Floating current source

A

alan

Guest
Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.
 
On Sat, 22 May 2004 01:41:06 -0700, the renowned alan
<no-longer-valid@yahoo.com> wrote:

Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.
Click to expand...
What kind of bandwidth? Galvanic isolation is one obvious way. If you
tranformer-isolate the power supply (for example, use an isolated
DC-DC converter) and use an isolation amplfier to get the input
voltage across then it's pretty straightforward, at least for
relatively low frequencies.

If you want to avoid that stuff, it'll depend a lot more on the
specifics of your situation.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On a sunny day (Sat, 22 May 2004 01:41:06 -0700) it happened alan
<no-longer-valid@yahoo.com> wrote in <c8n40j$q5d$1@news.Stanford.EDU>:

Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.
Some light bulbs (filament) behave like this, can't remember the name.
Click to expand...
Do not even know if they are still made.
But you could try a normal one.
JP
 
alan wrote:
Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.
Click to expand...
It's floating but "not really floating"- you need to get your head
screwed on straight before you sully the NG with this kind of confusion,
or take it to sci.basics.
 
On Sat, 22 May 2004 15:52:15 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com>
posted this:

On a sunny day (Sat, 22 May 2004 01:41:06 -0700) it happened alan
no-longer-valid@yahoo.com> wrote in <c8n40j$q5d$1@news.Stanford.EDU>:

Hello,

I am looking to design a voltage controlled current source.

Some light bulbs (filament) behave like this, can't remember the name.
Do not even know if they are still made.
But you could try a normal one.
JP
Click to expand...
AFAIK, those special light bulbs, barriter(sp) elements, are more or
less fixed at one current and not voltage controlled.

Jim
 
On Sat, 22 May 2004 01:41:06 -0700, alan <no-longer-valid@yahoo.com>
wrote:

Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.
Click to expand...
Switching power supply so that current "source" isn't bound to ground.

Optical command path. (Or possibly contained within the switching
mechanism.)

You might post "some other part of the circuit" so we can see what you
are trying to do. It might not need to "float" in the sense you are
thinking.

Sounds like (for a change) an actual DESIGN challenge. I hope you are
able to post specifics.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
On a sunny day (Sat, 22 May 2004 16:38:05 GMT) it happened James Meyer
<jmeyer@nowhere.com> wrote in <csuua09aom4uf8u7bgkos615a7kk61al1n@4ax.com>:

On Sat, 22 May 2004 15:52:15 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com
posted this:

On a sunny day (Sat, 22 May 2004 01:41:06 -0700) it happened alan
no-longer-valid@yahoo.com> wrote in <c8n40j$q5d$1@news.Stanford.EDU>:

Hello,

I am looking to design a voltage controlled current source.

Some light bulbs (filament) behave like this, can't remember the name.
Do not even know if they are still made.
But you could try a normal one.
JP

AFAIK, those special light bulbs, barriter(sp) elements, are more or
less fixed at one current and not voltage controlled.

Jim
I was my understanding that he needed 4 fixed currents, so 4 bulbs?
Click to expand...
JP
 
On Sat, 22 May 2004 17:18:23 GMT, the renowned Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

On a sunny day (Sat, 22 May 2004 16:38:05 GMT) it happened James Meyer
jmeyer@nowhere.com> wrote in <csuua09aom4uf8u7bgkos615a7kk61al1n@4ax.com>:

On Sat, 22 May 2004 15:52:15 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com
posted this:

On a sunny day (Sat, 22 May 2004 01:41:06 -0700) it happened alan
no-longer-valid@yahoo.com> wrote in <c8n40j$q5d$1@news.Stanford.EDU>:

Hello,

I am looking to design a voltage controlled current source.

Some light bulbs (filament) behave like this, can't remember the name.
Do not even know if they are still made.
But you could try a normal one.
JP

AFAIK, those special light bulbs, barriter(sp) elements, are more or
less fixed at one current and not voltage controlled.

