Single Op-Amp Current Amplifier
in [Design]
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From: Andy on 9 Sep 2005 13:45 <jpopelish(a)rica.net> wrote in message news:1126280554.538567.229710(a)g43g2000cwa.googlegroups.com... > > 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. > 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.
From: Robert Baer on 9 Sep 2005 22:54 Andy wrote: > "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 > > Maybe the Norton amplifier (LM2900, LM3900)?
From: Andy on 10 Sep 2005 10:05 "Robert Baer" <robertbaer(a)earthlink.net> wrote in message news:oVrUe.8030$4P5.857(a)newsread2.news.pas.earthlink.net... > Andy wrote: > >> "Ken Smith" <kensmith(a)green.rahul.net> wrote in message >> news:dfqjth$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 > Maybe the Norton amplifier (LM2900, LM3900)? 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.
From: John Woodgate on 10 Sep 2005 12:00
I read in sci.electronics.design that Andy <andrewpreece(a)onetel.com> wrote (in <4322e81a(a)212.67.96.135>) about 'Single Op-Amp Current Amplifier', on Sat, 10 Sep 2005: >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! 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. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |