AoE x-Chapters, High-Speed op-amps section, DRAFT

[John Larkin]

On Sat, 27 Apr 2019 12:55:00 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

lřrdag den 27. april 2019 kl. 20.09.02 UTC+2 skrev piglet:
On 27/04/2019 17:50, Winfield Hill wrote:

I thought it was in AoE-III, but it was one of the
"advanced" things that got moved to the x-Chapters,
when we thought they were still going to be part
of the main book. Sorry. Here's a link:

https://www.dropbox.com/s/gburz2nrfpdqvri/mirror_with-extra-BJT.JPG?dl=0


That is truly *beautiful* - wonderfully elegant.

Thanks Win, not seen that before.

piglet

https://wiki.analog.com/university/courses/electronics/text/chapter-11#buffered_feedback_current_mirror

?

Some people seem to fall in love with current mirrors. They rarely
make sense off-chip.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Sat, 27 Apr 2019 12:55:00 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

lřrdag den 27. april 2019 kl. 20.09.02 UTC+2 skrev piglet:
On 27/04/2019 17:50, Winfield Hill wrote:

I thought it was in AoE-III, but it was one of the
"advanced" things that got moved to the x-Chapters,
when we thought they were still going to be part
of the main book. Sorry. Here's a link:

https://www.dropbox.com/s/gburz2nrfpdqvri/mirror_with-extra-BJT.JPG?dl=0


That is truly *beautiful* - wonderfully elegant.

Thanks Win, not seen that before.

piglet

https://wiki.analog.com/university/courses/electronics/text/chapter-11#buffered_feedback_current_mirror

?

Interesting. Trying to get kids interested in Real Electronics.

But this is outrageous:

https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter11.pdf

The word "latchup" is not mentioned.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

Lasse Langwadt Christensen wrote...

den 27. april 2019 kl. 20.09.02 UTC+2 skrev piglet:
On 27/04/2019 17:50, Winfield Hill wrote:

I thought it was in AoE-III, but it was one of the
"advanced" things that got moved to the x-Chapters,
when we thought they were still going to be part
of the main book. Sorry. Here's a link:

https://www.dropbox.com/s/gburz2nrfpdqvri/mirror_with-extra-BJT.JPG

That is truly *beautiful* - wonderfully elegant.
Thanks Win, not seen that before.

https://wiki.analog.com/university/courses/electronics/text/chapter-11#buff
ered_feedback_current_mirror

No, it's not a Wilson mirror, and really not at all
like it, each mirror output would take lots of parts,
and would fail to work up close to the rail.


--
Thanks,
- Win

 

[Winfield Hill]

John Larkin wrote...

Interesting. Trying to get kids interested in Real Electronics.

But this is outrageous:

https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter11.pdf

The word "latchup" is not mentioned.

And the output-saturation problem we've been discussing,
which is certainly an overvoltage situation, or maybe
an excess voltage (beyond-range), is also not mentioned.
Not to mention integrator windup that occurs. I dunno
if these guys have much real experience.


--
Thanks,
- Win

 

[Lasse Langwadt Christense]

lørdag den 27. april 2019 kl. 23.09.10 UTC+2 skrev Winfield Hill:

Lasse Langwadt Christensen wrote...

den 27. april 2019 kl. 20.09.02 UTC+2 skrev piglet:
On 27/04/2019 17:50, Winfield Hill wrote:

I thought it was in AoE-III, but it was one of the
"advanced" things that got moved to the x-Chapters,
when we thought they were still going to be part
of the main book. Sorry. Here's a link:

https://www.dropbox.com/s/gburz2nrfpdqvri/mirror_with-extra-BJT.JPG

That is truly *beautiful* - wonderfully elegant.
Thanks Win, not seen that before.

https://wiki.analog.com/university/courses/electronics/text/chapter-11#buff
ered_feedback_current_mirror

No, it's not a Wilson mirror, and really not at all
like it, each mirror output would take lots of parts,
and would fail to work up close to the rail.

the one above the Wilson, "11.7.1 Buffered Feedback current mirror"
it's just upside down

 

On Saturday, 27 April 2019 21:29:05 UTC+1, John Larkin wrote:

On 27 Apr 2019 09:50:44 -0700, Winfield Hill
hill@rowland.harvard.edu> wrote:
John Larkin wrote...
On 26 Apr 2019, Winfield Hill wrote:

Yes, but it's trivial to add one transistor, and
circumvent the problem. I felt we should blame Jim,
rather than have him blaming us for our standard use.

