On Friday, December 15, 2023 at 3:54:16 PM UTC-5, Phil Hobbs wrote:
> On 2023-12-15 12:31, Fred Bloggs wrote:
> > On Friday, December 15, 2023 at 1:45:17 AM UTC-5, Phil Hobbs wrote:
> >> On 2023-12-14 20:41, Fred Bloggs wrote:
> >>> On Monday, December 11, 2023 at 6:06:08 PM UTC-5, Phil Hobbs
> >>> wrote:
> >>>> On 2023-12-11 12:09, Fred Bloggs wrote:
> >>>>> On Monday, December 11, 2023 at 10:50:04 AM UTC-5, legg
> >>>>> wrote:
> >>>>>> On Mon, 11 Dec 2023 15:01:25 +0000, Clive Arthur
> >>>>>> <cl...@nowaytoday.co.uk> wrote:
> >>>>>>
> >>>>>>> On 11/12/2023 14:05, legg wrote:
> >>>>>>>> On Wed, 06 Dec 2023 23:22:26 GMT, Glen Walpert
> >>>>>>>> <nos...@null.void> wrote:
> >>>>>>>
> >>>>>>> <snip>
> >>>>>>>
> >>>>>>>>> Some years ago Jim Thompson posted an audio
> >>>>>>>>> amplifier design which used current mirrors to
> >>>>>>>>> provide bias to the output transistors for the
> >>>>>>>>> express purpose of keeping crossover distortion low
> >>>>>>>>> over a large temperature range. He claimed it was the
> >>>>>>>>> bees knees, but a quick search failed to turn it up.
> >>>>>>>>> Perhaps someone else saved it or remembers the
> >>>>>>>>> thread?
> >>>>>>>>>
> >>>>>>>>> Glen
> >>>>>>>>
> >>>>>>>> From old LTSpice trash here;
> >>>>>>>>
> >>>>>>>> http://ve3ute.ca/query/Half_Bridge_for_77_280Z_thompson.pdf
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>
> >>>>>>>>
> RL
> >>>>>>>
> >>>>>>> Thanks!
> >>>>>>>
> >>>>>>> Looks like the top output Darlington is AC coupled and
> >>>>>>> when the comparator detects a quiescent current through
> >>>>>>> the output resistors transitioning to less than some
> >>>>>>> value, it pumps the upper Darlington base voltage up a
> >>>>>>> bit, otherwise, the upper Darlington base voltage drifts
> >>>>>>> down.
> >>>>>>>
> >>>>>>> Is that about right?
> >>>>>>>
> >>>>>>> Not sure it would work in my application as my signal
> >>>>>>> isn't continuous - it spends some proportion of the time
> >>>>>>> idling at half supply. Still, I could probably arrange a
> >>>>>>> clock to force a comparator sample somehow.
> >>>>>>>
> >>>>>>> Or maybe make the adjustment non-volatile (digipot?) and
> >>>>>>> clock it both up and down. The signal comes from a DAC,
> >>>>>>> so I do have access to timing signals.
> >>>>>> A lot of the bumph is dedicated only to biasing and it
> >>>>>> would take some doing to get it to work over temperature
> >>>>>> given those polarized cap sizes. Integrated darlingtons are
> >>>>>> also best avoided. By 'wide range', the author was talking
> >>>>>> standard industrial temperatures.
> >>>>>>
> >>>>>> You'd also have to do some thin'in around the gain-setting
> >>>>>> regime. Doubt this was a consideration in this drawing ( .
> >>>>>> . . 'or' . . .), nor was 100KHz ( hence zobel network ).
> >>>>>>
> >>>>>> I don't see quiescent conditions being an issue, but
> >>>>>> start-up and shutdown could be surprising. Not sure that
> >>>>>> was Thompson's strong point.
> >>>>>>
> >>>>>> RL
> >>>>>
> >>>>> That 100u base-to-base bias cap is probably needed to
> >>>>> dominant poll stabilize the LM311 amp more than anything
> >>>>> else.
> >>>>>
> >>>> I'm not sure that we're looking at the same schematic. In the
> >>>> one I have, "Half_Bridge_for_77_280Z_thompson.pdf", the LM311
> >>>> is a weird sort of switching bias supervisor, not a linear amp.
> >>>> It's running as a normal open-collector comparator, with its
> >>>> output wire-ORed with the shutdown transistor Q7.
> >>>
> >>> That's crazy. For one thing, audio speakers do not respond well
> >>> to discontinuities in the drive voltage. They do things like
> >>> snap, crackle and pop.
> >>>
> >>>>
> >>>> When it fires, or the shutdown line is high, it steals Q3's
> >>>> base bias. That causes Darlington Q5/Q6 to turn on more, with a
> >>>> TC of about 2 seconds. That reduces the quiescent bias.