Jim
I was my understanding that he needed 4 fixed currents, so 4 bulbs?
JP
Click to expand...
He said "four point" measurement- for example:

http://bridgetec.com/s-301.pdf

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On a sunny day (Sat, 22 May 2004 18:21:52 GMT) it happened Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote in
<i17va0d2rohous97jop1kmjm2tkltapv6f@4ax.com>:

AFAIK, those special light bulbs, barriter(sp) elements, are more or
less fixed at one current and not voltage controlled.

Jim
I was my understanding that he needed 4 fixed currents, so 4 bulbs?
JP

He said "four point" measurement- for example:

http://bridgetec.com/s-301.pdf

Best regards,
Spehro Pefhany
OK, I looked at that thing, for wafers?
Click to expand...
But this guy -the way I understand his post (I just reread it for the 3rd
time) will need 2 separate voltage sources, with 2 bulbs and 4 pins,
if 'all' is floating.
We probably need a lot more data on what is going on.
JP
 
On Sat, 22 May 2004 20:09:34 GMT, the renowned Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

On a sunny day (Sat, 22 May 2004 18:21:52 GMT) it happened Spehro Pefhany
speffSNIP@interlogDOTyou.knowwhat> wrote in
i17va0d2rohous97jop1kmjm2tkltapv6f@4ax.com>:

AFAIK, those special light bulbs, barriter(sp) elements, are more or
less fixed at one current and not voltage controlled.

Jim
I was my understanding that he needed 4 fixed currents, so 4 bulbs?
JP

He said "four point" measurement- for example:

http://bridgetec.com/s-301.pdf

Best regards,
Spehro Pefhany
OK, I looked at that thing, for wafers?
But this guy -the way I understand his post (I just reread it for the 3rd
time) will need 2 separate voltage sources, with 2 bulbs and 4 pins,
if 'all' is floating.
Click to expand...
One op-amp and a series voltage reference and a resistor, at a
minimum, once the power and input signal isolation is dealt with
(which can be either a project to sink your teeth into or a "pick out
the building blocks from the catalogs" exercise, depending on the
specifics of the requirements.

We probably need a lot more data on what is going on.
JP
Click to expand...
Bulbs?

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On a sunny day (Sat, 22 May 2004 20:17:51 GMT) it happened Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote in
<6pdva01h4np6dcmc0k32r30fcatkktkrgv@4ax.com>:
One op-amp and a series voltage reference and a resistor, at a
minimum, once the power and input signal isolation is dealt with
(which can be either a project to sink your teeth into or a "pick out
the building blocks from the catalogs" exercise, depending on the
specifics of the requirements.

We probably need a lot more data on what is going on.
JP

Bulbs?
Light bulbs, as current source..
Click to expand...
When the wire in the buklb gets hot, its resistance increases (a lot),
so you get some current stabilization.
JP
 
alan wrote:
Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.
Click to expand...
I have erad the various responses, and have a different "solution".
Use a Depletion-mode FET with switchable source resistors; switches
can be reed relays if need good leakage isolation.
If the current limiter must work in both polarities, then put two
back-to-back (add diodes to get rid of the resistor drop) OR use a FWB.
 
Spero Pefany wrote:
On Sat, 22 May 2004 01:41:06 -0700, the renowned alan
no-longer-valid@yahoo.com> wrote:


Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.


What kind of bandwidth?
Click to expand...
1kHz

Galvanic isolation is one obvious way. If you
tranformer-isolate the power supply (for example, use an isolated
DC-DC converter) and use an isolation amplfier to get the input
voltage across then it's pretty straightforward, at least for
relatively low frequencies.
Click to expand...
Yeah, that sounds about right...

Anyway, I was hoping that there would be a way to do it without
physically isolating things.
It just seems that it shouldn't be that hard to take an existing
circuit for a current source with one end grounded, but then make that
end float as well. Of course, I could build two opposite current
sources, but I don't think I could match up the source and sink currents
exactly.
 