What would that transistor do?

I thought it was in AoE-III, but it was one of the
"advanced" things that got moved to the x-Chapters,
when we thought they were still going to be part
of the main book. Sorry. Here's a link:

https://www.dropbox.com/s/gburz2nrfpdqvri/mirror_with-extra-BJT.JPG?dl=0

Seems easier to buy a decent opamp than to kluge
a bad one.

That extra transistor isn't a kludge, and I'll
bet it's actually in most modern op-amps, etc.

Sorry, I thought you meant to add an external transistor somehow.

Transistors are so cheap in an IC, why share current mirrors between
opamp sections?

LM324 is ancient, full of hazards.

Yep. 1972?


NT

 

[Phil Hobbs]

On 4/27/19 5:48 AM, Allan Herriman wrote:

On Fri, 26 Apr 2019 07:10:37 -0700, Winfield Hill wrote:

Allan Herriman wrote...

On Wed, 24 Apr 2019 17:19:53 -0700, John Miles, KE5FX wrote:

On Wednesday, April 24, 2019 at 4:04:34 AM UTC-7, Winfield Hill wrote:
Read and comment. AoE x-Chapters,

You might add the LMH6733 to your table. 3 wideband CFB amps with
shutdown pins, somewhat similar to OPA3695.

Also could be worth mentioning that the shutdown pins on many of these
parts are referenced to the positive rail, not ground. That just
hosed me the other day. The shutdown pin on the LMH6733 and similar
parts is best thought of as an analog input, not a digital one.
Some chips like the newer THS3491 have a separate reference pin just
for that purpose, but most don't. For the LMH6733 the disable/enable
thresholds are specified at 3.2V and 3.6V for 5V rails, and you need
to add a volt to them if you max out the rails at +/- 6 volts.

That could be (and was) a problem when relying on an open-drain output
to pull the shutdown pin up to 5 volts, since the current drawn by the
shutdown pin is not negligible. (Worse, the open-drain pin in
question was on an I2C extender whose data sheet specifies 5.5V
compatibility at the GPIO pins without regard to its own supply
voltage, but didn't mention the ESD diode that limits the "open drain"
voltage to 1 volt above the Vdd rail, which was 3.3V in my case.)

-- john, KE5FX

LTC6228 has a multifunction shutdown pin. Different voltages (wrt the
positive rail) will shutdown the whole opamp, or enable / disable the
input bias current cancellation circuit (allowing one to trade input
current noise for input bias current).

Wow, Allan, that's one fascinating op-amp. It's new and I had missed
it, thanks!

For me, the fascination is why they thought a sub-nV/rtHz opamp would be
marketable with a 1/f noise corner so high.

Perhaps I misunderstood the intended application.

Allan

A 1-MHz 1/f corner in a GHz-class op amp is probably OK for a lot of
applications. I did an ultralow-noise preamp for Samsung's research lab
that used an ATF38143 pHEMT, whose 1/f corner was about 10 MHz but whose
flatband noise was about 0.35 nV/sqrt(Hz). It was a big win.

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

 

[Phil Hobbs]

On 4/27/19 3:08 PM, John Larkin wrote:

On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...
I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

I use LM358s routinely in super-low-cost circuits. One of my favourite
sports is doing amazing things(*) with next to zero apparatus.

Cheers

Phil Hobbs

(*) For sufficiently permissive definitions of 'amazing'.

--
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

 

[John Larkin]

On Sat, 27 Apr 2019 20:06:37 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 3:08 PM, John Larkin wrote:
On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...
I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

If you dump enough current, you can almost make an LM324 into an
acceptable audio amplifier. Almost. Sort of. For AM radio sound maybe.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

On Sunday, 28 April 2019 01:55:10 UTC+1, John Larkin wrote:

On Sat, 27 Apr 2019 20:06:37 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:
On 4/27/19 3:08 PM, John Larkin wrote:
On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...

I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

If you dump enough current, you can almost make an LM324 into an
acceptable audio amplifier. Almost. Sort of. For AM radio sound maybe.

I've done just that. Sounds decent, though P_out is not too good. Substituting lower power versions of 324 always degraded the sound (LP324?).


NT

 

[John Larkin]

On Sun, 28 Apr 2019 16:06:12 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 8:55 PM, John Larkin wrote:
On Sat, 27 Apr 2019 20:06:37 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 3:08 PM, John Larkin wrote:
On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...
I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

If you dump enough current, you can almost make an LM324 into an
acceptable audio amplifier. Almost. Sort of. For AM radio sound maybe.



