Power Amplifier for 100kHz.

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
>
>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.


Is it ONLY 100 kHz you want to amplify ?

If so, you might incorporate a filter on the output.

There are other ways to make 100 kHz too if you think about it for a
while.

boB

In article <d785b331-3ea1-58b4-75ae-02ba04c75680@electrooptical.net>,
Phil Hobbs  <pcdhSpamMeSenseless@electrooptical.net> wrote:

>Some local feedback around the output stage would get my vote.  The 
>Sziklai pairs have their own local feedback, but that doesn't fix the 
>crossover problem.

Doug Self has a version of this in his audio-amplifier book (not sure if
he mentions it in his online materials).  He refers to it as "output-
inclusive compensation".

Briefly, you split the Cdom capacitor into a series pair (each being
twice the nominal Cdom value), and connect the junction of the two to
the output node via a resistor.  Over an intermediate range of
frequencies, the Vas feedback is partly local (collector-to-base
via the two halves of Cdom) and partially from the output stage.
Self claims that this can knock down residual crossover distortion
to negligible (even unmeasurable) levels.

I used this technique in an audio amplifier I finished building this
year.  It seems to work, and I've been unable to detect any adverse
effect on stability (either in SPICE simulation, or in measurement of
the finished amp).

On Thursday, December 7, 2023 at 2:27:07 AM UTC+11, Clive Arthur 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. 

Few power transistors are specified  for operation at 180C and the audio amplifier market doesn't cater to people who want that kind of operating temperature.

You might look at using power MOSFETs, and you might think about going for a class D (switching) amplifier. Some of the new SiC.MOSFET power switching transistors can be switched at a quite few MHz.

-- 
Bill Sloman, Sydney

On a sunny day (Wed, 6 Dec 2023 16:48:28 +0000) it happened Clive Arthur
<clive@nowaytoday.co.uk> wrote in <ukq8ku$rbhh$2@dont-email.me>:

>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.

Would it not be simpler to use the signal to FM modulate say a 100 MHz carrier
and detect that at the other end?
Low power required.
100 kHz bandwidth at 100 MHz (or higher) should be no problem.
Sensitive receiver is standard stuff, use RTL_SDR stick if needed, demodulation in software
(or any other modulation method, but FM is simple, some LC and varicaps..).
Transmitter can be a one transistor oscillator.
Can your ?underground? cable carry 100 MHz?

On 07/12/2023 08:57, Jan Panteltje wrote:

<snip>
> 
> Would it not be simpler to use the signal to FM modulate say a 100 MHz carrier
> and detect that at the other end?

The 'cable' is horrible.  There are many different types, absolutely and 
completely non-negotiable, but for the longest, getting 100kHz through 
is often not achievable.  The signal looks like white noise.  Many 
sinusoids from 5kHz to 100kHz.

-- 
Cheers
Clive

On Thu, 7 Dec 2023 10:30:58 +0000, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 07/12/2023 08:57, Jan Panteltje wrote:
>
><snip>
>> 
>> Would it not be simpler to use the signal to FM modulate say a 100 MHz carrier
>> and detect that at the other end?
>
>The 'cable' is horrible.  There are many different types, absolutely and 
>completely non-negotiable, but for the longest, getting 100kHz through 
>is often not achievable.  The signal looks like white noise.  Many 
>sinusoids from 5kHz to 100kHz.

It sounds to me like you should not be using an ancient audio
amplifier. There is lots of digital data equilization technology
around. Some of the fast signals used in PCs and telecom systems look
like noise after they pass through cables and traces but can be
equalized back to beautiful binary data. 

On Thursday, December 7, 2023 at 11:28:54 PM UTC+11, John Larkin wrote:
> On Thu, 7 Dec 2023 10:30:58 +0000, Clive Arthur  <cl...@nowaytoday.co.uk> wrote: 
> >On 07/12/2023 08:57, Jan Panteltje wrote: 
> > 
> ><snip> 
> >> 
> >> Would it not be simpler to use the signal to FM modulate say a 100 MHz carrier 
> >> and detect that at the other end? 
> > 
> >The 'cable' is horrible. There are many different types, absolutely and completely non-negotiable, but for the longest, getting 100kHz through 
> >is often not achievable. The signal looks like white noise. Many 
> >sinusoids from 5kHz to 100kHz.
> >
> It sounds to me like you should not be using an ancient audio amplifier. 

Doug Self's stuff isn't all that ancient. Bit no audio amplifier is designed to drive long lengths of arbitrary cable,  or to cope with a 180C ambient.

> There is lots of digital data equilization technology  around. Some of the fast signals used in PCs and telecom systems look  like noise after they pass through cables and traces but can be equalized back to beautiful binary data.

Why concentrate on digital equalisation? If the delay and attenuation through the cable is frequency dependent you may have to pre-distort the signal to get an output that keeps the customer happy, and that's more likely to need analog techniques.

-- 
Bill Sloman, Sydney

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.

No you haven't. Not using conventional components.

RL

On 07/12/2023 15:04, legg 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.
> 
> No you haven't. Not using conventional components.
> 
> RL

Bugger!  I could have sworn it was working at 180'C (along with all the 
other parts of the system), but it seems you know better.  I must have a 
faulty oven.

I'd better warn all the other downhole instrumentation companies too!

But yes, selected conventional components, analog and digital.  And yes, 
I know that if you extrapolate the graphs, most of the parts de-rate to 
negative power dissipation.

-- 
Cheers
Clive

On Wed, 6 Dec 2023 22:49:40 +0000, Clive Arthur
<clive@nowaytoday.co.uk> wrote:

>On 06/12/2023 21:19, Phil Hobbs wrote:
>> On 2023-12-06 10:26, Clive Arthur 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
>>>
>>> Cdom needs to come down for this higher frequency application, but too 
>>> far and you have oscillation.&#4294967295; 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.&#4294967295; Seems 
>>> stable and not slew rate limiting.&#4294967295; 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.&#4294967295; 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).&#4294967295; 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?&#4294967295; The signal is OFDM, so pretty much a load of 
>>> 'random' steps, some of which may be small and at a crossover point.
>>>
>> Some local feedback around the output stage would get my vote.&#4294967295; The 
>> Sziklai pairs have their own local feedback, but that doesn't fix the 
>> crossover problem.


I used to build giant NMR gradient amplifiers. I used one opamp per
mosfet to make a sloppy fet into a nearly perfect device.

https://www.dropbox.com/scl/fi/1c06h0u101c9cyh2d6ifi/Z556_1.jpg?rlkey=mswvdbxm2q9m9ks7un90cvax9&raw=1