Constant current to capacifor / rising voltage...

[sonnic...@gmail.com]

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?

 

[Jan Panteltje]

On a sunny day (Mon, 19 Jun 2023 01:43:18 -0700 (PDT)) it happened
\"sonnic...@gmail.com\" <sonnichjensen@gmail.com> wrote in
<eed6b6ab-4f16-402c-95eb-5dfcbdc3dd00n@googlegroups.com>:

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?

If you have a realy high voltage, say 1000V and only need a 1 V ramp,
then just a resistor will do, as I will not change much (1 part in 1000) in that case.


JFETs also make decent current sources, this is for a negatve ramp (J2):
https://panteltje.nl/panteltje/scope_tv/index.html

 

[John Larkin]

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

That works but won\'t be very temperature stable. A zener or bandgap
instead of the two diodes would improve it.

An LED can be the voltage reference instead of two diodes. That can
temperature compensate Vbe. And it lights up! I think I did that a
while back and it worked pretty well.

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

That may oscillate. There are fixes for that. Use a bandgap as the
reference. And a mosfet to eliminate base current error.

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

A depletion fet, LND150 type, can be a simple constant-current source.
LND150 is about 1.6 mA but a source resistor can scale that down.

A 3-terminal voltage regulator, LM317 type, and a resistor make a
constant-current source.

An opamp follower and a bootstrap current source is nice. That can
make nanosecond linear ramps. Bootstraps are fun.

An opamp integrator makes a nice ramp.

Just a voltage, a resistor, and a cap make an exponential rise, which
is sorta linear. That\'s easy and can go very fast. Just live with or
compensate for the curvature. Add an inductor to make that more
linear.

If someone suggests a current mirror, it\'s probably a bad idea.

The ultimate horror: a PNP operating in beta-limited constant-current
mode. One transistor and one resistor. Beta-bracketed transistors like
BCX71 make that slightly less awful.

I did a linear ramp recently with a PV optocoupler driving a cap and
an n-channel mosfet follower, with amps of drive capability. It ramps
all the way up to the 48-volt power rail. It\'s a soft-start circuit
for a power inverter.

 

[Anthony William Sloman]

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.

--
Bill Sloman, Sydney

 

[Anthony William Sloman]

On Monday, June 19, 2023 at 8:25:27 PM UTC+10, John Larkin wrote:

On Mon, 19 Jun 2023 01:43:18 -0700 (PDT), \"sonnic...@gmail.com\"
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
That works but won\'t be very temperature stable. A zener or bandgap
instead of the two diodes would improve it.

An LED can be the voltage reference instead of two diodes. That can
temperature compensate Vbe. And it lights up! I think I did that a
while back and it worked pretty well.

Also a similar thing can be done with an op-amp
https://www.instructables.com/Constant-Current-Source-with-Operational-Amplifier/
That may oscillate. There are fixes for that. Use a bandgap as the
reference. And a mosfet to eliminate base current error.

But I was just wondering, are there other ways of doing this?
A depletion fet, LND150 type, can be a simple constant-current source.
LND150 is about 1.6 mA but a source resistor can scale that down.

A 3-terminal voltage regulator, LM317 type, and a resistor make a
constant-current source.

An opamp follower and a bootstrap current source is nice. That can
make nanosecond linear ramps. Bootstraps are fun.

An opamp integrator makes a nice ramp.

Just a voltage, a resistor, and a cap make an exponential rise, which
is sorta linear. That\'s easy and can go very fast. Just live with or
compensate for the curvature. Add an inductor to make that more
linear.

If someone suggests a current mirror, it\'s probably a bad idea.

John Larkin thinks that he can design electronic circuits. People who actually can are more fond of current mirrors, but they tend to recommend the three transistor Wilson current mirror or it\'s four transistor elaboration. Integrated circuits are full of current mirrors, but their designers have a lot more things that they can play with

> The ultimate horror: a PNP operating in beta-limited constant-current mode. One transistor and one resistor. Beta-bracketed transistors like BCX71 make that slightly less awful.

It is the sort of gross over-simplification that only gets mentioned for completeness.

> I did a linear ramp recently with a PV optocoupler driving a cap and an n-channel mosfet follower, with amps of drive capability.

MOSFETs can need a gate-stopper resistor - 10R/22R mostly - in series with the gate to discourage them from oscillating.

> It ramps all the way up to the 48-volt power rail. It\'s a soft-start circuit for a power inverter.

A rather expensive one.

--
Bill Sloman, Sydney

 

[Jasen Betts]

On 2023-06-19, 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)

So, a \"ramp generator\".

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?

Many, so many.

