Problem with current mirrors
in [Design]
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From: whit3rd on 23 Feb 2010 12:25 On Feb 23, 5:39 am, Remco Poelstra <remco.poelstra+albas...(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 It looks like Q12, Q13, Q9 are all unnecessary; take collector current of Q2 instead directly to emitter of Q8, and ground the base of Q8. I'm also unclear on the expected behavior, but is R37 really the right size to carry the desired current? Your Q2 might just be saturating instead of being in the gain region.
From: Jim Thompson on 23 Feb 2010 12:35 On Tue, 23 Feb 2010 12:16:00 -0500, Hammy <spam(a)spam.com> wrote: >On Tue, 23 Feb 2010 10:04:11 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Tue, 23 Feb 2010 11:56:42 -0500, Hammy <spam(a)spam.com> wrote: >> >>[snip] >>> >>>That is a pretty elaborate cct. to just switch a Highside PFET and a >>>low side NFET. If your willing to use all those components just to >>>switch a HS PFET you might as well use a NFET on the HS and bootstrap >>>the drive. The whole reason you would use a PFET on the HS is to >>>reduce the component count and complexity you defiantly aren't doing >>>that. >>> >> >>Odd! I didn't find the OP's post to be "defiant" ;-) >> >>But he indeed didn't ask for alternate circuits, he asked _why_ his >>mirrors were inaccurate. >> >> ...Jim Thompson > >I know my spelling has really gone downhill. :-) > >To much relying on software spell checkers. Yep. Too many "fro" accepted for "for" :-( ...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 16:14 On Tue, 23 Feb 2010 07:53:39 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.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 > >Early effect. Change of collector current with collector voltage. > >Cure it by dropping several volts in the emitter path... add R's in >series to Q1:E, Q2:E, Q13:E, Q12:E, etc. > > ...Jim Thompson An extreme demonstration, I picked a transistor with VAF=20V, then showed mirrors with and without emitter "ballast" resistors... <http://analog-innovations.com/SED/EarlyVoltage(fromSED).pdf> ...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: Robert Baer on 24 Feb 2010 00:37 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? > > Kind regards, > > Remco Poelstra Well, it might help to use Vbe matched transistors so that the collector currents are reasonably matched. Also, i note there are about 3 times the transistors needed to do the job.
From: Robert Baer on 24 Feb 2010 00:40
John Larkin 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. > Cool! Not many use common base... |