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On Thursday, March 2, 2023 at 12:07:47 PM UTC-5, whit3rd wrote:
>>Snip>>

> It'll probably work. That doesn't mean it's good design practice to depend on it.

That is my gut feeling too. I was looking for better corroboration of if it's a good idea to design into something that can go months before anyone comes by to service the flaky analog input section. 

Don

On Wednesday, March 29, 2023 at 11:46:17 PM UTC+11, neo5...@gmail.com wrote:
> On Thursday, March 2, 2023 at 12:07:47 PM UTC-5, whit3rd wrote: 

<snip>

> > It'll probably work. That doesn't mean it's good design practice to depend on it.
> That is my gut feeling too. I was looking for better corroboration of if it's a good idea to design into something that can go months before anyone comes by to service the flaky analog input section. 

There's nothing flaky about the approach

National Application note AN-74

https://www.ti.com/lit/an/snoa654a/snoa654a.pdf?ts=1677789876245&ref_url=https%253A%252F%252Fwww.google.com%252F

specifies the National Semiconductor process 21 transistor 2N2369. The 2N3904 is process 23, which is much the same but on a slightly bigger chip with more interdigitation. Both are gold-doped.

It's at Figure 37. It's not going to require anybody to service it.

It's an unconventional application of the part, so you can't sue anybody if stops working, but it won't. It's essentially the same idea as the 1N829 precision voltage reference diode, but it offers a close to zero temperature coefficient at 1.5mA rather than the 7.5mA you need with the 1N829.

-- 
Bill Sloman, Sydney

On 30/03/2023 4:20 pm, Anthony William Sloman wrote:
> On Wednesday, March 29, 2023 at 11:46:17 PM UTC+11, neo5...@gmail.com wrote:
>> On Thursday, March 2, 2023 at 12:07:47 PM UTC-5, whit3rd wrote:
> 
> <snip>
> 
>>> It'll probably work. That doesn't mean it's good design practice to depend on it.
>> That is my gut feeling too. I was looking for better corroboration of if it's a good idea to design into something that can go months before anyone comes by to service the flaky analog input section.
> 
> There's nothing flaky about the approach
> 
> National Application note AN-74
> 
> https://www.ti.com/lit/an/snoa654a/snoa654a.pdf?ts=1677789876245&ref_url=https%253A%252F%252Fwww.google.com%252F
> 
> specifies the National Semiconductor process 21 transistor 2N2369. The 2N3904 is process 23, which is much the same but on a slightly bigger chip with more interdigitation. Both are gold-doped.
> 
> It's at Figure 37. It's not going to require anybody to service it.
> 
> It's an unconventional application of the part, so you can't sue anybody if stops working, but it won't. It's essentially the same idea as the 1N829 precision voltage reference diode, but it offers a close to zero temperature coefficient at 1.5mA rather than the 7.5mA you need with the 1N829.
> 

Do not try the C-B junction to compensate the E-B breakdown trick on a 
2N3904. It has too much gain. As I wrote up thread on 3 March the 
reverse gain needs to be well below unity otherwise transistor action 
will create interesting effects. If you really want to have a forward 
junction compensate the reverse junction either use a device with low 
reverse beta (as mentioned in the app note) or use a second 2N3904 to 
compensate the first.

piglet

On Thu, 30 Mar 2023 22:11:56 +0100, piglet <erichpwagner@hotmail.com>
wrote:

>On 30/03/2023 4:20 pm, Anthony William Sloman wrote:
>> On Wednesday, March 29, 2023 at 11:46:17?PM UTC+11, neo5...@gmail.com wrote:
>>> On Thursday, March 2, 2023 at 12:07:47?PM UTC-5, whit3rd wrote:
>> 
>> <snip>
>> 
>>>> It'll probably work. That doesn't mean it's good design practice to depend on it.
>>> That is my gut feeling too. I was looking for better corroboration of if it's a good idea to design into something that can go months before anyone comes by to service the flaky analog input section.
>> 
>> There's nothing flaky about the approach
>> 
>> National Application note AN-74
>> 
>> https://www.ti.com/lit/an/snoa654a/snoa654a.pdf?ts=1677789876245&ref_url=https%253A%252F%252Fwww.google.com%252F
>> 
>> specifies the National Semiconductor process 21 transistor 2N2369. The 2N3904 is process 23, which is much the same but on a slightly bigger chip with more interdigitation. Both are gold-doped.
>> 
>> It's at Figure 37. It's not going to require anybody to service it.
>> 
>> It's an unconventional application of the part, so you can't sue anybody if stops working, but it won't. It's essentially the same idea as the 1N829 precision voltage reference diode, but it offers a close to zero temperature coefficient at 1.5mA rather than the 7.5mA you need with the 1N829.
>> 
>
>Do not try the C-B junction to compensate the E-B breakdown trick on a 
>2N3904. It has too much gain. As I wrote up thread on 3 March the 
>reverse gain needs to be well below unity otherwise transistor action 
>will create interesting effects. If you really want to have a forward 
>junction compensate the reverse junction either use a device with low 
>reverse beta (as mentioned in the app note) or use a second 2N3904 to 
>compensate the first.
>
>piglet

Buy a bandgap reference. The transistor zener thing is risky.