In article <c8n40j$q5d$1@news.Stanford.EDU>,
alan <no-longer-valid@yahoo.com> wrote:

I am looking to design a voltage controlled current source. I
would like the current source to be bipolar and fully floating,
i.e. both the source and sink are totally floating. Or in other
words, the current that goes out the source HAS to come back
through the sink. The input is an AC voltage source (not really
floating). This circuit is used in a four-point measurement,
but some other part of the circuit "pushes the voltages around",
thus the need for the current source to be floating.
Click to expand...
I'll stick my neck out to get the discussion going.
Assuming the OP needs an AC sinewave current stimulus.

+---|>|---+---+---------+
| D1 | | |
| | )+ \
| | )sec1 /R1
| | )- \
| +| | D2 |
| C1=== +---|>|---+
| | | |
| | | |/e
| | '-------|PNP Q1
| | |\c
+ | | |
)|| | | +------>
)|| | +sec3- | | Iout
pri)|| +-//////--+-----/-------------->
)|| | | ======CT |
)|| | <--------+////+ |
+ | CT | | |
| <-------------' |/c
| | ,-------|NPN Q2
| | | |\e
| +| | D3 |
| C2=== +---|<|---+
| | | |
| | )- \
| | )sec2 /R2
| | )+ \
| D4 | | |
+--|<|----+---+---------+

It's a transformer with a pri and 3x secs. Two of the
secs (1 and 2) each drive a common base transistor,
PNP and NPN to provide alternating-polarity half-sine
output currents. The pri is voltage-driven by some
unspecified amplifier.

The main sec (sec3) delivers the total output power
needed for the PNP or NPN stages, plus the Vpk/Ipk
required by the unknown load. sec3 output is phased
with the sec1 and sec2 so that each stage is powered
up during each respective half-sine.

D1 and D4 prevent reverse currents flowing when
either transistor stage has a reverse voltage
across it. C1/C2 provide a small amount of dc
bias across a transistor stage at the start of
each half-sine.

Because it is a relatively open loop circuit it
would be prudent to measure the actual ac output
current and factor it's value (and actual shape)
into the unknown resistance calculation. A small
current-transformer (CT) provides this signal.

--
Tony Williams.
 
Jim Thompson wrote:

On Sat, 22 May 2004 01:41:06 -0700, alan <no-longer-valid@yahoo.com
wrote:


Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.


Switching power supply so that current "source" isn't bound to ground.

Optical command path. (Or possibly contained within the switching
mechanism.)

You might post "some other part of the circuit" so we can see what you
are trying to do.
Click to expand...
The setup is a four-point AC measurement across a sample, with some
extra goodies thrown in. There is an additional scanning probe that
touches down somewhere along the sample. Another lead that is connected
to the end of the sample pushes the sample to whatever voltage such that
the current coming out of the first lead I meantioned is zero. (In
other words, zeros out the voltage in the middle of the sample) That is
why the current souce needs to be floating. This is a circuit for
scanning potentiometry.

It might not need to "float" in the sense you are
thinking.
Click to expand...
That's true - I'll have to think of alternatives.
 
On Sun, 23 May 2004 04:07:59 -0700, alan <no-longer-valid@yahoo.com>
wrote:

Jim Thompson wrote:

On Sat, 22 May 2004 01:41:06 -0700, alan <no-longer-valid@yahoo.com
wrote:


Hello,

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.


Switching power supply so that current "source" isn't bound to ground.

Optical command path. (Or possibly contained within the switching
mechanism.)

You might post "some other part of the circuit" so we can see what you
are trying to do.

The setup is a four-point AC measurement across a sample, with some
extra goodies thrown in. There is an additional scanning probe that
touches down somewhere along the sample. Another lead that is connected
to the end of the sample pushes the sample to whatever voltage such that
the current coming out of the first lead I meantioned is zero. (In
other words, zeros out the voltage in the middle of the sample) That is
why the current souce needs to be floating. This is a circuit for
scanning potentiometry.

It might not need to "float" in the sense you are
thinking.