--
Jasen.
🇺🇦 Слава Україні

 

[Don Y]

On 6/19/2023 1:43 AM, 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)

How much control (repeatability, etc.) do you want over the current\'s
magnitude? Over how short of a time interval do you want to have this
occur? How linear must it be (or, just monotonically increasing?)

WHY do you want it?

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?

 

[Fred Bloggs]

On Monday, June 19, 2023 at 4:43:23 AM UTC-4, 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?

Make your question more specific by stating just exactly how \"really slow\" the ramp should be, the low to high voltage range of the ramp, the load using the ramp, the ultimate performance accuracy you\'re going for, circuit power supplies available, whether you want really simple low cost, easy to build, or not.

 

[Phil Hobbs]

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.

Cheers

Phil Hobbs

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

 

[John Larkin]

On Mon, 19 Jun 2023 13:35:11 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> 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.

I do that for my fastest linear ramps, nanosecond stuff. It avoids
making a really good fast low-capacitance constant-current source,
which can be a nuisance.

Some fraction of the world\'s fine-pitch ICs are fabbed by a controller
that includes this:

https://www.dropbox.com/scl/fi/shk7hp4d1t4t0nml3wtfb/TEM2_Time_Stamp.jpg?dl=0&rlkey=ktld7d8uwy5amt4ecbq2bobwv

Apologies for the trimpot; we had to do this in a rush.

Here\'s a later variant with two time ranges:

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?dl=0&rlkey=t3yz6hjhjv88juqexd384b5ar


Incidentally, does this link work?

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?raw=1

Dropbox is being obtuse again.

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.

Cheers

Phil Hobbs

 

[Don]

sonnic... 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 can concoct a current source out of a three-terminal voltage
regulator. Connect the regulator\'s Vout to the top of a reference
resistor and its Gnd/Adj/Set to the bottom. Then Iout = Vout / Rref.

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.

 

[Fred Bloggs]

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

--
Bill Sloman, Sydney

 

[John Larkin]

On Mon, 19 Jun 2023 08:57:16 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.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.

 

[Brian Gregory]

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.

--
Brian Gregory (in England).

 

[Fred Bloggs]

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.

 

[ehsjr]

On 6/19/2023 10:44 AM, John Larkin wrote:

<snip>

Incidentally, does this link work?

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?raw=1

Dropbox is being obtuse again.

Does not work.
Ed

 

[Phil Hobbs]

On 2023-06-19 10:44, John Larkin wrote:

On Mon, 19 Jun 2023 13:35:11 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> 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.


I do that for my fastest linear ramps, nanosecond stuff. It avoids
making a really good fast low-capacitance constant-current source,
which can be a nuisance.

Some fraction of the world\'s fine-pitch ICs are fabbed by a controller
that includes this:

https://www.dropbox.com/scl/fi/shk7hp4d1t4t0nml3wtfb/TEM2_Time_Stamp.jpg?dl=0&rlkey=ktld7d8uwy5amt4ecbq2bobwv

Apologies for the trimpot; we had to do this in a rush.

Here\'s a later variant with two time ranges:

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?dl=0&rlkey=t3yz6hjhjv88juqexd384b5ar

Fun. You don\'t have a nonlinear capacitance problem with the 74AUC2G07s?

Incidentally, does this link work?

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?raw=1

Dropbox is being obtuse again.

Wants a login. The others are OK in Firefox if I change them to \'dl=1\'
and use evince or gwenview.

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 14:00:11 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-19 10:44, John Larkin wrote:
On Mon, 19 Jun 2023 13:35:11 -0000 (UTC), Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> 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.


I do that for my fastest linear ramps, nanosecond stuff. It avoids
making a really good fast low-capacitance constant-current source,
which can be a nuisance.

Some fraction of the world\'s fine-pitch ICs are fabbed by a controller
that includes this:

https://www.dropbox.com/scl/fi/shk7hp4d1t4t0nml3wtfb/TEM2_Time_Stamp.jpg?dl=0&rlkey=ktld7d8uwy5amt4ecbq2bobwv

Apologies for the trimpot; we had to do this in a rush.

Here\'s a later variant with two time ranges:

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?dl=0&rlkey=t3yz6hjhjv88juqexd384b5ar


Fun. You don\'t have a nonlinear capacitance problem with the 74AUC2G07s?

Probably some. We did a polynomial calibration so a little curvature
is OK.

Incidentally, does this link work?

https://www.dropbox.com/scl/fi/10zr76x347kxktukb6msp/Fast_Ramp.jpg?raw=1

Dropbox is being obtuse again.

Wants a login. The others are OK in Firefox if I change them to \'dl=1\'
and use evince or gwenview.

Cheers

Phil Hobbs

 

[John Larkin]

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.

 

[John Larkin]

On Mon, 19 Jun 2023 09:09:18 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.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?