Current divider mirror

Tim Williams
Guest

Thu Feb 04, 2010 2:08 am

I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

I'd just do it with one of those ten emitter transistors that ICs are so
fond of using, but they aren't variable geometry...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

John Larkin
Guest

Thu Feb 04, 2010 3:31 am

On Wed, 3 Feb 2010 18:08:04 -0600, "Tim Williams"
<tmoranwms_at_charter.net> wrote:

Quote:

I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

I'd just do it with one of those ten emitter transistors that ICs are so
fond of using, but they aren't variable geometry...

Tim

Do you mean things like this?

ftp://jjlarkin.lmi.net/Mirror1.JPG

ftp://jjlarkin.lmi.net/Mirror2.JPG

John

Jim Thompson
Guest

Thu Feb 04, 2010 3:43 am

On Wed, 03 Feb 2010 17:31:16 -0800, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

Quote:

On Wed, 3 Feb 2010 18:08:04 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:

I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

I'd just do it with one of those ten emitter transistors that ICs are so
fond of using, but they aren't variable geometry...

Tim

Do you mean things like this?

ftp://jjlarkin.lmi.net/Mirror1.JPG

ftp://jjlarkin.lmi.net/Mirror2.JPG

John

Jamie
Guest

Thu Feb 04, 2010 4:03 am

Tim Williams wrote:

Quote:

I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

I'd just do it with one of those ten emitter transistors that ICs are so
fond of using, but they aren't variable geometry...

Tim

Are you looking for a totem pole current mirror?

Tim Williams
Guest

Thu Feb 04, 2010 4:32 am

On Feb 3, 7:31 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

Quote:

Do you mean things like this?

ftp://jjlarkin.lmi.net/Mirror1.JPG

ftp://jjlarkin.lmi.net/Mirror2.JPG

No, offset is an issue. Example: a bipolar current mirror will work
over as many decades as the transistors do (assuming equal
temperatures, which works for monolithic, not as well for discrete,
but is capable of working roughly), but an op-amp mirror gets lost in
Vos. I'd rather not tune microvolts of offset, or amplify the noise.
A bipolar mirror would work if I basically had a 1:100 variable
emitter width.

Specifically I want something that works well from 1uA to 5mA within
+/-5 or 10V rails.

Tim

John Larkin
Guest

Thu Feb 04, 2010 5:12 am

On Wed, 3 Feb 2010 18:28:40 -0800 (PST), Tim Williams
<tmoranwms_at_gmail.com> wrote:

Quote:

On Feb 3, 7:31 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Do you mean things like this?

ftp://jjlarkin.lmi.net/Mirror1.JPG

ftp://jjlarkin.lmi.net/Mirror2.JPG

No, offset is an issue. Example: a bipolar current mirror will work
over as many decades as the transistors do (assuming equal
temperatures, which works for monolithic, not as well for discrete,
but is capable of working roughly), but an op-amp mirror gets lost in
Vos. I'd rather not tune microvolts of offset, or amplify the noise.
A bipolar mirror would work if I basically had a 1:100 variable
emitter width.

Specifically I want something that works well from 1uA to 5mA within
+/-5 or 10V rails.

Tim

Specs? You have specs? How about noise, bamdwidth, cost? Do you intend
it to be an adjustable ratio? How would it be adjusted?

Use a chopper amp and biggish resistors to keep the offset low.

John

Tim Williams
Guest

Thu Feb 04, 2010 6:34 am

"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:cjekm59tmt7i00sp9dd4fqfc1n1t3ch2ut_at_4ax.com...

Quote:

Specs? You have specs? How about noise, bamdwidth, cost?

Noise: low enough. It's charging a timing capacitor, so HF noise doesn't
matter as much as 1/f noise. Let's say current noise of 2nA or less.
Likewise, the current itself is a DC signal (< 100Hz), bandwidth doesn't
really matter.
The output needs ~MHz compliance, which won't matter around a drain /
collector output (if nothing else, it can be cascoded).
Cost: low, and should be simple (i.e., that rules out a logarithmic
ADC/DSP/DAC).

Quote:

Do you intend
it to be an adjustable ratio? How would it be adjusted?

