Current divider mirror
in [Design]
|
From: John Larkin on 5 Feb 2010 11:24 On Fri, 5 Feb 2010 00:48:09 -0800 (PST), whit3rd <whit3rd(a)gmail.com> wrote: >On Feb 3, 4:08�pm, "Tim Williams" <tmoran...(a)charter.net> 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. > >The schemes John Larkin gave are unipolar, and 'current from +V or -V' >sounds like bipolar to me. > >If it's for bipolar current, use one op amp as current/voltage >converter (this >means the pseudoground input is your input load, which is ideal), then >a second op amp with current mirrors on its power pins to convert back >to a current; the ratio of resistors sets the current gain > >(warning bad ASCII art follows) > > > > > +-----Rf----+ > | |\ | >I input +--------+---|- \ | > | >--+--+ > GND--|+ / | > |/ | > | > | > | > +---------------------+ > | (+V) > | | > | {PNP mirror} > | | | > | |\| | > +------|+ \ +------- I_out= I_input*Rf/Rg > | >-+ | > GND --Rg---+----|- / | | > | |/| | | > | | | | > +-----------+ | > | | > {NPN mirror} > | > (-V) > > That's the topology of my MRI gradient amps. We sell units that go from 3 amps to 120 amps peak output. The big one peaks at about 20 kilowatts out. That wouldn't be a bad audio amp topology, either. You can make the crossover essentially perfect, and it uses 100% of the available supplies, all biasing is tightly controlled, fet load matching is excellent, and it has some nice supervision/protection hooks. No, on second thought, this is way too precise for audio. John
From: Tim Williams on 5 Feb 2010 12:21 "whit3rd" <whit3rd(a)gmail.com> wrote in message news:152e5f52-6af1-4cb3-aa39-0ecf74e30db1(a)q4g2000yqm.googlegroups.com... > 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/tmoranwms/Circuits_2008/Current_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
From: JosephKK on 5 Feb 2010 13:03 On Wed, 3 Feb 2010 18:28:40 -0800 (PST), Tim Williams <tmoranwms(a)gmail.com> wrote: >On Feb 3, 7:31 pm, John Larkin ><jjlar...(a)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?
From: whit3rd on 5 Feb 2010 14:20 On Feb 5, 8:24 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 5 Feb 2010 00:48:09 -0800 (PST), whit3rd <whit...(a)gmail.com> > wrote: [current amplifier with complementary current mirrors as final stage] > That wouldn't be a bad audio amp topology, either.... No, on > second thought, this is way too precise for audio. It does very much look odd, the first time, looking at that big resistor to ground, and nothing else on the 'output' pin. There's a bit of difficulty if the intended output is a low current; the quiescent op amp drain and the deviation from unity of the mirror gain interact. That makes an output offset current of Iq* (Gpnp - Gnpn). To trim that, you put a pot from the op amp V+ supply to the V- supply and ground the wiper (through a big resistor). The original inquiry mentioned a range in the low microamps, will probably require such trim. It looks as odd as the 'output' resistor.
From: John Larkin on 5 Feb 2010 14:39
On Fri, 5 Feb 2010 11:20:02 -0800 (PST), whit3rd <whit3rd(a)gmail.com> wrote: >On Feb 5, 8:24�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Fri, 5 Feb 2010 00:48:09 -0800 (PST), whit3rd <whit...(a)gmail.com> >> wrote: > >[current amplifier with complementary current mirrors as final stage] > >> That wouldn't be a bad audio amp topology, either.... No, on >> second thought, this is way too precise for audio. > >It does very much look odd, the first time, looking >at that big resistor to ground, and nothing else on the 'output' pin. > >There's a bit of difficulty if the intended output is a low current; >the quiescent >op amp drain and the deviation from unity of the mirror gain interact. >That makes an output offset current of Iq* (Gpnp - Gnpn). >To trim that, you put a pot from the op amp V+ supply to the V- supply >and ground the wiper (through a big resistor). > >The original inquiry mentioned a range in the low microamps, will >probably >require such trim. It looks as odd as the 'output' resistor. I use active mirrors, very precise, with an opamp per fet, so all the fet currents are very well controlled and matched. Various units use one to 16 fets per mirror. ftp://jjlarkin.lmi.net/Amp.jpg My quiescent current problem is the opposite: most opamps have too much Iq compared to Imax, so my mirror idle currents get too high. That's fixed by finding the right opamp and adding some off-direction offset to the mirrors. I do a pot from opamp V+ to V- to set fet idle currents. The crossover graphs are esentially perfect. John |