Maker Pro
Maker Pro

Adjustable current mirror

T

Tim Williams

Jan 1, 1970
0
Leading from earlier discussion, I have drawn this:
http://webpages.charter.net/dawill/Images/Variable_Mirror.png

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
 
F

Fred Bartoli

Jan 1, 1970
0
Tim Williams a écrit :
Leading from earlier discussion, I have drawn this:
http://webpages.charter.net/dawill/Images/Variable_Mirror.png

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.


Why not something like this ?


Ip 0.1 Ip to 10 Ip

V V
| |
| |/
+------------------------------|
| |>
| |
| .-----+
| | |
| /| | |/
\| /+|---------+---|
|--------+--< | |>
<| | \-|-. |
| | \| | |
| | ___ | ===
=== '-|___|-+ GND
GND 1K |
'----< -60uA < I < +60uA

Should track reasonably well.
And Diode has some nice DMMT3904 and DMMT5551 matched pairs.
 
T

Tim Williams

Jan 1, 1970
0
Fred Bartoli said:
Why not something like this ?

(snip)

Should track reasonably well.
And Diode has some nice DMMT3904 and DMMT5551 matched pairs.

Yes, that looks even better!

Tim
 
J

Joerg

Jan 1, 1970
0
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.

Pondering further, adding a gate resistor in that configuration might
get you _into_ the soup ;-)

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 :)
 
J

Joerg

Jan 1, 1970
0
Jim said:
Jim said:
On Wed, 10 Feb 2010 17:55:19 -0700, Jim Thompson

Jim Thompson wrote:
On Thu, 11 Feb 2010 01:19:49 +0100, Fred Bartoli <" "> wrote:

Tim Williams a écrit :
Leading from earlier discussion, I have drawn this:
http://webpages.charter.net/dawill/Images/Variable_Mirror.png

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.

Why not something like this ?


Ip 0.1 Ip to 10 Ip

V V
| |
| |/
+------------------------------|
| |>
| |
| .-----+
| | |
| /| | |/
\| /+|---------+---|
|--------+--< | |>
<| | \-|-. |
| | \| | |
| | ___ | ===
=== '-|___|-+ GND
GND 1K |
'----< -60uA < I < +60uA

Should track reasonably well.
And Diode has some nice DMMT3904 and DMMT5551 matched pairs.
If you want accuracy, use OpAmp-driven MOSFET's...

http://analog-innovations.com/SED/CurrentSourceConceptual.pdf

But watch out that it oscillateth not. Some opamps react with mild
indigestion when having to drive a gate capacitance directly, meaning
sans resistor.
But not in that configuration... think about it for awhile before
jumping to that conclusion ;-)

...Jim Thompson
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.

Pondering further, adding a gate resistor in that configuration might
get you _into_ the soup ;-)
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 :)

Aha! You discovered the Bojangles hyper-instability ?:)

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.
 
S

Spehro Pefhany

Jan 1, 1970
0
Pondering further, adding a gate resistor in that configuration might
get you _into_ the soup ;-)

...Jim Thompson

Provided the source resistor isn't too low (like <100 ohms, depending
on the op-amp, with ultra-low-power ones being the most skittish) the
gate capacitance won't make things oscillate.
 
T

Tim Williams

Jan 1, 1970
0
John Larkin said:
Do classic transistor mirrors really hold up at uA and nA currents?

Well, in that triangle wave generator I posted, I observed reasonable (i.e.
not ludicrously bad) results. It did 1Hz to 4.5MHz on 680pF and about
3Vp-p, what's C*dV/dt on that? The duty cycle was perhaps 40-60% at the
lowest frequency (nearly 50% higher up), which means the currents were still
matched to the tune of +/-10%.

Since everything is leaking, that could just as well be the difference
between leakages of the 2N4401, 2N4403, 1N914 and 2N4093. I guess I would
expect leakages to be more mismatched than that, but I don't really know.
At any rate, it looked good *near* cutoff, so even a discrete current mirror
(at least with "lucky" parts choice?) can work quite low.

I should go put that circuit back together. I have access to all the parts
in the EE lab. I don't seem to have any scope shots of the low frequency
waveform from back when.

Tim
 
F

Fred Bartoli

Jan 1, 1970
0
Jim Thompson a écrit :
Bipolar mirrors flake out, due mostly to the ISE parameter Kirwan
asked about (recombination). CMOS mirrors can get down there, but
physical offsets start raring their ugly heads.

Some behave really well.
While my next prototype is baking at 125°C I just pulled a random BC548C
out of the junk box and tested it.

