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Adjustable current mirror

Leading from earlier discussion, I have drawn this:

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Assuming thermal currents and voltages are equal, analysis suggests that Ic is variable from Ip/2 to Ip/200 (i.e., a range of 1:100) for base voltages of 0 to -5.29*Vt. Meanwhile, the diode supplies about -28*Vt, so R2/(R1+R2) = 28/5.29, or picking values, R1 = 4.29k, R2 = 1k is good enough.

Vt and Is cancel out, so it should be stable to a first approximation. I know there's a factor of alpha in everything, which can be mitigated with more transistors (e.g., Wilson mirror, darlington LTP), but that's a small error I won't bother with anyway. More important is, I don't know how much effect imbalance will have on temperature compensation. How sucky is a diode compensated, unbalanced LTP?

The other issue is getting 20*Ip into the tail. I don't want to stack up 20 transistors just to do that, that would be silly. AoE suggests resistors, which obviously doesn't work at low currents where r_e is way larger than R_E. The other choice is finding potentially wildly different transistors and hoping their emitters are different sized (e.g. 2N5088 vs. TIP31?), which is also rather silly, but has the distiction of potentially working.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams
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Tim Williams a écrit :

Why not something like this ?

Ip 0.1 Ip to 10 Ip

V V | | | |/ +------------------------------| | |>

| | | .-----+ | | | | /| | |/ \| /+|---------+---| |--------+--< | |>

Reply to
Fred Bartoli

Yes, that looks even better!

Tim

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

If you want accuracy, use OpAmp-driven MOSFET's...

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...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.
Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Accuracy at 0.1uA to 50mA?

Tim

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

But watch out that it oscillateth not. Some opamps react with mild indigestion when having to drive a gate capacitance directly, meaning sans resistor.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

R2/(R1+R2)

20

But not in that configuration... think about it for awhile before jumping to that conclusion ;-) ...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.
Reply to
Jim Thompson

Ic

R2/(R1+R2)

much

20

Pondering further, adding a gate resistor in that configuration might get you _into_ the soup ;-) ...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.
Reply to
Jim Thompson

That's starting to look like a Gilbert cell. I think there may be an IC that does something similar... Analog Devices maybe.

The best way to do the bottom mirror, over a large current range and

20:1 mirror ratio, is an opamp thing.

ftp://jjlarkin.lmi.net/Mirror2.JPG

John

Reply to
John Larkin

Ic

voltages

R2/(R1+R2)

small

much

20

resistors,

transistors

It depends on the load, I have seen such current sources doing tarantella dances and had to fix them. Often the oscillations were subtle. One client called me in with a "wee noise problem". Sure enough there was a whiff of a carrier at around 2MHz. I also noticed that an AM radio [1] they listened to in the lab went *POCK* everytime I turned the unit on. Sniffed around a bit and found an opamp driving a FET as a current source, singin' da blues.

Well yeah, if you just slap a gate resistor in there. Got to spring for another (larger) one between source and IN- and a wee capacitor from IN- of the opamp to its output.

[1] No, it was not tuned to Rush. Although I did turn it to a country station later in the evening because I couldn't listen to Mr.Bojangles one more time :-) 
--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

Ic

voltages

R2/(R1+R2)

I

with

small

much

up 20

resistors,

than

transistors

working.

Aha! You discovered the Bojangles hyper-instability ?:-) ...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.
Reply to
Jim Thompson

that Ic

voltages

R2/(R1+R2)

I

with

small

much

up 20

resistors,

than

transistors

working.

This banter reminded me of being in a "hot" building... the steel studs were resonant at the frequency of a nearby FM station... drove me crazy for awhile until I figured out what it was :-(

Then all rules of bypass must deviate to handle the local situation. ...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.
Reply to
Jim Thompson

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

ide quoted text -

Ahh, that's just beautiful!! I'll put it in my Thompson folder. Thanks!

I've got a circuit fragment scribbled in some notebook of an opamp driving fets push pull. But I figured the 'cross-over' distortion would be terrible and it never went further.

George H.

FET's and opamps go nicely together.

Reply to
George Herold

quoted text -

I did this whan I was young and foolish..

ftp://jjlarkin.lmi.net/L700_splitter.jpg

But this topology...

ftp://jjlarkin.lmi.net/Isource.JPG

is better. It is simple and has no crossover distortion, because with zero input the quiescent current of the opamp gets dumped into both mirrors, and then any signal ramps one up but leaves the other at Iq. There's no deadband.

If the mirrors are something better, like closed-loop opamp things, this gets very, very precise.

John

Reply to
John Larkin

quoted text -

If you're lucky, and have a high belief system ;-) Stones tomorrow... it's wine time :-) ...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.
Reply to
Jim Thompson

that Ic

voltages

R2/(R1+R2)

I

with

small

much

up 20

resistors,

than

transistors

working.

It can be worse in Europe. The ladies in production often listen to chanson hitparades all day long, the really schmaltzy heart-throb stuff. Drives me up the walls. Much better in the US although I can't stand

50's and 60's music for more than a couple hours. Or Motown, just not my stuff.

Luckily my wife and I both like country and bluegrass so we listen to that in the evenings. Good old Americana. You late father would have liked it.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

=A0 =A0 =A0 ...Jim Thompson

=A0 =A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0 =A0|

=A0 =A0 =A0 =A0 |

=A0|

|

- Hide quoted text -

OK the first one will take some study.... but the second is totally weird! Does that really work?

(I'm afraid I started electronics late in life and will remain old and foolish. But it's still fun.)

George H.

Reply to
George Herold

quoted text -

Absolutely. It prefers an opamp that has wide bandwidth, low Iq, and high peak current capability, so you can have a relatively low quiescent current if you want. But that can be worked around, too.

This gets close to the perfect class AB biasing. The mirror currents, as a function of input signal, can be made to look like this...

/ / / / upper mirror current /

--------------------- Iq

---------------------+------------------------- input voltage

-Iq -------------------------- / / / lower mirror current / /

with essentially perfect behavior around zero.

It's interesting to consider a low-output-impedance version, like the classic Vbe-multiplier biased complementary emitter followers, but an arrangement where turning one on harder leaves the other at Iq, and has similarly excellent crossover behavior.

Electronics is fun.

John

Reply to
John Larkin

e quoted text -

"> Absolutely. It prefers an opamp that has wide bandwidth, low Iq, and

Sounds like one of my fav's the OPA134. (Or is that to slow?) It's got a full power band width above 1MHz.

Hmmm I'm not sure about the Iq...

Are there any good 'lab power amps' being made. I'm thinking of something that's got 10-100kHz of bandwidth and 10's of volts at a several amps. I'm picturing this supply thing made by (Keepco?)? sorta a power supply and power amplifier.

George H.

Reply to
George Herold

quoted text -

Kepco used to, may still, make 4-quadrant active power supplies. A lab box like that would be useful. Maybe a medium-power opamp like an OPA552 with some transistor boosters. Then you get to use my favorite class B circuit, complementary source followers with no bias but a sneak b-e bypass resistor.

How about constant-current and constant-voltage modes, with external signal in or a fixed vef. A 10-turn pot would set the gain or the constant voltage or current.

Oh, here they are, BOPs...

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for a mere $6975,

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or a little one for $2500.

John

Reply to
John Larkin

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