Constant current to capacifor / rising voltage...

[Fred Bloggs]

On Monday, June 19, 2023 at 2:19:02 PM UTC-4, John Larkin wrote:

On Mon, 19 Jun 2023 09:09:18 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Monday, June 19, 2023 at 12:08:02?PM UTC-4, John Larkin wrote:
On Mon, 19 Jun 2023 08:57:16 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:

On Monday, June 19, 2023 at 6:39:41?AM UTC-4, Anthony William Sloman wrote:
On Monday, June 19, 2023 at 6:43:23?PM UTC+10, sonnic...@gmail.com wrote:
Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?
You want to use a dual transistor so both transistor junctions are at the same temperature. By cascoding one or both of the transistors you can do better.

https://en.wikipedia.org/wiki/Wilson_current_mirror

It all depends on what kind of performance you need.

Simplest low cost with linear components is the dual opamp difference amp with buffered feedback. So-called current setting reference voltage is entirely flexible. You don\'t need to use the AD parts or transistor called out in app note, single supply jfet input OAs are usually enough:

https://www.analog.com/en/analog-dialogue/articles/diff-amp-heart-of-precision-current-source.html
The OP wants a current source to charge a cap. That circuit isn\'t well
suited to that.

The hell if it isn\'t. Obviously you put the cap as the load-sheesh.
Sense resistor below the cap?

It\'s a simple difference amplifier that applies a stiff scaled input difference voltage across the output resistor. The buffer is there to eliminate error current due to the feedback resistor divider.

It\'s the \"Two-Op-Amp Topology\" here:

https://www.allaboutcircuits.com/technical-articles/how-to-design-a-precision-current-pump-with-op-amps/

 

[Phil Hobbs]

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid-dead-dontuse@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.


TI TL011-014. Long obsolete, alas.

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

 

[Phil Hobbs]

On 2023-06-19 09:35, Phil Hobbs wrote:

sonnic...@gmail.com <sonnichjensen@gmail.com> wrote:
Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?



Relying on the collector impedance of a BJT over a wide voltage range will
get you roughly 1-10% nonlinearity, depending on the device and conditions,
on account of the Early effect.

A big resistor in series with the emitter helps a lot, but doesn’t fix the
next problem, which is that the beta also varies with Vce.

You can fix that problem by using a FET instead. Their nonlinearity is much
worse than a BJT’s, so you absolutely need an op amp wrapped round it, with
close attention to stability as JL says.

Another approach to slowish ramps is a bootstrap current source. You hang a
voltage reference on the output of an op amp follower, and connect a
resistor from the reference to the follower’s input. In principle, that
makes a perfectly constant current.

You can make it pretty nearly perfect in real life too, by choosing a FET
op amp with really good common mode rejection, such as the OPA140.

The two remaining problems are the reset circuit and dielectric absorption
in the capacitor.

For a slow ramp, you want a polypropylene or (better) polystyrene cap.
I recently built a slowish, very linear, adjustable ramp generator for
characterizing a 12-bit ADC on a chip we\'ve been collaborating on.

I started with wound mylar (polyster) caps, and found that the after a
100-us ramp reset, the output waveform exhibited absorption tails about
2 ms long. (That is, the voltage just sort of sat there for a couple of
milliseconds before starting to rise again.)

It wasn\'t the current source--that was clean as a whistle--but rather
dielectric absorption (aka soakage) in the capacitor.

Replacing it with a 630V polyprop Y cap (0.22 uF iirc) reduced but
didn\'t eliminate the tails, so I just made the reset pulse 5 ms long.
That reduced the maximum rep rate, but cleaned up the ramp amazingly.

(BTW Newark has some nice-looking 10 nF polyprop caps on super deal just
now.)

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

 

[John Larkin]

On Mon, 19 Jun 2023 18:29:56 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-19 09:35, Phil Hobbs wrote:
sonnic...@gmail.com <sonnichjensen@gmail.com> wrote:
Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?



Relying on the collector impedance of a BJT over a wide voltage range will
get you roughly 1-10% nonlinearity, depending on the device and conditions,
on account of the Early effect.

A big resistor in series with the emitter helps a lot, but doesn’t fix the
next problem, which is that the beta also varies with Vce.

You can fix that problem by using a FET instead. Their nonlinearity is much
worse than a BJT’s, so you absolutely need an op amp wrapped round it, with
close attention to stability as JL says.

Another approach to slowish ramps is a bootstrap current source. You hang a
voltage reference on the output of an op amp follower, and connect a
resistor from the reference to the follower’s input. In principle, that
makes a perfectly constant current.

You can make it pretty nearly perfect in real life too, by choosing a FET
op amp with really good common mode rejection, such as the OPA140.

The two remaining problems are the reset circuit and dielectric absorption
in the capacitor.

For a slow ramp, you want a polypropylene or (better) polystyrene cap.
I recently built a slowish, very linear, adjustable ramp generator for
characterizing a 12-bit ADC on a chip we\'ve been collaborating on.

I started with wound mylar (polyster) caps, and found that the after a
100-us ramp reset, the output waveform exhibited absorption tails about
2 ms long. (That is, the voltage just sort of sat there for a couple of
milliseconds before starting to rise again.)

It wasn\'t the current source--that was clean as a whistle--but rather
dielectric absorption (aka soakage) in the capacitor.

Replacing it with a 630V polyprop Y cap (0.22 uF iirc) reduced but
didn\'t eliminate the tails, so I just made the reset pulse 5 ms long.
That reduced the maximum rep rate, but cleaned up the ramp amazingly.

(BTW Newark has some nice-looking 10 nF polyprop caps on super deal just
now.)

Cheers

Phil Hobbs

One could use a good DAC to test an ADC. DAC1220 is linear to 15 PPM,
monotonic at 20 bits.

 

[Phil Hobbs]

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 19 Jun 2023 18:29:56 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-19 09:35, Phil Hobbs wrote:
sonnic...@gmail.com <sonnichjensen@gmail.com> wrote:
Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?



Relying on the collector impedance of a BJT over a wide voltage range will
get you roughly 1-10% nonlinearity, depending on the device and conditions,
on account of the Early effect.

A big resistor in series with the emitter helps a lot, but doesn’t fix the
next problem, which is that the beta also varies with Vce.

You can fix that problem by using a FET instead. Their nonlinearity is much
worse than a BJT’s, so you absolutely need an op amp wrapped round it, with
close attention to stability as JL says.

Another approach to slowish ramps is a bootstrap current source. You hang a
voltage reference on the output of an op amp follower, and connect a
resistor from the reference to the follower’s input. In principle, that
makes a perfectly constant current.

You can make it pretty nearly perfect in real life too, by choosing a FET
op amp with really good common mode rejection, such as the OPA140.

The two remaining problems are the reset circuit and dielectric absorption
in the capacitor.

For a slow ramp, you want a polypropylene or (better) polystyrene cap.
I recently built a slowish, very linear, adjustable ramp generator for
characterizing a 12-bit ADC on a chip we\'ve been collaborating on.

I started with wound mylar (polyster) caps, and found that the after a
100-us ramp reset, the output waveform exhibited absorption tails about
2 ms long. (That is, the voltage just sort of sat there for a couple of
milliseconds before starting to rise again.)

It wasn\'t the current source--that was clean as a whistle--but rather
dielectric absorption (aka soakage) in the capacitor.

Replacing it with a 630V polyprop Y cap (0.22 uF iirc) reduced but
didn\'t eliminate the tails, so I just made the reset pulse 5 ms long.
That reduced the maximum rep rate, but cleaned up the ramp amazingly.

(BTW Newark has some nice-looking 10 nF polyprop caps on super deal just
now.)

Cheers

Phil Hobbs

One could use a good DAC to test an ADC. DAC1220 is linear to 15 PPM,
monotonic at 20 bits.

Yeah, there a number of ways to do it. This one was a low voltage gizmo
where the main worry was DNL. Using an integrator ramp made it easy to sort
the DNL out from the noise.

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

 

[Anthony William Sloman]

On Tuesday, June 20, 2023 at 2:09:06 AM UTC+10, Brian Gregory wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.
Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

That why you build them from dual transistors, as I mentioned. Element-14/Newark has them in stock.

https://4donline.ihs.com/images/VipMasterIC/IC/ONSM/ONSM-S-A0013579433/ONSM-S-A0013749973-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

This is a close-matched PNP pair. There are others.