VCE=5V,
IC Ib Beta
5.3m 10u 530
1m 2u 500
489u 1u 489
90.5u 200n 452
43.2u 100n 432
20.5u 50n 410
3.44u 10n 344
1.55u 5n 310
523n 2n 262
228n 1n 228
112n 500p 224
38n 200p 190
117n 80p 146
62n 50p 124
1.8n 20p 90
740p 10p 74
330p 5p 66
130p 2p 65


It's not that bad.
 
F

Fred Bartoli

Jan 1, 1970
0
Fred Bartoli a écrit :
Jim Thompson a écrit :

Some behave really well.
While my next prototype is baking at 125°C I just pulled a random BC548C
out of the junk box and tested it.

VCE=5V,
IC Ib Beta
5.3m 10u 530
1m 2u 500
489u 1u 489
90.5u 200n 452
43.2u 100n 432
20.5u 50n 410
3.44u 10n 344
1.55u 5n 310
523n 2n 262
228n 1n 228
112n 500p 224
38n 200p 190
117n 80p 146
62n 50p 124
1.8n 20p 90
740p 10p 74
330p 5p 66
130p 2p 65


It's not that bad.

BTW it's fun having a silicon BJT working with something like 12mV base
bias :)
 
F

Fred Bartoli

Jan 1, 1970
0
Jim Thompson a écrit :
Repeat at +125ºC ;-)

Yes, of course, but not too many got that high.
And the oven is occupied to something that earns money. But I'll do it
when it idles...
 
T

Tim Williams

Jan 1, 1970
0
Fred Bartoli said:
BTW it's fun having a silicon BJT working with something like 12mV base
bias :)

Picoamps don't perform work. :p

Tim
 
F

Fred Bartoli

Jan 1, 1970
0
Tim Williams a écrit :
Picoamps don't perform work. :p

Hey, 12mV*2pA=24fW
Since the begin of universe that's about 10kJ. Not nothing :)

Giving me some idea. I might try to build the world's lowest current,
lowest supply voltage, hence lowest power, low frequency astable
oscillator, just to set the record and use those drained batteries to
something really useful, for still a long time.
 
T

Tim Williams

Jan 1, 1970
0
Fred Bartoli said:
Hey, 12mV*2pA=24fW
Since the begin of universe that's about 10kJ. Not nothing :)

Giving me some idea. I might try to build the world's lowest current,
lowest supply voltage, hence lowest power, low frequency astable
oscillator, just to set the record and use those drained batteries to
something really useful, for still a long time.


I seem to recall JT had posted something like that. I think it had ~20G
resistors, so current ~nA.

Funny thing about "0.7V" is it's just because we like Manly currents, like
one miliampere. A bunch of decades down, and it's a lot teensier, roughly
ln(1e9) times. Mmm, that's not quite correct, and 12mV sounds way too low.
Lesse. If Vbe = 0.7V at 1mA, then Is = 1mA * exp(-0.7V / 26mV) = 2fA. So
then at 1pA, V = 26mV * ln(1pA/2fA) = 0.16V.

Did you actually measure 12mV? If so, then I guess that means it'll have to
be underbiased a bit -- like germanium. In fact, same physics, just a
wildly different order of magnitude. Pick the right current range and
temperature, and you can "zero bias" a BJT, just like in the germanium days.

I'd like to see an LC oscillator at this current. It's gonna need about a
million turns, of course, but maybe that wouldn't be too awful to print.
Let's see, for Fo = 1Hz, Rloss ~ 1G and Q = 10, parallel resonant, so Xc =
X_L < 0.1G, or let's say L = 16MH and C = 1.6nF. If you can make a small
core with A_L = 1uH/T^2 (I'd recommend sputtered permalloy), then you'll
only need 4 megaturns. At 1nA and a typical current density of 155A/cm^2,
you're looking at about 650 nm^2 cross section, or a 25 nm square hunk of
copper, which is well within modern process capacity. Let's say you "wind"
100 planar layers, each containing 40,000 turns. Including a generous 25nm
of insulation in all directions, you need 2mm width and 10um height, plus
whatever size the ferrite gets to.

You probably don't have to worry about saturation too much. Magnetization
is 1nA * 4Mt = 4mAt, and if the path is about 5mm, you get only 0.8 A/m.
Even supermalloy takes 1-2A/m to get into saturation, so that's not too bad.
Oh, as long as I'm diddling with geometry, I might as well verify
inductivity: if B = 1T from H = 1 A/m, and the core cross section is maybe 2
x 2 mm square, then the flux*turns is 16 V*s, which should be plenty. The
inductance is 16 V*s / 1nA = 16GH, so a smaller core or fewer turns could be
used.