Potentiometer, or something else panel adjustable. Doesn't have to be
electronically controlled.

Quote:

Use a chopper amp and biggish resistors to keep the offset low.

I suppose a chopper amp could be filtered to remove its noise. Something
less than 1k would generate somewhat less than 5V at full current, and less
than 100uV at the lowest range (assuming something like the
opamp-FET-feedback resistor circuit).

On principle, I don't like using something additive (a linear amp) when the
nature of the signal is logarithmic. So the obvious answer is a logarithmic
converter, which is basically a glorified diode. Maybe a simplified log-exp
amp would work anyway with reasonable stability, while keeping noise and
offset sensible? Gain would also be electronically controllable by adding
to the log signal. Of course, the simplest log-exp converter is the current
mirror, but unequal current densities aren't temp compensated. Maybe they
can be. Temp compensation isn't something I've explored much and may be
worth a romp to figure out...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Robert Baer
Guest

Thu Feb 04, 2010 11:56 am

Tim Williams wrote:

Quote:

I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

I'd just do it with one of those ten emitter transistors that ICs are so
fond of using, but they aren't variable geometry...

Tim

That is what a variable resistor in the emitter is for..

Jim Thompson
Guest

Thu Feb 04, 2010 4:40 pm

On Thu, 04 Feb 2010 01:56:38 -0800, Robert Baer
<robertbaer_at_localnet.com> wrote:

Quote:

Tim Williams wrote:
I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

I'd just do it with one of those ten emitter transistors that ICs are so
fond of using, but they aren't variable geometry...

Tim

That is what a variable resistor in the emitter is for..

whit3rd
Guest

Fri Feb 05, 2010 11:48 am

On Feb 3, 4:08 pm, "Tim Williams" <tmoran...@charter.net> wrote:

Quote:

I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

The schemes John Larkin gave are unipolar, and 'current from +V or -V'
sounds like bipolar to me.

If it's for bipolar current, use one op amp as current/voltage
converter (this
means the pseudoground input is your input load, which is ideal), then
a second op amp with current mirrors on its power pins to convert back
to a current; the ratio of resistors sets the current gain

(warning bad ASCII art follows)

+-----Rf----+
| |\ |
I input +--------+---|- \ |
| >--+--+
GND--|+ / |
|/ |
|
|
|
+---------------------+
| (+V)
| |
| {PNP mirror}
| | |
| |\| |
+------|+ \ +------- I_out= I_input*Rf/Rg
| >-+ |
GND --Rg---+----|- / | |
| |/| | |
| | | |
+-----------+ |
| |
{NPN mirror}
|
(-V)

John Larkin
Guest

Fri Feb 05, 2010 6:24 pm

On Fri, 5 Feb 2010 00:48:09 -0800 (PST), whit3rd <whit3rd_at_gmail.com>
wrote:

Quote:

On Feb 3, 4:08 pm, "Tim Williams" <tmoran...@charter.net> wrote:
I need a current mirror that has a current coming in from +V or -V,
reproduces that current from +V, and produces a current to -V which is about
0.1 to 10 times the input current.

The schemes John Larkin gave are unipolar, and 'current from +V or -V'
sounds like bipolar to me.

If it's for bipolar current, use one op amp as current/voltage
converter (this
means the pseudoground input is your input load, which is ideal), then
a second op amp with current mirrors on its power pins to convert back
to a current; the ratio of resistors sets the current gain

(warning bad ASCII art follows)

+-----Rf----+
| |\ |
I input +--------+---|- \ |
| >--+--+
GND--|+ / |
|/ |
|
|
|
+---------------------+
| (+V)
| |
| {PNP mirror}
| | |
| |\| |
+------|+ \ +------- I_out= I_input*Rf/Rg
| >-+ |
GND --Rg---+----|- / | |
| |/| | |
| | | |
+-----------+ |
| |
{NPN mirror}
|
(-V)

That's the topology of my MRI gradient amps. We sell units that go
from 3 amps to 120 amps peak output. The big one peaks at about 20
kilowatts out.

That wouldn't be a bad audio amp topology, either. You can make the
crossover essentially perfect, and it uses 100% of the available
supplies, all biasing is tightly controlled, fet load matching is
excellent, and it has some nice supervision/protection hooks. No, on
second thought, this is way too precise for audio.