On Friday, March 31, 2023 at 8:20:30 AM UTC+11, John Larkin wrote:
> On Thu, 30 Mar 2023 22:11:56 +0100, piglet <erichp...@hotmail.com> 
> wrote:
> >On 30/03/2023 4:20 pm, Anthony William Sloman wrote:
> >> On Wednesday, March 29, 2023 at 11:46:17?PM UTC+11, neo5...@gmail.com wrote:
> >>> On Thursday, March 2, 2023 at 12:07:47?PM UTC-5, whit3rd wrote: 
> >> 
> >> <snip> 
> >> 
> >>>> It'll probably work. That doesn't mean it's good design practice to depend on it. 
> >>> That is my gut feeling too. I was looking for better corroboration of if it's a good idea to design into something that can go months before anyone comes by to service the flaky analog input section. 
> >> 
> >> There's nothing flaky about the approach 
> >> 
> >> National Application note AN-74 
> >> 
> >> https://www.ti.com/lit/an/snoa654a/snoa654a.pdf?ts=1677789876245&ref_url=https%253A%252F%252Fwww.google.com%252F 
> >> 
> >> specifies the National Semiconductor process 21 transistor 2N2369. The 2N3904 is process 23, which is much the same but on a slightly bigger chip with more interdigitation. Both are gold-doped. 
> >> 
> >> It's at Figure 37. It's not going to require anybody to service it. 
> >> 
> >> It's an unconventional application of the part, so you can't sue anybody if stops working, but it won't. It's essentially the same idea as the 1N829 precision voltage reference diode, but it offers a close to zero temperature coefficient at 1.5mA rather than the 7.5mA you need with the 1N829. 
> >> 
> > 
> >Do not try the C-B junction to compensate the E-B breakdown trick on a 
> >2N3904. It has too much gain. As I wrote up thread on 3 March the 
> >reverse gain needs to be well below unity otherwise transistor action 
> >will create interesting effects. If you really want to have a forward 
> >junction compensate the reverse junction either use a device with low 
> >reverse beta (as mentioned in the app note) or use a second 2N3904 to 
> >compensate the first. 

It probably won't be difficult to select for a reverse beta of less that 0.25 (as specified in AN74). Using the base diode in a  second 2N3904 as the compensation diode for a reverse biased base-emitted junction in another 2N3904 avoids the problem, but the two diodes aren't guaranteed to be at the same temperature, which was the whole point of the trick

> Buy a bandgap reference. The transistor zener thing is risky.

It's using a part in a way that wasn't intended and exploiting a a feature that isn't specified by the manufacturer, but John Larkin does that all the time and boasts about it. 2N3904s are cheaper than well-specified reference voltage sources, and the performance should be better than a bandgap reference.

-- 
Bill Sloman Sydney

On Thursday, March 30, 2023 at 2:20:30 PM UTC-7, John Larkin wrote:
> On Thu, 30 Mar 2023 22:11:56 +0100, piglet <erichp...@hotmail.com> 
> wrote:

> >Do not try the C-B junction to compensate the E-B breakdown trick on a 
> >2N3904. It has too much gain. As I wrote up thread on 3 March the 
> >reverse gain needs to be well below unity otherwise transistor action 
> >will create interesting effects. If you really want to have a forward 
> >junction compensate the reverse junction either use a device with low 
> >reverse beta (as mentioned in the app note) or use a second 2N3904 to 
> >compensate the first. 

> Buy a bandgap reference. The transistor zener thing is risky.

Another non-risky option is uA723 or LM723 or MC1723; the innards is basically a few
general  purpose transistors, plus a temperature compensated zener.
That was the old-school part, with LOTS of second sources.

On Fri, 31 Mar 2023 23:43:22 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Thursday, March 30, 2023 at 2:20:30?PM UTC-7, John Larkin wrote:
>> On Thu, 30 Mar 2023 22:11:56 +0100, piglet <erichp...@hotmail.com> 
>> wrote:
>
>> >Do not try the C-B junction to compensate the E-B breakdown trick on a 
>> >2N3904. It has too much gain. As I wrote up thread on 3 March the 
>> >reverse gain needs to be well below unity otherwise transistor action 
>> >will create interesting effects. If you really want to have a forward 
>> >junction compensate the reverse junction either use a device with low 
>> >reverse beta (as mentioned in the app note) or use a second 2N3904 to 
>> >compensate the first. 
>
>> Buy a bandgap reference. The transistor zener thing is risky.
>
>Another non-risky option is uA723 or LM723 or MC1723; the innards is basically a few
>general  purpose transistors, plus a temperature compensated zener.
>That was the old-school part, with LOTS of second sources.

723 has had an amazing lifetime. It's still being designed into power
supplies.

On Sunday, April 2, 2023 at 1:25:09 AM UTC+11, John Larkin wrote:
> On Fri, 31 Mar 2023 23:43:22 -0700 (PDT), whit3rd <whi...@gmail.com> 
> wrote:
> >On Thursday, March 30, 2023 at 2:20:30?PM UTC-7, John Larkin wrote: 
> >> On Thu, 30 Mar 2023 22:11:56 +0100, piglet <erichp...@hotmail.com> 
> >> wrote: 
> > 
> >> >Do not try the C-B junction to compensate the E-B breakdown trick on a 
> >> >2N3904. It has too much gain. As I wrote up thread on 3 March the 
> >> >reverse gain needs to be well below unity otherwise transistor action 
> >> >will create interesting effects. If you really want to have a forward 
> >> >junction compensate the reverse junction either use a device with low 
> >> >reverse beta (as mentioned in the app note) or use a second 2N3904 to 
> >> >compensate the first. 
> > 
> >> Buy a bandgap reference. The transistor zener thing is risky. 
> > 
> >Another non-risky option is uA723 or LM723 or MC1723; the innards is basically a few 
> >general purpose transistors, plus a temperature compensated zener. 
> >That was the old-school part, with LOTS of second sources.
>
> 723 has had an amazing lifetime. It's still being designed into power  supplies.

A legacy part, used by legacy designers, in much the same way that the NE555 still has it's fans. You can do better, but often you don't have to bother.

-- 
Bill Sloman, Sydney