Problem with current mirrors
in [Design]
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From: Jim Thompson on 23 Feb 2010 11:22 On Tue, 23 Feb 2010 08:11:02 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 23 Feb 2010 14:39:27 +0100, Remco Poelstra ><remco.poelstra+albasani(a)duran-audio.com> wrote: > >>Hi, >> >>I'm trying to operate MOSFET devices with current mirrors. I want a >>maximum Vgs, to make sure they have minimum Rds_on. The supply rails on >>the other hand vary a lot, so I decided to use current sources to >>develop the required Vgs across a resistor. >>See http://remco.beryllium.net/Sheet1.pdf for a schematic >> >>Q7 and Q5 make up the current source, using R12 to switch M2. Logic to >>switch the current source off has been removed. M2 is switched perfectly >>using this construct. >>Q6 uses the current source reference output to generate another current >>of about 1mA. >>The other BJT's are current mirrors to copy the same current to R48, to >>switch M6. >>The problem is that the current through R37 is way larger than the >>current through R27. Q12,9 and 8 also do not seem to be able to settle >>at the correct value. >>I'm wondering why it doesn't work correctly. >>Can someone help? >> >>Kind regards, >> >>Remco Poelstra > >What is the overall circuit supposed to do? > >Discrete-transistor current mirrors like this tend to be very >inaccurate. > >John Not if you know what you're doing. See my post... From: Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> Newsgroups: sci.electronics.design Subject: Re: Problem with current mirrors Date: Tue, 23 Feb 2010 07:53:39 -0700 Message-ID: <dkq7o5l53efq7qrnb4l6vqnfhm8asv27cc(a)4ax.com> ...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: Vladimir Vassilevsky on 23 Feb 2010 11:24 Remco Poelstra wrote: > Hi, > > I'm trying to operate MOSFET devices with current mirrors. I want a > maximum Vgs, to make sure they have minimum Rds_on. The supply rails on > the other hand vary a lot, so I decided to use current sources to > develop the required Vgs across a resistor. > See http://remco.beryllium.net/Sheet1.pdf for a schematic > > Q7 and Q5 make up the current source, using R12 to switch M2. Logic to > switch the current source off has been removed. M2 is switched perfectly > using this construct. > Q6 uses the current source reference output to generate another current > of about 1mA. > The other BJT's are current mirrors to copy the same current to R48, to > switch M6. > The problem is that the current through R37 is way larger than the > current through R27. Q12,9 and 8 also do not seem to be able to settle > at the correct value. > I'm wondering why it doesn't work correctly. > Can someone help? This circuit won't work without global feedback keeping it in place. Current mirrors are not going to be any accurate. VLV
From: Fred Bartoli on 23 Feb 2010 11:36 John Larkin a �crit : > On Tue, 23 Feb 2010 14:39:27 +0100, Remco Poelstra > <remco.poelstra+albasani(a)duran-audio.com> wrote: > >> Hi, >> >> I'm trying to operate MOSFET devices with current mirrors. I want a >> maximum Vgs, to make sure they have minimum Rds_on. The supply rails on >> the other hand vary a lot, so I decided to use current sources to >> develop the required Vgs across a resistor. >> See http://remco.beryllium.net/Sheet1.pdf for a schematic >> >> Q7 and Q5 make up the current source, using R12 to switch M2. Logic to >> switch the current source off has been removed. M2 is switched perfectly >> using this construct. >> Q6 uses the current source reference output to generate another current >> of about 1mA. >> The other BJT's are current mirrors to copy the same current to R48, to >> switch M6. >> The problem is that the current through R37 is way larger than the >> current through R27. Q12,9 and 8 also do not seem to be able to settle >> at the correct value. >> I'm wondering why it doesn't work correctly. >> Can someone help? >> >> Kind regards, >> >> Remco Poelstra > > > How about something simpler? Try this for starters [1] maybe: > > ftp://jjlarkin.lmi.net/MirrorFets.JPG > > That's fine but why the reference and biasing/depletion Fet? Loose them and just use the input logic levels. One might want to add some VGS clamping zener diodes to allow maximum FET drive in all temperature conditions. -- Thanks, Fred.
From: Jim Thompson on 23 Feb 2010 11:35 On Tue, 23 Feb 2010 10:24:24 -0600, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >Remco Poelstra wrote: > >> Hi, >> >> I'm trying to operate MOSFET devices with current mirrors. I want a >> maximum Vgs, to make sure they have minimum Rds_on. The supply rails on >> the other hand vary a lot, so I decided to use current sources to >> develop the required Vgs across a resistor. >> See http://remco.beryllium.net/Sheet1.pdf for a schematic >> >> Q7 and Q5 make up the current source, using R12 to switch M2. Logic to >> switch the current source off has been removed. M2 is switched perfectly >> using this construct. >> Q6 uses the current source reference output to generate another current >> of about 1mA. >> The other BJT's are current mirrors to copy the same current to R48, to >> switch M6. >> The problem is that the current through R37 is way larger than the >> current through R27. Q12,9 and 8 also do not seem to be able to settle >> at the correct value. >> I'm wondering why it doesn't work correctly. >> Can someone help? > >This circuit won't work without global feedback keeping it in place. >Current mirrors are not going to be any accurate. > > >VLV The OP is referencing to the Vbe of Q7, probably stable enough if temperature range isn't an issue. Add a zener under the emitter of Q7 if you fret; or replace the Q5/Q7 assembly with a TL431. ...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: Jim Thompson on 23 Feb 2010 11:42
On Tue, 23 Feb 2010 08:21:35 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Tue, 23 Feb 2010 14:39:27 +0100, Remco Poelstra ><remco.poelstra+albasani(a)duran-audio.com> wrote: > >>Hi, >> >>I'm trying to operate MOSFET devices with current mirrors. I want a >>maximum Vgs, to make sure they have minimum Rds_on. The supply rails on >>the other hand vary a lot, so I decided to use current sources to >>develop the required Vgs across a resistor. >>See http://remco.beryllium.net/Sheet1.pdf for a schematic >> >>Q7 and Q5 make up the current source, using R12 to switch M2. Logic to >>switch the current source off has been removed. M2 is switched perfectly >>using this construct. >>Q6 uses the current source reference output to generate another current >>of about 1mA. >>The other BJT's are current mirrors to copy the same current to R48, to >>switch M6. >>The problem is that the current through R37 is way larger than the >>current through R27. Q12,9 and 8 also do not seem to be able to settle >>at the correct value. >>I'm wondering why it doesn't work correctly. >>Can someone help? >> >>Kind regards, >> >>Remco Poelstra > > >How about something simpler? Try this for starters [1] maybe: > >ftp://jjlarkin.lmi.net/MirrorFets.JPG > > >John > > >[1] Note that I said "for starters" to reduce some of the predictable >whining. "[1]" is called an ego protector in case John is wrong... as he often is ;-) ...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. |