--
Bill Sloman, Sydney

 

[Anthony William Sloman]

On Tuesday, June 20, 2023 at 4:18:25 AM UTC+10, John Larkin wrote:

On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

https://4donline.ihs.com/images/VipMasterIC/IC/ONSM/ONSM-S-A0013579433/ONSM-S-A0013749973-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

You do need to read the data sheets.

> Zetex sells a sort of current mirror thing. Somebody should sell a good packaged IC mirror but I don\'t know of one.

No surprise there.

--
Bill Sloman, sydney

 

[whit3rd]

On Monday, June 19, 2023 at 1:43:23 AM UTC-7, sonnic...@gmail.com wrote:

Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?

The op amp circuit is OK, but would be better with a reference voltage (TL431
gives a good 2.5V value) instead of R1, and a PMOS transistor instead of the PNP 2N3906.

For best compliance, use +30V or so (the TL082 can take it).

But, if you want REALLY slow, this is about ramping up the voltage on a capacitor?
An integrator (current source into op amp(-), capacitor from output to op amp (-),
and opamp (+) grounded) gives op amp output that follows the slow ramp rule..

It only takes a voltage source and resistor to drive that circuit, because the
op amp (-) stays at GND due to the feedback, so a resistor and known voltage
IS an accurate current source for that load. I\'ve made ramp generators that run for
days by feeding low voltage (0.04V) into a few-megohms resistor in such a circuit.

The drawbacks: after it hits the power supply limit, use a reset switch (across
the capacitor) to discharge it. And, the \'grounded\' op amp (+) input terminal
ought to be connected to GND through a resistor that matches the current source
resistor... that compensates for any bias current.

 

[Gerhard Hoffmann]

Am 19.06.23 um 23:30 schrieb Phil Hobbs:

John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid-dead-dontuse@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.


TI TL011-014. Long obsolete, alas.

infineon BCV61 ?

I\'m not yet awake.

Gerhard

BCV-xxx has a lot of 2-transistor thingies.

 

[Anthony William Sloman]

On Tuesday, June 20, 2023 at 1:18:34 PM UTC+10, Gerhard Hoffmann wrote:

Am 19.06.23 um 23:30 schrieb Phil Hobbs:
John Larkin <jla...@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.


TI TL011-014. Long obsolete, alas.

infineon BCV61 ?

I\'m not yet awake.

Gerhard

BCV-xxx has a lot of 2-transistor thingies.

Motorola - ON Semiconductor - does seem to do monolithic duals with close matching. I pulled this one off the elemt-14/Farnell/Newark web-site.

https://4donline.ihs.com/images/VipMasterIC/IC/ONSM/ONSM-S-A0013579433/ONSM-S-A0013749973-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

Infineon and Nexperia seem to mostly put two different transistors into a single package. Not quite the same thing.

Analog Devices used to sell tightly matched dual transistors but they weren\'t cheap.

--
Bill Sloman, Sydney

 

[whit3rd]

On Monday, June 19, 2023 at 11:18:25 AM UTC-7, John Larkin wrote:

On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.
Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature.

That\'s because EEs more often use ICs than actually design them.
Few if any op amps are made without a few current mirror sources.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.

LM13700 is a dual packaged IC mirror, if you drive the differential input with 2 or 3 volts.
You get your choice of current sense, positive or negative, from a negative-rail
current receiver.

 

[John Larkin]

On Tue, 20 Jun 2023 05:18:27 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

Am 19.06.23 um 23:30 schrieb Phil Hobbs:
John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid-dead-dontuse@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.


TI TL011-014. Long obsolete, alas.


infineon BCV61 ?

That seems to be a genuine matched-pair monolithic dual, unlike most
dual transistors that are two dies in a package. And it\'s available
and cheap.

Thermal image of a dual transistor:

https://www.dropbox.com/s/dd072w1z2gmfpbt/Dual_NPN.jpg?dl=0

I\'m not yet awake.

I do some of my best work when I\'m asleep. That\'s sure easy.

It\'s 3 AM here and I woke up with an idea to research. I\'m hungry too.

Gerhard

BCV-xxx has a lot of 2-transistor thingies.

 

[legg]

On Mon, 19 Jun 2023 01:43:18 -0700 (PDT), \"sonnic...@gmail.com\"
<sonnichjensen@gmail.com> wrote:

Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?

