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>
On Thu, 14 Dec 2023 22:40:23 +0000, piglet <erichpwagner@hotmail.com>
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>
Most of the audio amp circuits posted here seem to have been designed
before the invention of the opamp.
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?
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.
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.
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.
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.
Mike Monett VE3BTI <spamme@not.com> wrote:
We don't need no stinking automatic bias supply for class B operation.
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.
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.
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:Probably me. I invented that way back in my youth.
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.
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
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
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
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.
On 2023-12-15 09:16, Mike Monett VE3BTI wrote:THS4631,
Mike Monett VE3BTI <spamme@not.com> wrote:We like the
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.
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
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
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
WIRE 528 -640 528 -672
WIRE 720 -640 720 -688
WIRE 1088 -640 1088 -672
WIRE 640 -592 592 -592
WIRE 896 -592 784 -592
WIRE 1024 -592 896 -592
WIRE 1888 -576 1888 -624
WIRE 1888 -576 1680 -576
WIRE -512 -560 -512 -624
WIRE -464 -560 -512 -560
WIRE 1680 -528 1680 -576
WIRE -464 -512 -464 -560
WIRE 1616 -480 1536 -480
WIRE 640 -464 640 -592
WIRE 896 -464 896 -592
WIRE 896 -464 640 -464
WIRE 1088 -464 1088 -544
WIRE 1088 -464 896 -464
WIRE -512 -432 -512 -560
WIRE 1888 -432 1888 -576
WIRE 1088 -416 1088 -464
WIRE 1264 -416 1264 -544
WIRE 1680 -384 1680 -432
WIRE 1824 -384 1680 -384
WIRE 1088 -320 1088 -336
WIRE 1264 -320 1264 -336
WIRE -512 -272 -512 -352
WIRE 1024 -272 928 -272
WIRE 1888 -272 1888 -336
WIRE 1888 -272 1328 -272
WIRE 1888 -240 1888 -272
WIRE 1168 -224 1088 -224
WIRE 1264 -224 1168 -224
WIRE 928 -192 928 -272
WIRE 1168 -176 1168 -224
WIRE 1680 -112 1680 -384
WIRE 1888 -112 1888 -160
WIRE 2080 -112 1888 -112
WIRE 2208 -112 2080 -112
WIRE 1888 -64 1888 -112
WIRE 1168 -32 1168 -96
WIRE 928 64 928 -112
WIRE 1888 64 1888 16
WIRE 1888 64 928 64
WIRE 720 128 720 -544
WIRE 864 128 720 128
WIRE 1536 128 1536 -480
WIRE 1536 128 864 128
WIRE 720 160 720 128
WIRE -112 240 -112 176
WIRE -496 256 -624 256
WIRE -304 256 -416 256
WIRE -144 256 -304 256
WIRE 864 256 864 128
WIRE 224 272 -80 272
WIRE 720 272 720 240
WIRE 720 272 224 272
WIRE 2208 272 2208 -112
WIRE -144 288 -224 288
WIRE 720 304 720 272
WIRE 1888 304 1888 64
WIRE 1680 352 1680 -32
WIRE 1824 352 1680 352
WIRE -224 368 -224 288
WIRE -112 384 -112 304
WIRE 1680 384 1680 352
WIRE -624 432 -624 256
WIRE 528 432 528 -544
WIRE 624 432 528 432
WIRE 720 432 720 384
WIRE 864 432 864 320
WIRE 864 432 720 432
WIRE 1616 432 864 432
WIRE 2208 464 2208 352
WIRE 528 480 528 432
WIRE 720 480 720 432
WIRE 1680 512 1680 480
WIRE 1888 512 1888 400
WIRE 1888 512 1680 512
WIRE 624 528 624 432
WIRE 624 528 592 528
WIRE 656 528 624 528
WIRE -304 560 -304 256
WIRE -48 560 -304 560
WIRE 224 560 224 272
WIRE 224 560 16 560
WIRE 528 608 528 576
WIRE 720 608 720 576
WIRE -624 656 -624 512
WIRE 1888 704 1888 512
WIRE 528 736 528 688
WIRE 720 736 720 688
WIRE -304 800 -304 560
WIRE -64 800 -304 800
WIRE 2080 800 2080 -112
WIRE 2080 800 16 800
FLAG -512 -800 P15
FLAG -112 176 P15
FLAG 1888 -624 P15
FLAG 1088 -800 P15
FLAG -512 -272 N15
FLAG -112 384 N15
FLAG 1888 704 N15
FLAG 1168 -32 N15
FLAG 2208 464 0
FLAG -624 656 0
FLAG -224 368 0
FLAG -464 -512 0
FLAG 720 736 N15
FLAG 528 736 N15
FLAG 720 -816 P15
FLAG 528 -816 P15
FLAG 1264 -544 P15
SYMBOL voltage -512 -720 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 15
SYMBOL voltage -512 -448 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 15
SYMBOL voltage -624 416 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value SINE(0 1 100k)
SYMBOL voltage 928 -208 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V4
SYMATTR Value 0.05
SYMBOL res 2192 256 R0
SYMATTR InstName R1
SYMATTR Value 8
SYMBOL pnp 1824 400 M180
SYMATTR InstName Q1
SYMATTR Value 2SAR533P
SYMBOL pnp 1616 480 M180
SYMATTR InstName Q2
