Current divider mirror

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

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
>

Are you looking for a totem pole current mirror?

"John Larkin" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> 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).

> 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.

> 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

"whit3rd" <(E-Mail Removed)> wrote in message
news:152e5f52-6af1-4cb3-aa39-(E-Mail Removed)...
> 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/t...t_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

On Wed, 3 Feb 2010 18:28:40 -0800 (PST), Tim Williams <(E-Mail Removed)> wrote:

>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?

"JosephKK" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Do you need continuously adjustable or can you live with discrete steps
> on the current ratio?

Continuous, or sufficiently continuous (>= 8 bits?).

I suppose an alternate formulation might be, if it can't be varied reliably
over a wide range, then it could be varied over a small dither range while
the switching handles the wide range, but that would quickly get ugly..

Tim

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

On Fri, 05 Feb 2010 21:02:46 -0800, John Larkin <(E-Mail Removed)> wrote:

>On Fri, 5 Feb 2010 11:21:05 -0600, "Tim Williams"
><(E-Mail Removed)> wrote:
>
>>"whit3rd" <(E-Mail Removed)> wrote in message
>>news:152e5f52-6af1-4cb3-aa39-(E-Mail Removed)....
>>> 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/t...t_Amplifer.gif
>>which isn't to be left alone on the breadboard unless the transistors are on
>>a fairly large heatsink. ;-)
>>
>>Tim
>
>Using the opamp rail currents gives essentially perfect crossover
>behavior. At zero signal, both mirrors run at their Iq. As you apply
>signal in one direction, one mirror current goes up and the other
>stays at Iq. That's different from most class AB stages, where as one
>side conducts, the other is turned off... which can lead to weird
>transient distortions.
>
>John

And to think i cribbed that trick from datasheets and application
notes in 1965 to build my first self designed stereo.

On Sat, 6 Feb 2010 19:42:35 -0600, "Tim Williams" <(E-Mail Removed)>wrote:

>"JosephKK" <(E-Mail Removed)> wrote in message
>news:(E-Mail Removed).. .
>> Do you need continuously adjustable or can you live with discrete steps
>> on the current ratio?
>
>Continuous, or sufficiently continuous (>= 8 bits?).
>
>I suppose an alternate formulation might be, if it can't be varied reliably
>over a wide range, then it could be varied over a small dither range while
>the switching handles the wide range, but that would quickly get ugly..
>
>Tim

For fully continuous i would look at a application of a Gilbert cell multiplier.
There are some relatively exposed Gilbert cell ICs available.

For a steppy solution an 8 or more bit (multiplying) current diverting DAC.
Though i am not sure if a DAC will work well over two decades of reference
current, you may have to subrange to two or three steps (that may overlap).

Both solutions will require some appropriate scaling before or after or both.

On Sat, 06 Feb 2010 20:01:31 -0800, John Larkin <(E-Mail Removed)> wrote:

>On Sat, 06 Feb 2010 19:45:38 -0800,
>"JosephKK"<(E-Mail Removed)> wrote:
>
>>On Fri, 05 Feb 2010 21:02:46 -0800, John Larkin <(E-Mail Removed)> wrote:
>>
>>>On Fri, 5 Feb 2010 11:21:05 -0600, "Tim Williams"
>>><(E-Mail Removed)> wrote:
>>>
>>>>"whit3rd" <(E-Mail Removed)> wrote in message
>>>>news:152e5f52-6af1-4cb3-aa39-(E-Mail Removed)....
>>>>> 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/t...t_Amplifer.gif
>>>>which isn't to be left alone on the breadboard unless the transistorsare on
>>>>a fairly large heatsink. ;-)
>>>>
>>>>Tim
>>>
>>>Using the opamp rail currents gives essentially perfect crossover
>>>behavior. At zero signal, both mirrors run at their Iq. As you apply
>>>signal in one direction, one mirror current goes up and the other
>>>stays at Iq. That's different from most class AB stages, where as one
>>>side conducts, the other is turned off... which can lead to weird
>>>transient distortions.
>>>
>>>John
>>
>>And to think i cribbed that trick from datasheets and application
>>notes in 1965 to build my first self designed stereo.
>
>This config is ideal for driving current into inductive loads, like
>MRI coils. The inherently high output impedance makes closed-loop
>dynamics mostly independent of load.
>
>For audio, where you want a low output impedance, it's not ideal. The
>fix is to apply massive negative feedback, which requires tons of GBW
>to waste at higher frequencies. That was probably hard to do in 1965.
>
>Got a schematic?
>
>John

I doubt if i still have the schematic. I should be able to do a fresh
instance with current parts. In a couple of days as an LTspice file OK.
The parts to the original may be unobtainium now.

"JosephKK" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> For fully continuous i would look at a application of a Gilbert cell
> multiplier.
> There are some relatively exposed Gilbert cell ICs available.

Hmm. I have some MC1496's laying around, that's halfway there. Or use a
TIA. I wouldn't need four quadrants, one would be enough. That would even
suggest a JFET. The problem is making the 0.1 to 10 ratio repeatable with
an output from 50nA to 50mA (maybe not 50 on the highest range, I could
settle for 5mA max.). That means putting the programming current into the
tail is tricky (a regular current mirror would do okay), and the output can
only be varied from 1 to 0.01 instead of 0.1 to 10 times (well, close
enough).

In this circuit,
http://webpages.charter.net/dawill/t...8/Triangle.gif
I did something kinda similar, but more simply adjustable, and obviously the
ratio is fixed. As I recall, duty cycle was fairly consistent across the
range, and at cutoff it was on the order of nanoamps through the 2N440x's.
If the multiplier's ratio is constant for a given differential voltage
offset (which will have to be temperature compensated, but that would simply
mean sourcing that offset voltage from a diode), then this might work fairly
well.

Thanks, this is something to think about.

Tim

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