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From: Andy on 9 Sep 2005 09:34

"Ken Smith" <kensmith(a)green.rahul.net> wrote in message
news:dfqjth$b6d$1(a)blue.rahul.net...
> In article <MPG.1d8a9732f331f74a989700(a)news.bright.net>,
> Ol' Duffer <DontSend(a)MeSpam.net> wrote:
> [...]
> Labels added:
>
>
> R1 Ra
>>Vin1---RRRR--+---RRRR--+---Iout
>> | |
>> | |\ R Rb
>> | | \ R
>> +---|+ \ |
>> | >-+
>> +---|- / |
>> | | / |
>> | |/ |
>> | |
>>Vin2---RRRR--+---RRRR--+
>> R2 R3
>
>>The equations tend to be messy since a portion of load current
>>also flows in the feedback,
>
> If you think of Ra and Rb as one resistor (r4) with a tab at the Iout, the
> math is a lot less messy.
>
> For infinite impedance:
>
> R1 = R2
> R3 = R4 = K*R1
>
> Iout = Vin * K / Ra
>
>

Thanks for that, yes, I did consider the Howland Current pump, but
didn't give it too much attention as it apparently has a voltage input,
however, rereading my own question, I see that I asked about
voltage-current and current-current amps, so the Howland would
qualify as a voltage-current amp with a load referenced to zero.
I shall look at it again.

That only leaves me with a proper single op-amp current-current
amp to find!

Andy


From: Fred Bloggs on 9 Sep 2005 09:46



>I just found it
> a little improbable that these circuits hadn't been invented years ago
> by some bright spark, and thought that they weren't shown in my text
> books because they were little used.

You are talking about a transconductance amplifier, voltage-to-current,
they are all smoke and mirrors, and a current-to-current is left as an
exercise for the student.


From: Ken Smith on 9 Sep 2005 10:43
In article <43218f71(a)212.67.96.135>, Andy <andrewpreece(a)onetel.com> wrote:

Modifies again:

>>>Iin1---------+---RRRR--+---Iout
>>> | |
>>> | |\ R Rb
>>> | | \ R
>>> +---|+ \ |
>>> | >-+
>>> +---|- / |
>>> | | / |
>>> | |/ |
>>> | |
>>>Iin2---------+---RRRR--+
>>> R2 R3
[...]
>That only leaves me with a proper single op-amp current-current
>amp to find!

--
--
kensmith(a)rahul.net forging knowledge

From: John Woodgate on 9 Sep 2005 11:19
I read in sci.electronics.design that Andy <andrewpreece(a)onetel.com>
wrote (in <43218f71(a)212.67.96.135>) about 'Single Op-Amp Current
Amplifier', on Fri, 9 Sep 2005:

>That only leaves me with a proper single op-amp current-current amp to
>find!

Ground V1in and leave out R2. Current-current!
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
From: jpopelish on 9 Sep 2005 11:42

Andy wrote:
(snip)
> That only leaves me with a proper single op-amp current-current
> amp to find!

It doesn't have a ground connected load (but a virtul ground connected
load), but an inverting amplifier with no input resistor and the load
as the feedbback resistor is a current to current configuration.

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