 33-V FET-input amp with 210 MHz GBW, 
1000 V/us slewing, and a 100-mA current limit.

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

On 2023-12-15 15:49, Phil Hobbs wrote:
> On 2023-12-15 09:16, Mike Monett VE3BTI wrote:
>> Mike Monett VE3BTI <spamme@not.com> wrote:
>>
>>> The op amp needs to have 100MHz bandwidth and slew rate, be able to 
>>> supply
>>> 50 ma output, and handle +/- 15V supplies. I tried searching 
>>> Octopart, but
>>> had no luck.
>>
>> I finally figured how to search. The first hit showed exactly what is 
>> needed.
>>
>> The TI THS4031 has 100MHz bandwidth and slew rate, +/- 15V supplies, 
>> and can
>> drive 90 ma. It is also low noise at 1.6 nV/Hz, $4.39
>>
>> https://www.ti.com/lit/ds/symlink/ths4031.pdf
>>
>> Further measurements show the difference between the input and output 
>> signals
>> is a 10mV sinusoid at 10V output.
>>
>> We don't need no stinking automatic bias supply for class B operation.
>>
>>
>>
> We like the 
THS4631,
> which is a 33-V FET-input amp with 210 MHz GBW, 
> 1000 V/us slewing, and a 100-mA current limit.
> 
> 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

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

>> We don't need no stinking automatic bias supply for class B operation.
>> 
>> 
>> 
> We like the THS4361, which is a 33-V FET-input amp with 210 MHz GBW, 
> 1000 V/us slewing, and a 100-mA current limit.
> 
> Cheers
> 
> Phil Hobbs

Thanks. I stopped at the first hit, but your suggestion is better. (I think 
you swapped the middle two digits.) 

https://www.ti.com/lit/ds/symlink/ths4631.pdf

- $7.51 at Mouser



-- 
MRM

On 14/12/2023 10:40 pm, piglet wrote:
> Inspired by the late Jim Thompson's autobias design that we saw in the 
> thread "Power Amplifier for 100kHz" I thought his use of a bang-bang 
> comparator in setting bias was non-optimal but I liked his idea of 
> running the pa in pure class B for large signals and setting a minimum 
> quiescent class A current for small signals.
> 
> My idea uses JT's way of sensing output device current falling below 
> threshold but instead of pumping up a "rubber Vbe diode" I pull the bias 
> point apart with resistors and a pair of current mirrors. As an IC 
> designer JT was liberal with current mirrors so I hope he would approve.
> 
> I expect over the decades many clever ways have been devised at 
> eliminating crossover distortion and autobiasing class A/B so I have no 
> shame in adding another. Does this look interesting or has it been done 
> already?
> 
> Here is a pdf schematic:
> 
> <https://www.dropbox.com/scl/fi/x1tw0nj4d44pgmutxw36p/classAB_rubber_mirrors_autobias.pdf?rlkey=0xr3fv4l2qswyhrombqtanwvo&raw=1>
> 
> Here is the LT Spice wirelist:
> 
> Version 4
> SHEET 1 2520 916
> WIRE 720 -768 720 -816
> WIRE 528 -752 528 -816
> WIRE 1088 -752 1088 -800
> WIRE -512 -704 -512 -800
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> SYMATTR Value 22
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> SYMATTR InstName R12
> SYMATTR Value 22
> SYMBOL res 704 144 R0
> SYMATTR InstName R7
> SYMATTR Value 220
> SYMBOL res 704 288 R0
> SYMATTR InstName R8
> SYMATTR Value 220
> SYMBOL res 1072 -432 R0
> SYMATTR InstName R14
> SYMATTR Value 1m
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> SYMATTR InstName R15
> SYMATTR Value 1m
> SYMBOL cap 848 256 R0
> SYMATTR InstName C2
> SYMATTR Value 100p
> TEXT -656 848 Left 2 !.tran 1m
> TEXT 2128 -728 Left 2 ;EPW SED DEC 2023
> TEXT 2128 -672 Left 2 ;CLASS AB AMPLIFIER
> TEXT 2128 -616 Left 2 ;CURRENT MIRROR ELASTIC BIAS
> 
> 
> piglet

The addition of a second current sense diff amp completely transforms 
the idea. The first design went into class A only around zero crossings 
or at very low levels and was in class B on large signals. As Mike 
Monett pointed out it created new distortion. The below adaptation 
senses both output devices currents and selects the lowest of the two to 
create the bias feedback - this keeps the output slightly out of pure 
class B.