I use them for stuff like temperature control, slow local feedback
networks, bias generators, that sort of thing. (Of course I mostly use
LM358s because it makes the layout a bit easier.)

My daughter Magdalen has just gone to work for Cirrus Logic, so we need
a new layout person.

Is she going to do PCB layout? The three best PCB layout people that
I've worked with were all women.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Phil Hobbs]

On 4/27/19 8:55 PM, John Larkin wrote:

On Sat, 27 Apr 2019 20:06:37 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 3:08 PM, John Larkin wrote:
On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...
I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

If you dump enough current, you can almost make an LM324 into an
acceptable audio amplifier. Almost. Sort of. For AM radio sound maybe.

I use them for stuff like temperature control, slow local feedback
networks, bias generators, that sort of thing. (Of course I mostly use
LM358s because it makes the layout a bit easier.)

My daughter Magdalen has just gone to work for Cirrus Logic, so we need
a new layout person.


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

 

[Phil Hobbs]

On 4/28/19 4:11 PM, John Larkin wrote:

On Sun, 28 Apr 2019 16:06:12 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 8:55 PM, John Larkin wrote:
On Sat, 27 Apr 2019 20:06:37 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 3:08 PM, John Larkin wrote:
On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...
I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

If you dump enough current, you can almost make an LM324 into an
acceptable audio amplifier. Almost. Sort of. For AM radio sound maybe.



I use them for stuff like temperature control, slow local feedback
networks, bias generators, that sort of thing. (Of course I mostly use
LM358s because it makes the layout a bit easier.)

My daughter Magdalen has just gone to work for Cirrus Logic, so we need
a new layout person.



Is she going to do PCB layout? The three best PCB layout people that
I've worked with were all women.

Yes, she's going to be doing demo boards for them. Cirrus seems is a
great place to work, by all accounts.

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

 

On Sun, 28 Apr 2019 16:42:05 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/28/19 4:11 PM, John Larkin wrote:
On Sun, 28 Apr 2019 16:06:12 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 8:55 PM, John Larkin wrote:
On Sat, 27 Apr 2019 20:06:37 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 4/27/19 3:08 PM, John Larkin wrote:
On Sat, 27 Apr 2019 12:54:22 -0500, "Tim Williams"
tiwill@seventransistorlabs.com> wrote:

"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message
news:sid7ce5hbeotn45ap3buoa1d6tnolkmf7v@4ax.com...
I've seen visible crossover distortion amplifying a 60 Hz sine
wave.


I like to put it this way: it's noticeable even in a control loop.

Tim

You should be happy to pay extra for integrated deadband.

Actually, it's professional malpractice to use an LM324 in a control
loop.


You just have to add a resistor so that it sources some Class A current.

If you dump enough current, you can almost make an LM324 into an
acceptable audio amplifier. Almost. Sort of. For AM radio sound maybe.



I use them for stuff like temperature control, slow local feedback
networks, bias generators, that sort of thing. (Of course I mostly use
LM358s because it makes the layout a bit easier.)

My daughter Magdalen has just gone to work for Cirrus Logic, so we need
a new layout person.



Is she going to do PCB layout? The three best PCB layout people that
I've worked with were all women.


Yes, she's going to be doing demo boards for them. Cirrus seems is a
great place to work, by all accounts.

Good products at great prices, too.

 

[whit3rd]

On Saturday, April 27, 2019 at 5:06:49 PM UTC-7, Phil Hobbs wrote:
> On 4/27/19 3:08 PM, John Larkin wrote:

[about LM358 and LM324 crossover distortion]

Actually, it's professional malpractice to use an LM324 in a control
loop.

You just have to add a resistor so that it sources some Class A current.

The resistor trick is fine for audio, but there's lots of control applications
(like voltage regulation) where no addition is necessary. Realistically,
any application where a human could twiddle knobs to keep something
steady, the kilohertz-and-up signal handling deficiencies are... well masked.

 

[George Herold]

On Monday, April 29, 2019 at 6:00:58 PM UTC-4, whit3rd wrote:

On Saturday, April 27, 2019 at 5:06:49 PM UTC-7, Phil Hobbs wrote:
On 4/27/19 3:08 PM, John Larkin wrote:

[about LM358 and LM324 crossover distortion]

Actually, it's professional malpractice to use an LM324 in a control
loop.

You just have to add a resistor so that it sources some Class A current.

