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From: Remco Poelstra on 23 Feb 2010 08:39
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 23 Feb 2010 09:53
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
--
| 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 23 Feb 2010 10:21
> 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.

Emitter resistors help, or you can cascode it (Wilson mirror, etc.), or
both.

Why use Q8, 9, 12 and 13, why not run Q2 straight into R40?

Why all the resistors? You can remove more than half with no change in
performance. R15, 16, 21, 22, 23, 37, 39 and 40, which basically leaves
R12, 19, 27, 28 and 48.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: John Larkin on 23 Feb 2010 11:11
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

From: John Larkin on 23 Feb 2010 11:21
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.

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