Adding outputs to a Wilson mirror - Page 1
Forum section: electronics / projects

author: Unknown
author_rank: Unknown
timestamp: June 26, 2011, 08:03:38 pm
content: In the Alex-CAD HD circuit attached is a 10uA current source feeding a Wilson current mirror.I would like to add a second 10uA output. An ideal situation would be to use the existing reference around the TL431 and mirror it one more time. Downgrading to a simple current mirror is not an option due to a very high output impedance requirement.Any suggestions much appreciated!

author: jimmc
author_rank: Frequent Contributor
timestamp: June 26, 2011, 09:03:10 pm
content: Tricky to modify your circuit.How about using the TL431 to provide a negative reference voltage wrt the 12v rail and then using an Op-Amp and P Channel FET to produce the constant current, duplicate the Op-Amp and FET to produce a many outputs as you require.Individual outputs could be trimmed by adjusting the individual source resistors or all currents adjusted together by varying the reference voltage.(Similar idea to Dave's Constant Current Dummy Loadhttp://www.eevblog.com/2010/08/01/eevblog-102-diy-constant-current-dummy-load-for-power-supply-and-battery-testing/but with the polarities reversed.)Jim

author: Unknown
author_rank: Unknown
timestamp: June 26, 2011, 10:49:55 pm
content: Thanks for the reply Jim.I modified the circuit to use P channel enchancement mode FETs. The idea was that since they draw almost no static gate current I would be able to tap into the feedback loop of the first mirror without disturbing it. Circuit attached.This works great on paper and on a simulator, but I doubt I will be able to achieve good performance without matched FETs. I was unable to find any on Farnell, only pairs of P FETs in the same package. I did however find matched PNPs (the ones on the first schematic). I am unsure as to how closely matched transistors will be to achieve a 1:1 ratio on all mirrors.I have also considered using the TL431 in series regulation from the 12V rail (not in shunt like now), but the internal voltage reference is connected to the collector of the internal transistor (the anode of the device) so the reference current -several uAs- is added to the output current! The LM334 is too slow at 20kHz and as a results its impedance drops to the kOhm range at 20kHz...The voltage reference/op-amp combo is a good idea, possibly cheaper compared to matched FETs in low volumes. There are a couple of points elsewhere in the circuit that require a voltage references so this approach will alow me to use the same voltage source with dividers. But you know the number of op-amps in analogue designs can very quickly go through the roof...it is getting there.Where are the 3 Ws when you need them?

author: gregariz
author_rank: Frequent Contributor
timestamp: June 26, 2011, 11:10:41 pm
content: Not sure what level of impedance you are looking for but just off hand what about downgrading to a simple current mirror arrangement but adding a decent level of emitter degeneration (your currents look small enough maybe) to give you the impedance.

author: Unknown
author_rank: Unknown
timestamp: June 27, 2011, 12:53:35 pm
content: Thanks for the reply gregariz. About 1.5MOhm at 20kHz is my requirement.I have drawn out both of your suggestions. The simple mirror has a small signal output resistance of 750kOhm at 20kHz at 10uA collector current. The op-amp solution has about 1Mohm at 20kHz, but I am sure I can get this much higher using a faster op-amp. The one drawn is a rather cheap dual bipolar op-amp.A variation of gregariz solution would be to use a cascode with a separate bandgap reference. It is easy to copy as needed. The fundamental issue I have is availability Vs price of quad matched transistors. The THAT320P14-U costs £6 on Farnell. Matched pairs are almost as expensive as an op-amp...For this reasons I am leaning towards the op-amp approach. There are 4 voltage references around the rest of the circuit and I will have to do those with an extra TL431 referenced to ground.

author: blackdog
author_rank: Frequent Contributor
timestamp: June 27, 2011, 01:21:32 pm
content: Hi Alex!Try this Quad transistor from Analog Devices, The MAT14, if you are a "Firm" than Sample it :-)Nice Aplication Note's in the Data Sheet.http://www.analog.com/en/special-linear-functions/matched-transistors/mat14/products/product.htmlKind Regarts,Bram

author: Unknown
author_rank: Unknown
timestamp: June 27, 2011, 02:18:12 pm
content: Hi Bram, thanks for that. Besides the fact that it is an NPN, I can sample it fine, but I am sure many others wont be able to (open hardware project). Looking on octopart.com, only digikey used to have it for $6. Out of stock now.