Power Amplifier for 100kHz.

I'm not an analog design expert, but needs must.

I recently adapted a Doug Self audio amplifier design for use on a 60V 
single supply, and to go up to 100kHz with a working ambient temperature 
of -20'C to 180'C.

I can't show the circuit, but it was based on Fig 1a here...
http://www.douglas-self.com/ampins/dipa/dipa.htm

Cdom needs to come down for this higher frequency application, but too 
far and you have oscillation.  Also, problems occur with slew rate 
limiting due to Cdom, the TR5 constant current, and the increase in hFE 
of TR4 with temperature.

So I added an emitter degeneration resistor to TR4 to tame the hFE 
variation, removed Cdom and put a smaller C across Rf1 instead.  Seems 
stable and not slew rate limiting.  Took a lot longer to do than that 
sentence might imply.

However.

The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably 
get hotter and their Vbe would drop by more than the diodes, so it's 
class B.  As it runs at a high temperature, I obviously want to reduce 
dissipation, and if I'd used say a rubber diode to get some quiescent 
current, I think it would be very difficult to control Iq well enough 
over the temperature range.

So I have a circuit which works well enough, but could be better with 
regard to crossover distortion (though it's lower than I would have 
thought).  In this application, the better the signal quality, the 
higher the data rate.

Any ideas for improving crossover distortion, bearing in mind the 
temperature range?  The signal is OFDM, so pretty much a load of 
'random' steps, some of which may be small and at a crossover point.

-- 
Cheers
Clive

On a sunny day (Wed, 6 Dec 2023 15:26:00 +0000) it happened Clive Arthur
<clive@nowaytoday.co.uk> wrote in <ukq3qb$qjik$1@dont-email.me>:

>I'm not an analog design expert, but needs must.
>
>I recently adapted a Doug Self audio amplifier design for use on a 60V 
>single supply, and to go up to 100kHz with a working ambient temperature 
>of -20'C to 180'C.
>
>I can't show the circuit, but it was based on Fig 1a here...
>http://www.douglas-self.com/ampins/dipa/dipa.htm
>
>Cdom needs to come down for this higher frequency application, but too 
>far and you have oscillation.  Also, problems occur with slew rate 
>limiting due to Cdom, the TR5 constant current, and the increase in hFE 
>of TR4 with temperature.
>
>So I added an emitter degeneration resistor to TR4 to tame the hFE 
>variation, removed Cdom and put a smaller C across Rf1 instead.  Seems 
>stable and not slew rate limiting.  Took a lot longer to do than that 
>sentence might imply.
>
>However.
>
>The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably 
>get hotter and their Vbe would drop by more than the diodes, so it's 
>class B.  As it runs at a high temperature, I obviously want to reduce 
>dissipation, and if I'd used say a rubber diode to get some quiescent 
>current, I think it would be very difficult to control Iq well enough 
>over the temperature range.
>
>So I have a circuit which works well enough, but could be better with 
>regard to crossover distortion (though it's lower than I would have 
>thought).  In this application, the better the signal quality, the 
>higher the data rate.
>
>Any ideas for improving crossover distortion, bearing in mind the 
>temperature range?  The signal is OFDM, so pretty much a load of 
>'random' steps, some of which may be small and at a crossover point.

My experience with this sort of transistor amplifiers is that it depends a lot on what transistors and what manufacturer 
you use.
Hard to tell this way
In the 3055 days one make transistor oscillated, the other was OK.
The temperatiure range you mention is extreme, not much power left at 180C!
Huge heatsink?
How much power output do you need?

On Wed, 6 Dec 2023 15:26:00 +0000, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>I'm not an analog design expert, but needs must.
>
>I recently adapted a Doug Self audio amplifier design for use on a 60V 
>single supply, and to go up to 100kHz with a working ambient temperature 
>of -20'C to 180'C.
>
>I can't show the circuit, but it was based on Fig 1a here...
>http://www.douglas-self.com/ampins/dipa/dipa.htm

That's really ancient. And what is fig 1b all about?

What are your requirements? Current, distortion, protections? What's
your load?

It won't work if you call the transistors TR; they feel insulted and
oscillate. They want to be called Q.

