Problem with current mirrors
in [Design]
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From: Remco Poelstra on 24 Feb 2010 03:52 Hi, Thanks for all replies! That Early voltage example is really an insight, thanks Jim. Explains why it isn't working indeed. As for the general idea, I just want to switch the FETs on and off. The remaining part of the circuit measures the current in the 80V lines and in the event of a short circuit, I want to disconnect the load. So just on or off. I wanted to use current sources and mirrors to make sure Vgs is always about (-)10V, a slight variation with temperature doesn't matter, as longs as it doesn't go over the absolute maximum rating of the FET's. I had not thought about the idea of using Q8 as current buffer in common base and thereby eleminating a few Q's. Given the amount of replies (and their content), it might be wise to ask for ideas for more alternative circuits :). The "MirrorFets.JPG" looks quite simple indeed. I don't think I can omit the bandgap reference (as Fred Bartoli suggested), since the supply lines vary a lot (+-30V each line). So I need some way to set the current through R12 and R48, just setting it by varying the series resistor in MirrorFets.JPG is not very accurate I think (unless I miss a good point somehwere). Kind regards, Remco Poelstra Remco Poelstra schreef: > 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
From: Jim Thompson on 24 Feb 2010 09:44 On Wed, 24 Feb 2010 09:52:44 +0100, Remco Poelstra <remco.poelstra+albasani(a)duran-audio.com> wrote: >Hi, > >Thanks for all replies! >That Early voltage example is really an insight, thanks Jim. Explains >why it isn't working indeed. [snip] You are quite welcome! And Thank YOU for posting a circuit problem! ...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: John Larkin on 24 Feb 2010 10:07 On Tue, 23 Feb 2010 21:40:20 -0800, Robert Baer <robertbaer(a)localnet.com> 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... I like that emitter-to-emitter thing. It's not often you get to use it. John
From: Jamie on 24 Feb 2010 18:50 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.
From: John Larkin on 24 Feb 2010 21:45
On Wed, 24 Feb 2010 18:50:28 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> 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. > But the go-up current and the go-down current will match to 1% or better. If the betas are equal, the currents match perfectly. Use mosfets and the currents *will* match. Hmmm, that would be a really interesting double servo. John |