SYMATTR Value 2N2907
SYMBOL pnp 784 -544 R180
SYMATTR InstName Q10
SYMATTR Value 2N3906
SYMBOL npn 1824 -432 R0
SYMATTR InstName Q3
SYMATTR Value 2SCR533P
SYMBOL npn 1616 -528 R0
SYMATTR InstName Q4
SYMATTR Value 2N2222
SYMBOL npn 1024 -320 R0
SYMATTR InstName Q5
SYMATTR Value 2N2222
SYMBOL npn 1328 -320 M0
SYMATTR InstName Q6
SYMATTR Value 2N2222
SYMBOL OpAmps\\UniversalOpAmp2 -112 272 R0
SYMATTR InstName U1
SYMBOL res 1872 -256 R0
SYMATTR InstName R2
SYMATTR Value 0.22
SYMBOL res 1872 -80 R0
SYMATTR InstName R3
SYMATTR Value 0.22
SYMBOL res 1696 -16 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 220
SYMBOL current 1168 -176 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value 10m
SYMBOL res 32 784 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL res -400 240 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 1k
SYMBOL cap -48 544 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 5p
SYMBOL npn 656 480 R0
SYMATTR InstName Q13
SYMATTR Value 2N3904
SYMBOL npn 592 480 M0
SYMATTR InstName Q12
SYMATTR Value 2N3904
SYMBOL pnp 1024 -544 M180
SYMATTR InstName Q9
SYMATTR Value 2N3906
SYMBOL pnp 592 -544 R180
SYMATTR InstName Q11
SYMATTR Value 2N3906
SYMBOL res 1072 -768 R0
SYMATTR InstName R9
SYMATTR Value 22
SYMBOL res 704 -784 R0
SYMATTR InstName R10
SYMATTR Value 22
SYMBOL res 512 -768 R0
SYMATTR InstName R11
SYMATTR Value 22
SYMBOL res 704 592 R0
SYMATTR InstName R13
SYMATTR Value 22
SYMBOL res 512 592 R0
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
SYMBOL res 1248 -432 R0
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
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>
<snip>
On 2023-12-14 17:40, 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>
<snip>
I spent a bit more time on JT's design, just for interest's sake. I replaced the op amp with a UniversalOpAmp2 that mimics a THS4631 (210
MHz GBW, 1000 V/us), and used the RH111 (an old rad-hard AD version of
the LM311).
<https://electrooptical.net/www/sed/JimThompsonAudioAmpWithWeirdBiasing.zip>
The bias doesn't become Class B for large signals--it's solid as a rock, weirdly. You just get a 300-ns-ish bias current pulse near the output
zero crossings. With a nearly full-scale output at 20 kHz, its
distortion is only 0.04%, not bad for having no local feedback.
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its
NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
On Mon, 18 Dec 2023 16:00:09 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2023-12-14 17:40, 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>
<snip>
I spent a bit more time on JT's design, just for interest's sake. I
replaced the op amp with a UniversalOpAmp2 that mimics a THS4631 (210
MHz GBW, 1000 V/us), and used the RH111 (an old rad-hard AD version of
the LM311).
<https://electrooptical.net/www/sed/JimThompsonAudioAmpWithWeirdBiasing.zip> >>
The bias doesn't become Class B for large signals--it's solid as a rock,
weirdly. You just get a 300-ns-ish bias current pulse near the output
zero crossings. With a nearly full-scale output at 20 kHz, its
distortion is only 0.04%, not bad for having no local feedback.
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its
NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
Why not try JT's updated design posted by myself and legg?
On 2023-12-14 17:40, 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>
<snip>
I spent a bit more time on JT's design, just for interest's sake. I
replaced the op amp with a UniversalOpAmp2 that mimics a THS4631 (210
MHz GBW, 1000 V/us), and used the RH111 (an old rad-hard AD version of
the LM311).
<https://electrooptical.net/www/sed/JimThompsonAudioAmpWithWeirdBiasing.zip>
The bias doesn't become Class B for large signals--it's solid as a rock, >weirdly. You just get a 300-ns-ish bias current pulse near the output
zero crossings. With a nearly full-scale output at 20 kHz, its
distortion is only 0.04%, not bad for having no local feedback.
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its
NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its
NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
On Mon, 18 Dec 2023 16:00:09 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2023-12-14 17:40, 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>
<snip>
I spent a bit more time on JT's design, just for interest's sake. I
replaced the op amp with a UniversalOpAmp2 that mimics a THS4631 (210
MHz GBW, 1000 V/us), and used the RH111 (an old rad-hard AD version of
the LM311).