>
>Cdom needs to come down for this higher frequency application, but too 
>far and you have oscillation.  Also, problems occur with slew rate 
>limiting due to Cdom, the TR5 constant current, and the increase in hFE 
>of TR4 with temperature.
>
>So I added an emitter degeneration resistor to TR4 to tame the hFE 
>variation, removed Cdom and put a smaller C across Rf1 instead.  Seems 
>stable and not slew rate limiting.  Took a lot longer to do than that 
>sentence might imply.
>
>However.
>
>The Vbias is a single diode, can't risk 2 diodes as TR6 & TR8 probably 
>get hotter and their Vbe would drop by more than the diodes, so it's 
>class B.  As it runs at a high temperature, I obviously want to reduce 
>dissipation, and if I'd used say a rubber diode to get some quiescent 
>current, I think it would be very difficult to control Iq well enough 
>over the temperature range.
>
>So I have a circuit which works well enough, but could be better with 
>regard to crossover distortion (though it's lower than I would have 
>thought).  In this application, the better the signal quality, the 
>higher the data rate.
>
>Any ideas for improving crossover distortion, bearing in mind the 
>temperature range?  The signal is OFDM, so pretty much a load of 
>'random' steps, some of which may be small and at a crossover point.

On 06/12/2023 16:19, John Larkin wrote:
> On Wed, 6 Dec 2023 15:26:00 +0000, Clive Arthur
> <clive@nowaytoday.co.uk> wrote:
> 
>> I'm not an analog design expert, but needs must.
>>
>> I recently adapted a Doug Self audio amplifier design for use on a 60V
>> single supply, and to go up to 100kHz with a working ambient temperature
>> of -20'C to 180'C.
>>
>> I can't show the circuit, but it was based on Fig 1a here...
>> http://www.douglas-self.com/ampins/dipa/dipa.htm
> 
> That's really ancient. And what is fig 1b all about?

It's old, but discrete and can take some pain.  Too many IC's have 
thermal limiting.  Fig 1b? dunno, I just Googled the circuit diagram as 
an example of the architecture.

> What are your requirements? Current, distortion, protections? What's
> your load?

It's a few watts, coupled capacitively to various very lossy lines, 
maybe 30R.  I just want to know if there are any bright ideas for 
improving it within the constraints.

> It won't work if you call the transistors TR; they feel insulted and
> oscillate. They want to be called Q.

No, PNP's are pink and NPN's are blue.  I'll have none of this LGBTQ 
nonsense.

<snip>

-- 
Cheers
Clive

On 06/12/2023 15:59, Jan Panteltje wrote:
> On a sunny day (Wed, 6 Dec 2023 15:26:00 +0000) it happened Clive Arthur
> <clive@nowaytoday.co.uk> wrote in <ukq3qb$qjik$1@dont-email.me>:
> 
>> I'm not an analog design expert, but needs must.
>>
<snip>

>> Any ideas for improving crossover distortion, bearing in mind the
>> temperature range?  The signal is OFDM, so pretty much a load of
>> 'random' steps, some of which may be small and at a crossover point.
> 
> My experience with this sort of transistor amplifiers is that it depends a lot on what transistors and what manufacturer
> you use.
> Hard to tell this way
> In the 3055 days one make transistor oscillated, the other was OK.
> The temperatiure range you mention is extreme, not much power left at 180C!
> Huge heatsink?
> How much power output do you need?

A few watts, the heatsink isn't huge, there's no room.  Tj on the output 
pair is probably around 210'C at 180'C ambient.  It doesn't come with a 
lifetime guarantee.

-- 
Cheers
Clive

On Wednesday, December 6, 2023 at 11:45:32 AM UTC-5, Clive Arthur wrote:
> On 06/12/2023 16:19, John Larkin wrote: 
> > On Wed, 6 Dec 2023 15:26:00 +0000, Clive Arthur 
> > <cl...@nowaytoday.co.uk> wrote: 
> > 
> >> I'm not an analog design expert, but needs must. 
> >> 
> >> I recently adapted a Doug Self audio amplifier design for use on a 60V 
> >> single supply, and to go up to 100kHz with a working ambient temperature 
> >> of -20'C to 180'C. 
> >> 
> >> I can't show the circuit, but it was based on Fig 1a here... 
> >> http://www.douglas-self.com/ampins/dipa/dipa.htm 
> > 
> > That's really ancient. And what is fig 1b all about?
> It's old, but discrete and can take some pain. Too many IC's have 
> thermal limiting. Fig 1b? dunno, I just Googled the circuit diagram as 
> an example of the architecture.

