Current divider mirror
in [Design]
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From: John Larkin on 3 Feb 2010 22:12 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 Specs? You have specs? How about noise, bamdwidth, cost? Do you intend it to be an adjustable ratio? How would it be adjusted? Use a chopper amp and biggish resistors to keep the offset low. John
From: Tim Williams on 3 Feb 2010 23:34 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:cjekm59tmt7i00sp9dd4fqfc1n1t3ch2ut(a)4ax.com... > Specs? You have specs? How about noise, bamdwidth, cost? Noise: low enough. It's charging a timing capacitor, so HF noise doesn't matter as much as 1/f noise. Let's say current noise of 2nA or less. Likewise, the current itself is a DC signal (< 100Hz), bandwidth doesn't really matter. The output needs ~MHz compliance, which won't matter around a drain / collector output (if nothing else, it can be cascoded). Cost: low, and should be simple (i.e., that rules out a logarithmic ADC/DSP/DAC). > Do you intend > it to be an adjustable ratio? How would it be adjusted? Potentiometer, or something else panel adjustable. Doesn't have to be electronically controlled. > Use a chopper amp and biggish resistors to keep the offset low. I suppose a chopper amp could be filtered to remove its noise. Something less than 1k would generate somewhat less than 5V at full current, and less than 100uV at the lowest range (assuming something like the opamp-FET-feedback resistor circuit). On principle, I don't like using something additive (a linear amp) when the nature of the signal is logarithmic. So the obvious answer is a logarithmic converter, which is basically a glorified diode. Maybe a simplified log-exp amp would work anyway with reasonable stability, while keeping noise and offset sensible? Gain would also be electronically controllable by adding to the log signal. Of course, the simplest log-exp converter is the current mirror, but unequal current densities aren't temp compensated. Maybe they can be. Temp compensation isn't something I've explored much and may be worth a romp to figure out... Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Robert Baer on 4 Feb 2010 04:56 Tim Williams 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. > > I'd just do it with one of those ten emitter transistors that ICs are so > fond of using, but they aren't variable geometry... > > Tim > That is what a variable resistor in the emitter is for..
From: Jim Thompson on 4 Feb 2010 09:40 On Thu, 04 Feb 2010 01:56:38 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: >Tim Williams 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. >> >> I'd just do it with one of those ten emitter transistors that ICs are so >> fond of using, but they aren't variable geometry... >> >> Tim >> > That is what a variable resistor in the emitter is for.. And introduces a TC if emitter:emitter and resistor:resistor aren't 1:1. OpAmp driven transistor is the only way a variable resistor scheme is accurate and temperature stable. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: whit3rd on 5 Feb 2010 03:48
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) |