<https://electrooptical.net/www/sed/JimThompsonAudioAmpWithWeirdBiasing.zip> >>
The bias doesn't become Class B for large signals--it's solid as a rock,
weirdly. You just get a 300-ns-ish bias current pulse near the output
zero crossings. With a nearly full-scale output at 20 kHz, its
distortion is only 0.04%, not bad for having no local feedback.
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its
NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
Phil,
What's the point of sticking an ideal fixed offset voltage
in a circuit that is attenpting to demonstrate practical
circuitry? It's what mirrors are for . . .
With all later revs of LTspiceXVII, you have to be pretty
careful when pulling parts from cmp\standard.xxx. This is
particularly true if demonstrating temperature effects.
I'm not sure what an ako:2n2904 (or even a stray 2n2906)
looks like on your machine.
The second sim in your zip produces an error in a not-recently-
updated LTSpiceXVII installation ;
Missing closing '}' in "{cb)"
. . . and won't start.
I see no obvious editable brackets to correct this.
On Mon, 18 Dec 2023 16:00:09 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
<snip>
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its
NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
You can get some idea of the flavors of different bipolar models
there are floating around in the LTSpice arena, often with the
same ident, from a spreadsheet thrown together last year:
http://ve3ute.ca/query/bjt_spice_parameter_221218.zip
RL
legg <legg@nospam.magma.ca> wrote:
On Mon, 18 Dec 2023 16:00:09 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2023-12-14 17:40, 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>
<snip>
I spent a bit more time on JT's design, just for interest's sake. I
replaced the op amp with a UniversalOpAmp2 that mimics a THS4631 (210
MHz GBW, 1000 V/us), and used the RH111 (an old rad-hard AD version of
the LM311).
<https://electrooptical.net/www/sed/JimThompsonAudioAmpWithWeirdBiasing.zip>
The bias doesn't become Class B for large signals--it's solid as a rock, >>> weirdly. You just get a 300-ns-ish bias current pulse near the output
zero crossings. With a nearly full-scale output at 20 kHz, its
distortion is only 0.04%, not bad for having no local feedback.
The LTspice 17.1 library has the D45H11 low-sat PNP, but weirdly not its >>> NPN complement, the D44H11. I put both libraries in the zipfile.
It works exactly the same with BCV61C duals for the mirrors, which
would save a couple of packages.
Cheers
Phil Hobbs
Phil,
What's the point of sticking an ideal fixed offset voltage
in a circuit that is attenpting to demonstrate practical
circuitry? It's what mirrors are for . . .
With all later revs of LTspiceXVII, you have to be pretty
careful when pulling parts from cmp\standard.xxx. This is
particularly true if demonstrating temperature effects.
I'm not sure what an ako:2n2904 (or even a stray 2n2906)
looks like on your machine.
The second sim in your zip produces an error in a not-recently-
updated LTSpiceXVII installation ;
Missing closing '}' in "{cb)"
. . . and won't start.
I see no obvious editable brackets to correct this.
Sorry about that. Just replace the closing paren with a right brace. I obviously didn’t save the file between fixing that and zipping it.
Re The bias voltage source: it makes zero difference to the performance of the circuit. It’s the comparator loop that interested me, and I thought it might interest others too.
I really like things that work very well using cheap components in non-obvious ways. I wouldn’t have expected 0.0004% THD for an 8V p-p output into 4 ohms.
Cheers
Phil Hobbs
I think the novel part of JT's design is that it controls what you
actually care about, namely the minimum quiescent bias under transient conditions. Linear methods only get you the average, which doesn't guarantee what happens with a large output signal. Mike's method has a large transient error at the zero crossing, which gets worse at smaller inputs.
On Thu, 14 Dec 2023 22:40:23 +0000, piglet <erichpwagner@hotmail.com>
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>
Most of the audio amp circuits posted here seem to have been designed
before the invention of the opamp.
On Thursday, December 14, 2023 at 6:05:59 PM UTC-5, John Larkin wrote:
On Thu, 14 Dec 2023 22:40:23 +0000, piglet <erichp...@hotmail.com>
wrote:
Inspired by the late Jim Thompson's autobias design that we saw in theMost of the audio amp circuits posted here seem to have been designed
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>
before the invention of the opamp.
They're a step up from tubes...
On 19/12/2023 5:20 pm, Phil Hobbs wrote:
I think the novel part of JT's design is that it controls what you
actually care about, namely the minimum quiescent bias under transient
conditions. Linear methods only get you the average, which doesn't
guarantee what happens with a large output signal. Mike's method has a
large transient error at the zero crossing, which gets worse at smaller
inputs.
No sure about that: JT's design has two big capacitors (100uF and 22uF I recall) in the bias control so it will always be sluggish on transients.
My second version has no capacitors in the bias control so should be
very adaptive?
piglet
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