This seems surprisingly doable, if rather expensive. The sheer number of
layers would be expensive, but at least the middle layer masks can be
identical. I wonder what the best construction method is -- I suppose you
wouldn't use plastic on the 25nm scale; then again, what's wrong with
directly using the UV cured mask resin as printed? Deposit some, sputter
some copper, cut off the peaks, deposit some more, etc. You'd start with a
"start winding" mask which takes the start lead out, and this layer is
deposited on top of a permalloy base (might as well make the whole thing out
of permalloy, plastic and copper). Same idea on the topmost layer: take the
winding out, then cover with a heavy layer of permalloy. Nice thing about
deposited metal, you can make the perfect pot core.

Such a device wouldn't be integrated on silicon, but it could be placed in
the package nearby. You aren't getting high density integration at this
current level, anyway; we're talking big (~1mm) transistor dies running at
picoamps here, so the current density is miniscule and the voltage drop is
small.

Speaking of low voltage stuff, it could be very interesting to use
transistors at currents near Is (voltages near Vt). Hook up a couple layer
thermocouple to such a chip, cool one side and heat the other, and *veeery
slowly*, have computation flow out of it. A true thermal computational
machine! It's too bad there's no coherence of physical states, it sounds a
lot like a quantum computer -- give it a teensy bit of heat energy and wait
for the equilibrium state to evolve!

Tim
 
F

Fred Abse

Jan 1, 1970
0
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.

I work to Willie Nelson, Buck Owens, Merle Haggard, et al. on CD.
 
F

Fred Abse

Jan 1, 1970
0
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...

http://www.kepcopower.com/bophi.htm

for a mere $6975,

http://www.kepcopower.com/bop.htm

or a little one for $2500.

I'm considering those versus Kikusui at the moment. Any comments, John?
 
J

Joerg

Jan 1, 1970
0
Fred said:
I work to Willie Nelson, Buck Owens, Merle Haggard, et al. on CD.

Great stuff. Jim's dad played in a Bluegrass band but not sure if any of
their pieces are online for lisnin' in. We only have one small local
station that plays Bluegrass once in a while but luckily there's plenty
of web streams.
 
F

Frank Buss

Jan 1, 1970
0
Joerg said:
Great stuff. Jim's dad played in a Bluegrass band but not sure if any of
their pieces are online for lisnin' in. We only have one small local
station that plays Bluegrass once in a while but luckily there's plenty
of web streams.

Do you mean something like this?


Nice piece and the cool look on the face of the banjo player in contrast to
the music is hilarious!

But currently I'm more a fan of classical English folk songs, as in my
latest piano performance:

 
J

JosephKK

Jan 1, 1970
0
I seem to recall JT had posted something like that. I think it had ~20G
resistors, so current ~nA.

Funny thing about "0.7V" is it's just because we like Manly currents, like
one miliampere. A bunch of decades down, and it's a lot teensier, roughly
ln(1e9) times. Mmm, that's not quite correct, and 12mV sounds way too low.
Lesse. If Vbe = 0.7V at 1mA, then Is = 1mA * exp(-0.7V / 26mV) = 2fA. So
then at 1pA, V = 26mV * ln(1pA/2fA) = 0.16V.

Did you actually measure 12mV? If so, then I guess that means it'll have to
be underbiased a bit -- like germanium. In fact, same physics, just a
wildly different order of magnitude. Pick the right current range and
temperature, and you can "zero bias" a BJT, just like in the germanium days.

I'd like to see an LC oscillator at this current. It's gonna need abouta
million turns, of course, but maybe that wouldn't be too awful to print.
Let's see, for Fo = 1Hz, Rloss ~ 1G and Q = 10, parallel resonant, so Xc =
X_L < 0.1G, or let's say L = 16MH and C = 1.6nF. If you can make a small
core with A_L = 1uH/T^2 (I'd recommend sputtered permalloy), then you'll
only need 4 megaturns. At 1nA and a typical current density of 155A/cm^2,
you're looking at about 650 nm^2 cross section, or a 25 nm square hunk of
copper, which is well within modern process capacity. Let's say you "wind"
100 planar layers, each containing 40,000 turns. Including a generous 25nm
of insulation in all directions, you need 2mm width and 10um height, plus
whatever size the ferrite gets to.

I would be wondering what the DCR would mount up to. If that gets too high
even a 600 mV source would not get to a dozen pA.
 
Top