That's true - I'll have to think of alternatives.
Click to expand...
(1) Post a drawing to alt.binaries.schematics.electronics

(2) Magnitude of current

(3) Maximum voltage drop across the sample

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
On a sunny day (Sun, 23 May 2004 11:14:14 +0100) it happened Tony Williams
<tonyw@ledelec.demon.co.uk> wrote in <4cb388dc5ctonyw@ledelec.demon.co.uk>:

In article <c8n40j$q5d$1@news.Stanford.EDU>,
alan <no-longer-valid@yahoo.com> wrote:

I am looking to design a voltage controlled current source. I
would like the current source to be bipolar and fully floating,
i.e. both the source and sink are totally floating. Or in other
words, the current that goes out the source HAS to come back
through the sink. The input is an AC voltage source (not really
floating). This circuit is used in a four-point measurement,
but some other part of the circuit "pushes the voltages around",
thus the need for the current source to be floating.

I'll stick my neck out to get the discussion going.
Assuming the OP needs an AC sinewave current stimulus.

+---|>|---+---+---------+
| D1 | | |
| | )+ \
| | )sec1 /R1
| | )- \
| +| | D2 |
| C1=== +---|>|---+
| | | |
| | | |/e
| | '-------|PNP Q1
| | |\c
+ | | |
)|| | | +------
)|| | +sec3- | | Iout
pri)|| +-//////--+-----/--------------
)|| | | ======CT |
)|| | <--------+////+ |
+ | CT | | |
| <-------------' |/c
| | ,-------|NPN Q2
| | | |\e
| +| | D3 |
| C2=== +---|<|---+
| | | |
| | )- \
| | )sec2 /R2
| | )+ \
| | | |
Click to expand...
| | |-/\/\- |
| D4 | | to+ |
| | --- |
| | \ / |
| | --- |
| | | |
+--|<|----+---+---------+
Click to expand...
OK, if we go this way, you have some cross-over distortion,
as the Q1 and Q2 need 0.7V to bias.

Since the be voltage is in the direction of the opposite
supply, perhaps you could add a diode in series with the 'cold'
sie (side connected to the supply lines) o fsec1 and sec3,
with a resistor to the opposite supply, to create a .7V bias.
(I have only drawn the negative side in the diagram).
 
On Sun, 23 May 2004 04:07:59 -0700, alan <no-longer-valid@yahoo.com> posted
this:


The setup is a four-point AC measurement across a sample, with some
extra goodies thrown in. There is an additional scanning probe that
touches down somewhere along the sample. Another lead that is connected
to the end of the sample pushes the sample to whatever voltage such that
the current coming out of the first lead I meantioned is zero. (In
other words, zeros out the voltage in the middle of the sample) That is
why the current souce needs to be floating. This is a circuit for
scanning potentiometry.

It might not need to "float" in the sense you are
thinking.

That's true - I'll have to think of alternatives.
Click to expand...
If you took an op-amp and added a current sense resistor to its output
and voltage sensors before the sense resistor and after the sense resistor, the
output of the whole thing could be programmed, or "servoed", to look like a
floating current source and a voltage sensing probe at the same time. Use as
many of those as you need to make your measurements. Of course there will be
common mode restraints associated with the op-amps, but all you have to do is
power them all from the same plus and minus supplies with the common of the
supplies connected anywhere on the sample.

Programmed properly, the output currents of the op-amps could be made to
follow each other closely enough, some sourcing and some sinking, so that no
current would flow in the sample-to-common connection lead and the whole thing
would look like it was completely floating as far as the sample were concerned.

Jim
 
On Sun, 23 May 2004 16:46:44 GMT, James Meyer <jmeyer@nowhere.com>
wrote:

On Sun, 23 May 2004 04:07:59 -0700, alan <no-longer-valid@yahoo.com> posted
this:



The setup is a four-point AC measurement across a sample, with some
extra goodies thrown in. There is an additional scanning probe that
touches down somewhere along the sample. Another lead that is connected
to the end of the sample pushes the sample to whatever voltage such that
the current coming out of the first lead I meantioned is zero. (In
other words, zeros out the voltage in the middle of the sample) That is
why the current souce needs to be floating. This is a circuit for
scanning potentiometry.