Look up \'Art of Electronics\'. Older editions are available on-line.

RL

 

[Don]

legg wrote:

sonnichjensen wrote:

Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?

Look up \'Art of Electronics\'. Older editions are available on-line.

Your followup motivated me to search for an online AoE 2nd edition. It
was my intention to reminisce about the 2nd\'s grey backgrounded, bright
bulbed, \"Circuit ideas\" before launching into a full blown gripe about
how AoE II\'s \"ultraprecise current source\" schematic shown in Figure
7.82H flows backwards.
Instead, an apparently unabridged online AoE 3rd edition was
discovered:

<https://archive.org/details/the-art-of-electronics-3rd-ed-2015_202008>

The legality of archive\'s AoE III is unknown to me.

With preliminaries now out of the way, allow me to note my semi-serious
quest to create a current source from a 555. Could such a circuit be the
cat\'s meow, or what?

Danke,

--
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

 

[Phil Hobbs]

On 2023-06-19 23:18, Gerhard Hoffmann wrote:

Am 19.06.23 um 23:30 schrieb Phil Hobbs:
John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid-dead-dontuse@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.


TI TL011-014. Long obsolete, alas.


infineon BCV61  ?

I\'m not yet awake.

Gerhard

BCV-xxx has a lot of 2-transistor thingies.

Nah, they\'re two separate dice. Almost no thermal coupling to speak
of--there\'s a spec in there that tells you how much dissipation you can
have in the output device before it runs away.

We had a very detailed thread on that some time back, but iirc there was
enough info to calculate that the thermal resistance die-to-die was no
better than the junction-to-ambient, i.e. 300 K/S or so.

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 2023-06-20 02:01, whit3rd wrote:

On Monday, June 19, 2023 at 11:18:25 AM UTC-7, John Larkin wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.
Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature.

That\'s because EEs more often use ICs than actually design them.
Few if any op amps are made without a few current mirror sources.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.

LM13700 is a dual packaged IC mirror, if you drive the differential input with 2 or 3 volts.
You get your choice of current sense, positive or negative, from a negative-rail
current receiver.

And if you don\'t mind 0.5 MHz bandwidth on a good day.

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 2023-06-20 10:21, Don wrote:

legg wrote:
sonnichjensen wrote:

Hi

I want to get a steady rising voltage, which is simple as constant current.
Say, like a saw tooth but only first rising part and really slow.
(I do not want a timer, but a rising voltage)

I was thinking of something like this using the Vbe of a transistor
https://electronics.stackexchange.com/questions/53615/constant-current-source

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/

But I was just wondering, are there other ways of doing this?

Look up \'Art of Electronics\'. Older editions are available on-line.

Your followup motivated me to search for an online AoE 2nd edition. It
was my intention to reminisce about the 2nd\'s grey backgrounded, bright
bulbed, \"Circuit ideas\" before launching into a full blown gripe about
how AoE II\'s \"ultraprecise current source\" schematic shown in Figure
7.82H flows backwards.
Instead, an apparently unabridged online AoE 3rd edition was
discovered:

https://archive.org/details/the-art-of-electronics-3rd-ed-2015_202008

The legality of archive\'s AoE III is unknown to me.

With preliminaries now out of the way, allow me to note my semi-serious
quest to create a current source from a 555. Could such a circuit be the
cat\'s meow, or what?

Danke,

A PWM current source isn\'t too hard, I shouldn\'t think. A _linear_
current source, now, that\'s the ticket.

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

 

[Anthony William Sloman]

On Wednesday, June 21, 2023 at 12:31:09 AM UTC+10, Phil Hobbs wrote:

On 2023-06-19 23:18, Gerhard Hoffmann wrote:
Am 19.06.23 um 23:30 schrieb Phil Hobbs:
John Larkin <jla...@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Non-EE academics seem fixated on current mirrors, as seen in the
scientific literature. They work fairly well inside ICs but not when
built with discretes. Even the mirrors inside ICs don\'t usually need
to be very good.

Most dual transistors are really two die, not thermally coupled.

Zetex sells a sort of current mirror thing. Somebody should sell a
good packaged IC mirror but I don\'t know of one.


TI TL011-014. Long obsolete, alas.


infineon BCV61 ?

I\'m not yet awake.