Fig 1b is Class A emitter follower, Fig 1a is a Class B, also emitter follower pair.

The TR8/9 and TR6/7 are well known composite emitter follower configurations. They should be wideband and not significantly affect loop phase at 100KHz. Dunno how you get crossover distortion with 1A, unless your 'VBIAS' is be too slight.

What exactly does '100KHz" refer to? Is this a bandwidth or CW frequency?

Do you have that right about 180oC? Seems extreme. 


> > What are your requirements? Current, distortion, protections? What's 
> > your load?
> It's a few watts, coupled capacitively to various very lossy lines, 
> maybe 30R. I just want to know if there are any bright ideas for 
> improving it within the constraints.
> > It won't work if you call the transistors TR; they feel insulted and 
> > oscillate. They want to be called Q.
> No, PNP's are pink and NPN's are blue. I'll have none of this LGBTQ 
> nonsense. 
> 
> <snip> 
> 
> -- 
> Cheers 
> Clive

On Wednesday, December 6, 2023 at 11:49:17 AM UTC-5, Clive Arthur wrote:
> On 06/12/2023 15:59, Jan Panteltje wrote: 
> > On a sunny day (Wed, 6 Dec 2023 15:26:00 +0000) it happened Clive Arthur 
> > <cl...@nowaytoday.co.uk> wrote in <ukq3qb$qjik$1...@dont-email.me>: 
> > 
> >> I'm not an analog design expert, but needs must. 
> >>
> <snip>
> >> Any ideas for improving crossover distortion, bearing in mind the 
> >> temperature range? The signal is OFDM, so pretty much a load of 
> >> 'random' steps, some of which may be small and at a crossover point. 
> > 
> > My experience with this sort of transistor amplifiers is that it depends a lot on what transistors and what manufacturer 
> > you use. 
> > Hard to tell this way 
> > In the 3055 days one make transistor oscillated, the other was OK. 
> > The temperatiure range you mention is extreme, not much power left at 180C! 
> > Huge heatsink? 
> > How much power output do you need?
> A few watts, the heatsink isn't huge, there's no room. Tj on the output 
> pair is probably around 210'C at 180'C ambient. It doesn't come with a 
> lifetime guarantee. 

There are metal packages rated for Tj,max of 200oC continuous.

> 
> -- 
> Cheers 
> Clive

On Wednesday 6 December 2023 at 17:08:28 UTC, Fred Bloggs wrote:
> On Wednesday, December 6, 2023 at 11:49:17 AM UTC-5, Clive Arthur wrote: 
> > On 06/12/2023 15:59, Jan Panteltje wrote: 
> > > On a sunny day (Wed, 6 Dec 2023 15:26:00 +0000) it happened Clive Arthur 
> > > <cl...@nowaytoday.co.uk> wrote in <ukq3qb$qjik$1...@dont-email.me>: 
> > > 
> > >> I'm not an analog design expert, but needs must. 
> > >> 
> > <snip> 
> > >> Any ideas for improving crossover distortion, bearing in mind the 
> > >> temperature range? The signal is OFDM, so pretty much a load of 
> > >> 'random' steps, some of which may be small and at a crossover point. 
> > > 
> > > My experience with this sort of transistor amplifiers is that it depends a lot on what transistors and what manufacturer 
> > > you use. 
> > > Hard to tell this way 
> > > In the 3055 days one make transistor oscillated, the other was OK. 
> > > The temperatiure range you mention is extreme, not much power left at 180C! 
> > > Huge heatsink? 
> > > How much power output do you need? 
> > A few watts, the heatsink isn't huge, there's no room. Tj on the output 
> > pair is probably around 210'C at 180'C ambient. It doesn't come with a 
> > lifetime guarantee.
> There are metal packages rated for Tj,max of 200oC continuous. 