It might not need to "float" in the sense you are
thinking.

That's true - I'll have to think of alternatives.

If you took an op-amp and added a current sense resistor to its output
and voltage sensors before the sense resistor and after the sense resistor, the
output of the whole thing could be programmed, or "servoed", to look like a
floating current source and a voltage sensing probe at the same time. Use as
many of those as you need to make your measurements. Of course there will be
common mode restraints associated with the op-amps, but all you have to do is
power them all from the same plus and minus supplies with the common of the
supplies connected anywhere on the sample.

Programmed properly, the output currents of the op-amps could be made to
follow each other closely enough, some sourcing and some sinking, so that no
current would flow in the sample-to-common connection lead and the whole thing
would look like it was completely floating as far as the sample were concerned.

Jim
Click to expand...
Absolutely... pretty much what I have been thinking. I don't even
think the extra probe is needed to establish "zeros out the voltage in
the middle of the sample".

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
alan wrote...
Spero Pefany wrote:
On Sat, 22 May 2004 01:41:06 -0700, the renowned alan wrote:

I am looking to design a voltage controlled current source. I would
like the current source to be bipolar and fully floating, i.e. both the
source and sink are totally floating. Or in other words, the current
that goes out the source HAS to come back through the sink. The input
is an AC voltage source (not really floating). This circuit is used in
a four-point measurement, but some other part of the circuit "pushes the
voltages around", thus the need for the current source to be floating.

The only circuits for (voltage controlled) current sources that I have
seen have one end of the load grounded. Also, I am not sure how to feed
in the controlling voltage and still have the circuit floating.

What kind of bandwidth?

1kHz

Galvanic isolation is one obvious way. If you tranformer-isolate
the power supply (for example, use an isolated DC-DC converter) and
use an isolation amplfier to get the input voltage across then it's
pretty straightforward, at least for relatively low frequencies.

Yeah, that sounds about right...

Anyway, I was hoping that there would be a way to do it without
physically isolating things. It just seems that it shouldn't be that
hard to take an existing circuit for a current source with one end
grounded, but then make that end float as well. Of course, I could
build two opposite current sources, but I don't think I could match
up the source and sink currents exactly.
Click to expand...
A few questions, and a few comments. How much current do you need?
With what voltage-compliance range? How "quiet" does it need to be?

As for bipolar current-sources, at low currents you can use OTA, or
transconductance oapmps. For moderate currents there are the Howland
(AoE pg 182) and diff-amp (AoE pg 254, L.) current-source circuits.
But these suffer the disadvantage of using opamp output stages: lower
output impedance at high frequencies (i.e. they then act more like
voltage sources than current sources). They can have weaknesses.
http://groups.google.com/groups?as_umsgid=49944ebb10tonyw%40ledelec.demon.co.uk

Tony Williams and I have posted several variants of a flexible bipolar
current-source circuit that maintains a good high output impedance to
higher frequencies (it has less apparent parallel capacitance). This
circuit is well-suited to operate to many amps and hundreds of volts.

You could use two such circuits, and adjust the "gain" of one side to
insure zero current into a grounded load. Or you could supplement a
trimmed higher-power current-source pair with a low-current OTA stage
to perfectly null the ground current.

p.s. The bipolar current-source circuit I refer to is not at all like
the one Tony just posted, e.g., see this article,
http://groups.google.com/groups?as_umsgid=3e852f41.0205030545.4751a62%40posting.google.com
(There are better postings of this circuit by Tony and me, including a
detailed design thread, but I couldn't find these just now on Google.)

If you like, I can email you a production drawing of this circuit,
designed to be operated as a pair in floating bridge mode. Actually,
I used four of them to create a 4-quadrant floating current source,
for externally controlling the vector of currents in a 2-D surface.
It was used last year for an interesting experiment with worms. :>)

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
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