Seems better and works like JT's but without the irregular comparator 
action.

PDF Schematic here:

<https://www.dropbox.com/scl/fi/1gr8mwj1zuw2j2x2jdpx5/dualsense_classAB_rubber_mirrors_autobias.pdf?rlkey=l0u5hrzz0jd2w0rvqm8uub6m6&raw=1>

LT Spice wirelist here:

Version 4
SHEET 1 2520 916
WIRE 512 -768 512 -816
WIRE 320 -752 320 -816
WIRE 1184 -752 1184 -800
WIRE 1776 -704 1776 -752
WIRE 1776 -704 1568 -704
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SYMATTR InstName Q1
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SYMATTR InstName Q9
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SYMATTR InstName Q4
SYMATTR Value 2SCR533P
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SYMATTR InstName Q3
SYMATTR Value 2N2222
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SYMATTR InstName Q6
SYMATTR Value 2N2222
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SYMATTR InstName Q7
SYMATTR Value 2N2222
SYMBOL OpAmps\\UniversalOpAmp2 0 272 R0
SYMATTR InstName U1
SYMBOL res 1760 -288 R0
SYMATTR InstName R2
SYMATTR Value 0.33
SYMBOL res 1760 -64 R0
SYMATTR InstName R3
SYMATTR Value 0.33
SYMBOL res 1584 304 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 220
SYMBOL res 144 784 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL res -288 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 1k
SYMBOL cap -16 544 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 5p
SYMBOL npn 448 480 R0
SYMATTR InstName Q11
SYMATTR Value 2N3904
SYMBOL npn 384 480 M0
SYMATTR InstName Q12
SYMATTR Value 2N3904
SYMBOL pnp 1120 -544 M180
SYMATTR InstName Q5
SYMATTR Value 2N3906
SYMBOL pnp 384 -544 R180
SYMATTR InstName Q10
SYMATTR Value 2N3906
SYMBOL res 1168 -768 R0
SYMATTR InstName R5
SYMATTR Value 22
SYMBOL res 496 -784 R0
SYMATTR InstName R9
SYMATTR Value 22
SYMBOL res 304 -768 R0
SYMATTR InstName R10
SYMATTR Value 22
SYMBOL res 496 592 R0
SYMATTR InstName R11
SYMATTR Value 22
SYMBOL res 304 592 R0
SYMATTR InstName R12
SYMATTR Value 22
SYMBOL res 496 144 R0
SYMATTR InstName R13
SYMATTR Value 220
SYMBOL res 496 288 R0
SYMATTR InstName R14
SYMATTR Value 220
SYMBOL res 1168 -496 R0
SYMATTR InstName Pleast
SYMATTR Value 1m
SYMBOL current 1296 -256 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value 10m
SYMBOL voltage 624 -192 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V5
SYMATTR Value 0.02
SYMBOL npn 656 -320 R0
SYMATTR InstName Q13
SYMATTR Value 2N2222
SYMBOL npn 976 -320 M0
SYMATTR InstName Q14
SYMATTR Value 2N2222
SYMBOL res 720 -432 R0
SYMATTR InstName Nleast
SYMATTR Value 1m
SYMBOL current 816 -176 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I2
SYMATTR Value 10m
TEXT -544 848 Left 2 !.tran 1m
TEXT 1856 -712 Left 2 ;EPW SED DEC 2023
TEXT 1856 -656 Left 2 ;CLASS AB AMPLIFIER
TEXT 1856 -600 Left 2 ;CURRENT MIRROR ELASTIC BIAS
TEXT 1856 -544 Left 2 ;REGULATE TO LEAST OF EACH PHASE


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