I guess that this will be driving a few km of cable from a very deep hole in the ground.

John

> > 
> > -- 
> > Cheers 
> > Clive

On Wed, 6 Dec 2023 16:42:32 +0000, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 06/12/2023 16:19, John Larkin wrote:
>> On Wed, 6 Dec 2023 15:26:00 +0000, Clive Arthur
>> <clive@nowaytoday.co.uk> wrote:
>> 
>>> I'm not an analog design expert, but needs must.
>>>
>>> I recently adapted a Doug Self audio amplifier design for use on a 60V
>>> single supply, and to go up to 100kHz with a working ambient temperature
>>> of -20'C to 180'C.
>>>
>>> I can't show the circuit, but it was based on Fig 1a here...
>>> http://www.douglas-self.com/ampins/dipa/dipa.htm
>> 
>> That's really ancient. And what is fig 1b all about?
>
>It's old, but discrete and can take some pain.  Too many IC's have 
>thermal limiting.  Fig 1b? dunno, I just Googled the circuit diagram as 
>an example of the architecture.
>
>> What are your requirements? Current, distortion, protections? What's
>> your load?
>
>It's a few watts, coupled capacitively to various very lossy lines, 
>maybe 30R.  I just want to know if there are any bright ideas for 
>improving it within the constraints.

Do you need a sine wave? Can you switch and not linear amplify? How
about switching followed by a passive LC filter?

GaN fets should be good for higher temp than silicon, although EPC
rates their parts for 150c. We're using d2pak mosfets rated for 175.

>
>> It won't work if you call the transistors TR; they feel insulted and
>> oscillate. They want to be called Q.
>
>No, PNP's are pink and NPN's are blue.  I'll have none of this LGBTQ 
>nonsense.

Don't offend the trans community.


>
><snip>

On Wed, 6 Dec 2023 09:04:53 -0800 (PST), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

>On Wednesday, December 6, 2023 at 11:45:32?AM UTC-5, Clive Arthur wrote:
>> On 06/12/2023 16:19, John Larkin wrote: 
>> > On Wed, 6 Dec 2023 15:26:00 +0000, Clive Arthur 
>> > <cl...@nowaytoday.co.uk> wrote: 
>> > 
>> >> I'm not an analog design expert, but needs must. 
>> >> 
>> >> I recently adapted a Doug Self audio amplifier design for use on a 60V 
>> >> single supply, and to go up to 100kHz with a working ambient temperature 
>> >> of -20'C to 180'C. 
>> >> 
>> >> I can't show the circuit, but it was based on Fig 1a here... 
>> >> http://www.douglas-self.com/ampins/dipa/dipa.htm 
>> > 
>> > That's really ancient. And what is fig 1b all about?
>> It's old, but discrete and can take some pain. Too many IC's have 
>> thermal limiting. Fig 1b? dunno, I just Googled the circuit diagram as 
>> an example of the architecture.
>
>Fig 1b is Class A emitter follower, 

Where is the follower? The pullup pair, TR5 and its pal, can source
about 12 mA max. TR4 and its friend are certainly not followers.

There was one Motorola power opamp that drew the output stage all
wrong. National or someone 2nd sourced it and drew theirs the same
wrong way.


>Fig 1a is a Class B, also emitter follower pair.

without current limiting.
>
>The TR8/9 and TR6/7 are well known composite emitter follower configurations. They should be wideband and not significantly affect loop phase at 100KHz. Dunno how you get crossover distortion with 1A, unless your 'VBIAS' is be too slight.
>
>What exactly does '100KHz" refer to? Is this a bandwidth or CW frequency?
>
>Do you have that right about 180oC? Seems extreme. 
>
>
>> > What are your requirements? Current, distortion, protections? What's 
>> > your load?
>> It's a few watts, coupled capacitively to various very lossy lines, 
>> maybe 30R. I just want to know if there are any bright ideas for 
>> improving it within the constraints.
>> > It won't work if you call the transistors TR; they feel insulted and 
>> > oscillate. They want to be called Q.
>> No, PNP's are pink and NPN's are blue. I'll have none of this LGBTQ 
>> nonsense. 
>> 
>> <snip> 
>> 
>> -- 
>> Cheers 
>> Clive