# Single Op-Amp Current Amplifier

• posted

The ideal current input amplifier has zero ohms input resistance, so it can pass whatever input current the source offers. The virtual ground of an inverting amplifier is a pretty fair approximation of such a beast.

A current output amplifier, ideally has infinite output resistance, and has feedback based on the current through the load, not the voltage across it. This feedback is often taken from the voltage drop across a low value resistor in series with the load current.

If you want a current to current amplifier, you may need to use two opamps, one to sum the input and feedback currents, and one to amplify the small drop across the current sense resistor and produce a current for the input current summing junction.

• posted

I've been revisiting my op-amp theory, and note that there are cicuits for voltage-voltage amplifiers (non-inverting ), current-voltage amplifiers ( see-saw or inverting amplifier ), but that it's a little more obscure with voltage-current and current-current amplifiers.

I've managed to derive the last two configurations but the load is floating in both cases. Of course I understand that, for instance, a voltage follower provides current gain, but it is not what I would call a voltage-current amplifier, the reason being that if you were to change the value of the output load, the output current would change too ( as the o/p voltage remains constant ).

A real voltage-current amplifier would give a gain of a constant value of Amps/Volt, even if the output load varied in resistance. Same with a current-current amplifier, it would have a constant gain of whatever Amps/Amp even if the load varied i.e it would apparently have a current source output.

So, has anyone come across either of these last two mythical beasts, with, like I say, a non-floating load ( i.e. one end referenced to ground ).

thanks,

Andy.

• posted

Yes, that idea ocurred to me though I haven't drawn it out. 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. Like I say, you can make current input and current output opamp circuits, but my intellect has only succeeded by allowing both ends of the load resistor to be floating, which must severely limit the utility of these circuits.

Perhaps it is something to do with the voltage amplifier nature of op-amps, after all with high input impedance and low input impedance they're the opposite of what you want in a current amplifier.

thanks,

Andy.

• posted

Have you heard of the Howland Current Pump? It is a 5-resistor configuration that drives current into a grounded load. The input is a floating voltage, so one side of it could be grounded if necessary.

Vin1---RRRR--+---RRRR--+---Iout | | | |\\ R | | \\ R +---|+ \\ | | >-+ +---|- / | | | / | | |/ | | | Vin2---RRRR--+---RRRR--+

From memory - I hope I got it right.

The equations tend to be messy since a portion of load current also flows in the feedback, but you can make the output sense resistor significantly lower in value than the feedback resistors and pre-calculate out the error.

Also, its input would ideally connect to a zero impedance source, but if the source happened to be a sense resistor of significantly lower value than the feedback resistors, it seems that it also could be pre-calculated out.

• posted

In article , Ol' Duffer wrote: [...] Labels added:

R1 Ra

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

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kensmith@rahul.net   forging knowledge```
• posted

Andy wrote: (snip)

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.

• posted

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.

• posted

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

• posted

Modifies again:

[...] >That only leaves me with a proper single op-amp current-current
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kensmith@rahul.net   forging knowledge```
• posted

I read in sci.electronics.design that Andy wrote (in ) about 'Single Op-Amp Current Amplifier', on Fri, 9 Sep 2005:

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.```
• posted

Ha, I spotted that one! In fact I drew a few block diagrams of a two input port/two output port amplifier and fiddled around with the feedback until I got a configuration that was current output, and when I translated it back to an op-amp I found I had what you suggest. The trouble with all my 'breakthroughs', is that they always have a snag, and it is the floating load in this case. Doubtless a useful circuit in special circumstances though.

I will look at the suggestions regarding the Howland current souce and modifying it, that might work as another poster suggested.

thanks All,

Andy.

• posted

Maybe the Norton amplifier (LM2900, LM3900)?

• posted

This is a different internal design to most coventional op-amps, no? I ought to say, that I m not actually trying to make a current-current op-amp, I'm sure it could be done with, say, two conventional op-amps, I'm merely trying to fill gaps in my knowledge, and it seemed to me that a single op-amp current-current amplifier must exist so I thought the quickest way to find out would be to ask, but it hasn't been that simple.

I imagine it wouldn't be any more difficult to design an op-amp that had zero-ish input impedance and infinite-ish output impedance, (Norton architecture ?) than it is to design the usual sort with infinite-ish input impedance and zero-ish output ipedance ( Thevenim architecture?). I'm only surprised there aren't equal numbers of both sorts in production. That's another question I'd like to hear the answer to!

Andy.

• posted

I read in sci.electronics.design that Andy wrote (in ) about 'Single Op-Amp Current Amplifier', on Sat, 10 Sep 2005:

Nobody understands low input-impedance (current-mode) technology. The LM13600 wasn't popular at all, but the LM13700 was a bit more successful.

```--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.```

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