> >>>>
> >>>> In small-signal conditions, that'll just oscillate irregularly
> >>>> and keep the class-A bias current of very roughly 60 mA. (*)
> >>>> In large-signal conditions, the comparator will be pulling low
> >>>> most of the time, which reduces the quiescent bias
> >>>> progressively. (If the gain of the bias loop is high enough, it
> >>>> may not drift that far, but I'd probably need to use SPICE to
> >>>> find that out.)
> >>>>
> >>>> When the shutdown pin is active, the class-A bias will
> >>>> gradually go to 0, turning the output totem pole into a really
> >>>> bad class B. (One gathers that the shutdown turns off the audio
> >>>> input as well.)
> >>>>
> >>>> One aspect that I don't understand well is Q3. With a 3k/100
> >>>> ohm voltage divider in the Q4 leg, ISTM that the base voltage
> >>>> of Q3 will be nearly the same as that of Q2, which is driven
> >>>> from a 620 ohm / 22 ohm divider (plus various V_BEs). That
> >>>> makes the current through the 220 ohm hard to estimate by
> >>>> eyeball. (The average current is obviously going to be small,
> >>>> on account of that 100k resistor.)
> >>>
> >>> I'll have to introduce some notation to explain what's going on.
> >>>
> >>> Isn is the speaker current supplied by the NPN Darlington.
> >>>
> >>> Isp is the speaker current drawn in reverse direction by the PNP
> >>> Darlington.
> >>>
> >>> Ven is NPN Darlington emitter voltage
> >>>
> >>> Vep is PNP Darlington emitter voltage
> >>>
> >>> IB is the common bias current supplied by the NPN emitter into
> >>> the PNP emitter sink.
> >>>
> >>> R is the value of the current sense resistors, 0.33R 2W in the
> >>> schematic, no designators given.
> >>>
> >>> Objective is to maintain IB constant, within reason, so as to
> >>> eliminate crossover distortion.
> >>>
> >>> Total voltage across the 2 R's at any instant is the
> >>> differential Ven- Vep = R x ( Isn + IB + IB + Isp)
> >>>
> >>> This rearranges to Ven- Vep = 2 x R x IB ( DC term) + R x ( Isn
> >>> + Isp) ( signal AC term)
> >>>
> >>> Note R x Isn-p is Ven-p relative to Voutput AC.
> >>>
> >>> Obviously a first requirement is to make d/dt ( Ven - Vep ) = 0,
> >>> making the DC term, and hence IB, a constant.
> >>>
> >>> One simple way, and simplest is best, is to make Vep follow Ven
> >>> during the positive half output cycles, and Ven follow Vep during
> >>> the negative half cycles.
> >>>
> >>> And, viola- there you have your perfect bias with zero
> >>> crossover.
> >> Nice arm waving there, 'Fred'. How exactly do you propose to do
> >> that?
> >
> > The circuit idea can be modernized, making for a big improvement over
> > that nearly 50 year old instantiation.
> Sure it can, but it would take more work than you suggest.
> >>
> >> Also it's the output current that makes the speaker move.
> >>> IB is set by that current sink drawing a stiff current through
> >>> the 27R in series with the LM311 (-) input.
> >>>
> >>> Now you should be able to revisit your analysis of all those
> >>> current mirrors and their cascades to see how those followers
> >>> are implemented. Be sure to watch for the linearity of the LM311
> >>> with its spec'd 200 V/mv gain.
> >> Gee, thanks. ;)
> >>>
> >>>>
> >>>> (I sort of gather that it's pretty tweaky, due to that
> >>>> scribbled-in 10k to ground from the bases of Q3 and Q4.)
> >>>
> >>> Did he say he built this?
> >>>
> >>> I'm pretty sure the performance claimed was all SPICE derived,
> >>> and not actually measured.
>
> >> Not in 1977, it wasn't. The first public release of the original
> >> Berkeley SPICE2 program was in 1972, and it was pretty
> >> primitive--FORTRAN, punch cards, running on a CDC 6400. SPICE
> >> didn't actually become useful until the early '80s.
> >
> > Yeah, it was a piece of junk, and a mess, written by undergrad
> > students at UCB.
> Laurence Nagel's thesis on SPICE2 was a nice piece of work actually.
> > The big semi houses most certainly had their proprietary IC sims
> > running well before SPICE.
> >
> > I'm sure Motorola had ample resources in this vein available at the
> > time.
> >
> I seriously doubt that. 1977 was a _long_ time ago--even the semis were
> still prototyping with kit parts and doing mask design with Rubylith.
> 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
https://www.computerhistory.org/siliconengine/computer-aided-design-tools-developed-for-ics/
They don't mention RCA, but they were very big at the time, and almost certainly used automated circuit simulation.