The resistor trick is fine for audio, but there's lots of control applications
(like voltage regulation) where no addition is necessary. Realistically,
any application where a human could twiddle knobs to keep something
steady, the kilohertz-and-up signal handling deficiencies are... well masked.

If zero volts has cross-over, do all class A, +24, -5 supplies.
I'm using a TCA3072 that way. Diode on the output, the 'plant' is
a resistor.

George H.

 

[John Larkin]

On Mon, 29 Apr 2019 18:35:09 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 6:00:58 PM UTC-4, whit3rd wrote:
On Saturday, April 27, 2019 at 5:06:49 PM UTC-7, Phil Hobbs wrote:
On 4/27/19 3:08 PM, John Larkin wrote:

[about LM358 and LM324 crossover distortion]

Actually, it's professional malpractice to use an LM324 in a control
loop.

You just have to add a resistor so that it sources some Class A current.

The resistor trick is fine for audio, but there's lots of control applications
(like voltage regulation) where no addition is necessary. Realistically,
any application where a human could twiddle knobs to keep something
steady, the kilohertz-and-up signal handling deficiencies are... well masked.

If zero volts has cross-over, do all class A, +24, -5 supplies.
I'm using a TCA3072 that way. Diode on the output, the 'plant' is
a resistor.

George H.

Only a few amps like LM324 were brain damaged this way. It's actually
class B, because all three (3!) output transistors have zero quiescent
current. There is three junction drops of deadband.

Silicon malpractice.

The TCA0372 data sheet shows three output bias diodes, but they drew
the upper darlington wrong.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Gerhard Hoffmann]

Am 27.04.19 um 00:37 schrieb tabbypurr@gmail.com:

One thing about the classic National LM324 is that if you rail any
section, it wrecks the shared bias supplies for the other sections. So
one comparator switching *really* messes up whatever the other three
amps are doing.

I think some peoples' later versions didn't do that.

The 324 is my favourite opamp ever. You get so much for so little.

Hm. It definitely needs better output biasing.

In a previous life I have made a LM124-like op amp on an
analog array, and also a LM139-like comparator.

Not a bad architecture when all the good PNP transistors you have
are substrate transistors. I.E. their collectors are preconnected
to the substrate. A extra mA and a VBE multiplier were essential.

Current mirrors with pnp lateral transistors are a joke.

Nevertheless it was surprising what could be done with
1 metal layer and a few predefined resistors and transistors.
And there was a 30 pF cap per cell. That one was huge.

regards,
Gerhard

 

[Gerhard Hoffmann]

Am 25.04.19 um 00:42 schrieb Joerg:

On 2019-04-24 14:17, Gerhard Hoffmann wrote:
Am 24.04.19 um 21:35 schrieb Joerg:

1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE
junction, even stuff at cell phone frequencies. This rectification or
demodulation is very inefficient but since that is inside the loop any
resultying baseband AM will hit at full tilt because it happens at
"open loop".

It's not that FETs are any worse at demodulating than BJTs, it's just
that they need a higher source impedance for the same dBms. Say, a
different cable transformation.


MOSFETs don't demodulate at all because there is no conducting diode
path. I have found cases with hardcore EM susceptibility to the point
where a person outside the concrete walls of the building could upset a
circuit operating at single-digit kHz range, just by turning on their
cell phone. GSM ones were especially bad. After switching to a CMOS
opamp ... nad, zilch, not even when holding a cell phone right above the
open circuit and then turning it on.

Are you really trying to tell me that a thing with a square law transfer
function cannot demodulate? That it takes a PN junction? I've seen it on
BF862 and the opa140 on my table (admittedly open) also dislikes my cell
phone.

For RF in, say, the cell phone range there is no or hardly any loop gain
unless you have a super-fast opamp.

The loop gain is not needed for the RF signal. It's enough if it
exists for the demodulated result of the input square law device.

2. Back-to-back input protection diodes between IN+ and IN-. Very
often overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V
diff limit. Aside from pouring gasoline on the above mentioned EMI
issue these diodes can really throw people a curve when using opamps
in an unorthodox way or as a comparator. Thou shalt not do that but ...

These diodes are anti-parallel, not an efficient rectifier.

Can be enough. There is always an offset.

100s of mV offset between the input pins of thing designed to have
near infinite gain. OMG.

                                                 ... It's a very
good thing that they are there. A low noise op amp with zenered BE
junctions at the input is no longer a low noise op amp. And an op amp
with 100 dB open loop gain and a Volt between its inputs is a design
error.