John

Tim Williams
Guest

Fri Feb 05, 2010 7:21 pm

"whit3rd" <whit3rd_at_gmail.com> wrote in message
news:152e5f52-6af1-4cb3-aa39-0ecf74e30db1_at_q4g2000yqm.googlegroups.com...

Quote:

The schemes John Larkin gave are unipolar, and 'current from +V or -V'
sounds like bipolar to me.

No, it's unipolar, I meant the currents are sourced from the +V rail, or
sunk into the -V rail.

Your circuit reminds me of,
http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Current_Amplifer.gif
which isn't to be left alone on the breadboard unless the transistors are on
a fairly large heatsink. ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

JosephKK
Guest

Fri Feb 05, 2010 8:03 pm

On Wed, 3 Feb 2010 18:28:40 -0800 (PST), Tim Williams <tmoranwms_at_gmail.com> wrote:

Quote:

On Feb 3, 7:31 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Do you mean things like this?

ftp://jjlarkin.lmi.net/Mirror1.JPG

ftp://jjlarkin.lmi.net/Mirror2.JPG

No, offset is an issue. Example: a bipolar current mirror will work
over as many decades as the transistors do (assuming equal
temperatures, which works for monolithic, not as well for discrete,
but is capable of working roughly), but an op-amp mirror gets lost in
Vos. I'd rather not tune microvolts of offset, or amplify the noise.
A bipolar mirror would work if I basically had a 1:100 variable
emitter width.

Specifically I want something that works well from 1uA to 5mA within
+/-5 or 10V rails.

Tim

Do you need continuously adjustable or can you live with discrete steps
on the current ratio?

John Larkin
Guest

Fri Feb 05, 2010 9:39 pm

On Fri, 5 Feb 2010 11:20:02 -0800 (PST), whit3rd <whit3rd_at_gmail.com>
wrote:

Quote:

On Feb 5, 8:24 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 5 Feb 2010 00:48:09 -0800 (PST), whit3rd <whit...@gmail.com
wrote:

[current amplifier with complementary current mirrors as final stage]

That wouldn't be a bad audio amp topology, either.... No, on
second thought, this is way too precise for audio.

It does very much look odd, the first time, looking
at that big resistor to ground, and nothing else on the 'output' pin.

There's a bit of difficulty if the intended output is a low current;
the quiescent
op amp drain and the deviation from unity of the mirror gain interact.
That makes an output offset current of Iq* (Gpnp - Gnpn).
To trim that, you put a pot from the op amp V+ supply to the V- supply
and ground the wiper (through a big resistor).

The original inquiry mentioned a range in the low microamps, will
probably
require such trim. It looks as odd as the 'output' resistor.

I use active mirrors, very precise, with an opamp per fet, so all the
fet currents are very well controlled and matched. Various units use
one to 16 fets per mirror.

ftp://jjlarkin.lmi.net/Amp.jpg

My quiescent current problem is the opposite: most opamps have too
much Iq compared to Imax, so my mirror idle currents get too high.
That's fixed by finding the right opamp and adding some off-direction
offset to the mirrors. I do a pot from opamp V+ to V- to set fet idle
currents.

The crossover graphs are esentially perfect.

John

whit3rd
Guest

Fri Feb 05, 2010 10:20 pm

On Feb 5, 8:24 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

Quote:

On Fri, 5 Feb 2010 00:48:09 -0800 (PST), whit3rd <whit...@gmail.com
wrote:

[current amplifier with complementary current mirrors as final stage]

Quote:

That wouldn't be a bad audio amp topology, either.... No, on
second thought, this is way too precise for audio.

It does very much look odd, the first time, looking
at that big resistor to ground, and nothing else on the 'output' pin.

There's a bit of difficulty if the intended output is a low current;
the quiescent
op amp drain and the deviation from unity of the mirror gain interact.
That makes an output offset current of Iq* (Gpnp - Gnpn).
To trim that, you put a pot from the op amp V+ supply to the V- supply
and ground the wiper (through a big resistor).

The original inquiry mentioned a range in the low microamps, will
probably
require such trim. It looks as odd as the 'output' resistor.