Gerhard

BCV-xxx has a lot of 2-transistor thingies.
Nah, they\'re two separate dice. Almost no thermal coupling to speak
of--there\'s a spec in there that tells you how much dissipation you can
have in the output device before it runs away.

We had a very detailed thread on that some time back, but iirc there was
enough info to calculate that the thermal resistance die-to-die was no
better than the junction-to-ambient, i.e. 300 K/S or so.

That seems to be true of the Infineon and Nexperia parts but
Motorola - ON Semiconductor - does seem to do monolithic duals with close matching. I pulled this one off the elemt-14/Farnell/Newark web-site.

https://4donline.ihs.com/images/VipMasterIC/IC/ONSM/ONSM-S-A0013579433/ONSM-S-A0013749973-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

If I remember rightly there\'s a least one NPN equivalent, and Analog Devices certainly used to do more tightly specified monolithic quads an duals at rather higher price.

The compound semiconductor transsotrs that you and John Larkin carry on about from tine to time - with the 300V Early voltage asnd tiny collector base capacitance might well work well as the cascode part in a Wilson three transistor current mirror or the four transistor elaboration of it.

--
Bil Sloman, Sydney

 

[whit3rd]

On Tuesday, June 20, 2023 at 7:31:09 AM UTC-7, Phil Hobbs wrote:

On 2023-06-19 23:18, Gerhard Hoffmann wrote:
Am 19.06.23 um 23:30 schrieb Phil Hobbs:
John Larkin <jla...@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Exact is a relative term; thermally linked is also a relative term, you can glue
two TO92\'s together and they ARE linked, thermally and physically: it
works better with C1841 NPN transistors than with PN2222\'s, because
the Japanese case has its metal tab oriented differently.


> >>> Most dual transistors are really two die, not thermally coupled.

That\'s a packaging issue.

Discrete transistor designs have the collector as the substrate, so
you can\'t easily electrically separate collectors of two transistors mounted
to the same metal plate. To make a good-quality dual, epitaxial
silicon grown over oxide can be electrically isolated, OR you need
a seven-pin package, so you can bias the substrate (as an IC would
do) to create a depletion region. Things like LM13700 have the
specified bias on that most-negative pin in order to work,
as did the (hard-to-find) transistor multiples of yesteryear (CA3046, anyone?).

 

[John Larkin]

On Tue, 20 Jun 2023 09:13:14 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Tuesday, June 20, 2023 at 7:31:09?AM UTC-7, Phil Hobbs wrote:
On 2023-06-19 23:18, Gerhard Hoffmann wrote:
Am 19.06.23 um 23:30 schrieb Phil Hobbs:
John Larkin <jla...@highlandSNIPMEtechnology.com> wrote:
On Mon, 19 Jun 2023 17:08:58 +0100, Brian Gregory
void-invalid...@email.invalid> wrote:

On 19/06/2023 11:25, John Larkin wrote:
If someone suggests a current mirror, it\'s probably a bad idea.

Yes. AIUI they\'re more something that gets built in to integrated
circuits since they don\'t work well when built from discrete components
because they\'re not exactly matched, and not thermally linked together.

Exact is a relative term; thermally linked is also a relative term, you can glue
two TO92\'s together and they ARE linked, thermally and physically: it
works better with C1841 NPN transistors than with PN2222\'s, because
the Japanese case has its metal tab oriented differently.


Most dual transistors are really two die, not thermally coupled.

That\'s a packaging issue.

Discrete transistor designs have the collector as the substrate, so
you can\'t easily electrically separate collectors of two transistors mounted
to the same metal plate. To make a good-quality dual, epitaxial
silicon grown over oxide can be electrically isolated, OR you need
a seven-pin package, so you can bias the substrate (as an IC would
do) to create a depletion region. Things like LM13700 have the
specified bias on that most-negative pin in order to work,
as did the (hard-to-find) transistor multiples of yesteryear (CA3046, anyone?).

A pure-silicon current mirror runs one transistor at 0.6ish volts and
low dissipation, and the other at whatever Vce and higher dissipation,
so tight thermal coupling is mandatory.

Really good monolithic pairs have many equivalent transistors
interleaved in a tricky pattern.

I guess you could bolt two TO-220s or TO-247s metal-to-metal with a
thin AlN insulator.