Not if you know what you are doing and the mfg has blessed railing the
output.


Using an op amp as a comparator is not unorthodox, it's wrong.


Nah, BTDT. You have to know the limits and not exceed reverse Vbe
anywhere. Which can be a mere -2V in RF stuff.


The diodes are there to protect other stuff from dying.


Real hot stuff doesn't have diodes. For example the SD5400 quad FET
array, a marvelous IC. One of my circuits almost drove a tech insane
because every one of them he soldered in died. Until I showed him how to
avoid every last bit of ESD. Once inside the circuit everything was safe.

The data sheet has a different opinion.

http://www.farnell.com/datasheets/69195.pdf

The idea of unprotected MOS gates died mostly with the RCA 40600
(verbatim). It had a nice bronze wire wrapped around its leads, to
be pulled away after soldering. It was soon away from the market
and made place for the 40673, 40841 and 3N140. I still have a few;
they will probably survive me unused in their box.


regards,
Gerhard

 

[Tim Williams]

"Gerhard Hoffmann" <dk4xp@arcor.de> wrote in message
news:qacg0j$geo$1@solani.org...

MOSFETs don't demodulate at all because there is no conducting diode
path. I have found cases with hardcore EM susceptibility to the point
where a person outside the concrete walls of the building could upset a
circuit operating at single-digit kHz range, just by turning on their
cell phone. GSM ones were especially bad. After switching to a CMOS opamp
... nad, zilch, not even when holding a cell phone right above the open
circuit and then turning it on.

Are you really trying to tell me that a thing with a square law transfer
function cannot demodulate? That it takes a PN junction? I've seen it on
BF862 and the opa140 on my table (admittedly open) also dislikes my cell
phone.

Right, it's an overly broad generalization, based on limited experience
rather than theory and regulation.

It is supported by these, but not in the strict way that it is presented
above.

To wit: if IEC 61000-4-3 says you need to handle 3 V/m at basically any
frequency, then expect on the order of 3V to get into your circuit (assuming
cables of about 1m length -- typical for a wired product). More with cable
resonances, less with short cables, and all other sorts of specifics that
vary with frequency and circuit.

Apply that 3V directly to a signal MOSFET, and it's sure as hell going to
demodulate it. Even small power transistors may; although not necessarily
at the frequencies tested, since gate impedance is so much lower. Again,
it's a matter of the whole circuit and environment, not of a single trifling
aspect of it.

Noteworthy, too, that when it comes to 61000-4-2 ESD testing, or -4 EFT,
anywhere you're getting ~3V on a gate in radiated, you're getting ~kV of
these surges. Maybe you put ESD diodes on there, but those just made
radiated worse (diode junctions!), if not at 3 V/m (say your supplies are
5V), but at some point they will detect it as well.

In this case, filtering is indicated (ground plane construction goes without
saying), or outright shielding.

Say you've got filtering, or short (or nonexistent) cable length, so that
your device generally isn't susceptible. Now you're getting good
attenuation, like 40dB+. Well, that's still 10s of mV of RF entering the
circuit. Apply that to a diode junction and you get meaningful demodulation
(poor performance, maybe 10-30%, but definitely detectable). Apply that to
a FET gate, and you probably won't get much (the quadratic response has a
much softer curve and looks flat for this arc length).

Is filtering or shielding still indicated? Depends. If the FET/MOS amp is
too noisy, or expensive, to meet the product's overall goals, then another
solution will be needed. If all requirements are still satisfied, then it
is certainly acceptable.

This is the theory behind the observation, and also illustrates the limits
where one expects to see variance from said observation.

Yes, even vacuum tubes will detect RF; it's just that it's very rare for --
well, for one it's very rare for audiophiles to submit their products to IEC
testing _at all_, but there are few circuits quite sensitive enough (phono
amps only deal with ~mV, not ~uV), for which typical grid bias (~volts) is
surpassed by enough ambient RF (~V/m), to cause perceptible detection.
That's even taking into consideration the shitty [RF] grounding that most
such amplifiers have. Rectification itself is also fairly slow (ordinary
receiving tubes do have bandwidth to the low GHz, but physics itself is
against them*).

*For example, I built a VHF oscillator with a 6C4 that generates harmonics
above the noise floor (RBW = 300kHz) out beyond 800MHz, and presumably at
lower RBW, to beyond 1GHz (I just didn't check at the time). This is far
out in the tail (-40dB/dec?) though, and I would expect similar performance
with respect to RF detection.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/