Problem with current mirrors
in [Design]
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From: Robert Baer on 25 Feb 2010 04:54 Jamie wrote: > Robert Baer wrote: >> 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... > that's because it doesn't offer you any current gain. In fact, you'll > lose a little. > > Yes, one DOES lose a little current gain, bit methinks that is not the reason one sees little use of common base circuitry.
From: Jim Thompson on 25 Feb 2010 10:45 On Thu, 25 Feb 2010 01:54:11 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: >Jamie wrote: >> Robert Baer wrote: >>> 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... >> that's because it doesn't offer you any current gain. In fact, you'll >> lose a little. >> >> > Yes, one DOES lose a little current gain, bit methinks that is not >the reason one sees little use of common base circuitry. Not MUCH! alpha = beta/(beta+1), so, if beta=100, you have less than 1% "lost". That NPN-PNP configuration used to be as common as dirt in bipolar I/C days. With the advent of CMOS encroaching analog, I last used the NPN-PNP thingy in 2002 (qrk's sonar stuff ;-) ...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: Tim Williams on 25 Feb 2010 11:32 "Robert Baer" <robertbaer(a)localnet.com> wrote in message news:0fmdneXbU5PE1RvWnZ2dnUVZ_q0AAAAA(a)posted.localnet... > Yes, one DOES lose a little current gain, bit methinks that is not the > reason one sees little use of common base circuitry. Bah, common base is great for saving parts. :) http://webpages.charter.net/dawill/Images/CC_Buck.gif The error amp / comparator 2N4403 has to be noninverting. Too bad about the 1k base voltage divider, it would be more efficient if it had complementary output, which could drive a 2N7000 FET instead of the 4401 switch. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 25 Feb 2010 11:45 On Thu, 25 Feb 2010 10:32:58 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"Robert Baer" <robertbaer(a)localnet.com> wrote in message >news:0fmdneXbU5PE1RvWnZ2dnUVZ_q0AAAAA(a)posted.localnet... >> Yes, one DOES lose a little current gain, bit methinks that is not the >> reason one sees little use of common base circuitry. > >Bah, common base is great for saving parts. :) >http://webpages.charter.net/dawill/Images/CC_Buck.gif >The error amp / comparator 2N4403 has to be noninverting. Hey, I can do that too! ftp://jjlarkin.lmi.net/Inverter.jpg A neat logic level shifter is TTL-resistor-PNPemitter with the PNP base grounded, collector going to somewhere negative. John
From: whit3rd on 25 Feb 2010 12:33
On Feb 25, 1:54 am, Robert Baer <robertb...(a)localnet.com> wrote: > Jamie wrote: > > Robert Baer wrote: > >> ... use common base... > > that's because it doesn't offer you any current gain. In fact, you'll > > lose a little. > Yes, one DOES lose a little current gain, bit methinks that is not > the reason one sees little use of common base circuitry. Every cascode pair is a 'use of common base', and lots of differential amps are, technically, a follower and a common-base transistor. Others will point this out, no doubt, but ... unless the gate resistors are high precision, a percent of the current lost is NOT going to do anything bad to this circuit. It'd be rather hard to find a small- signal transistor that would lose two percent in common-base connection. |