Output Z of CMOS Current Mirror ??

J

Jim Thompson

Guest
Anyone have experience at creating a CMOS current mirror with really
high output Z?

I'm trying attain about 1G-ohm, at 100Hz to 5KHz.

I can use cascodes and feedback loops.

Just thought I'd ask before I plunge in and shoot myself in the foot
;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Jim Thompson wrote:
Anyone have experience at creating a CMOS current mirror with really
high output Z?

I'm trying attain about 1G-ohm, at 100Hz to 5KHz.

I can use cascodes and feedback loops.

Just thought I'd ask before I plunge in and shoot myself in the foot
;-)
Click to expand...
Doesn't 1G-ohm at 5KHz imply no more than .03pf of parallel
capacitance?

--
John Popelish
 
"Jim Thompson" <thegreatone@example.com> wrote in message
news:5rvko05llf25rf1hiqt2loru9mdkf6gcc9@4ax.com...
Anyone have experience at creating a CMOS current mirror with really
high output Z?

I'm trying attain about 1G-ohm, at 100Hz to 5KHz.

I can use cascodes and feedback loops.

Just thought I'd ask before I plunge in and shoot myself in the foot
;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
Click to expand...
Thought you guys had it cracked long ago with the 100's of Mohms of cascoded
Wilson mirrors. Looks like you'll be marking out some virgin turf :).
regards
john
 
John Popelish wrote:
Jim Thompson wrote:

Anyone have experience at creating a CMOS current mirror with really
high output Z?

I'm trying attain about 1G-ohm, at 100Hz to 5KHz.

I can use cascodes and feedback loops.

Just thought I'd ask before I plunge in and shoot myself in the foot
;-)


Doesn't 1G-ohm at 5KHz imply no more than .03pf of parallel
capacitance?
Click to expand...
I once managed to screw up a pwm to +/-10V output circuit, with a mere
60fF of stray capacitance. I got 100mV spikes in my output, synchronous
with the pwm. Aha I sez to myself, a capacitive coupling issue. So I
looked at the pcb, and found the pwm signal ran near the opamp inverting
input - 8mil away, for 9mm or so. I did a quick calc, and found about
60fF. Rubbish I sez to myself, thats tiny, it cant be causing this. So I
went and measured PWM dV/dt and output spike amplitude. I ran backwards
thru the opamp to work out what current caused the V, and guess what -
60fF*dV/dt = just the right amount of current. A little surgery with a
small drill and a bit of wire-wrap wire, and voila - no problem. I moved
the trace on the pcb, and the production version worked a treat.

Cheers
Terry
 
Terry Given wrote:
the pwm signal ran near the opamp inverting
input - 8mil away, for 9mm or so.
Click to expand...
Would routing a ground trace between them have fixed it?
 
Clifford Heath wrote:

Terry Given wrote:

the pwm signal ran near the opamp inverting input - 8mil away, for 9mm
or so.


Would routing a ground trace between them have fixed it?
Click to expand...
Most certainly, although in this case I looked at the layout and changed
it quite significantly - it had been done by one of our techs (he did
most of the pcb layout) and he was more concerned with joining the dots
than thinking about the circuit he was building. The layout as
implemented was terrible, with really poor part placement leading to
nasty interconnects and many vias. A simple re-arrangement of top-layer
components made the design flow nicely, and removed a whole bunch of
vias and inner-layer traces. That was when I first started to approach
pcb layout as a circuit design problem. When I lay out a pcb nowadays I
typically spend 80-90% of my time placing components, and do the copper
last. Its perhaps a bit slower, but the circuits tend to work a lot better.

Cheers
Terry
 
Jim Thompson wrote:
Anyone have experience at creating a CMOS current mirror with really
high output Z?

I'm trying attain about 1G-ohm, at 100Hz to 5KHz.
Click to expand...
Why?

Maybe there another way to do achieve the same function.

Anyway, what am I missing? I just checked a standard 3 stack cascoded
current mirror, using my generic Bsim models, L=1u, w=1u, I=1ua, and got
345 Gohm < 100Hz falling to 29 Gohm at 10K. At 10ua it was flat to 10Khz
at 11 Gohms.

So, what current are you trying to do this at?

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
In sci.electronics.design Jim Thompson <thegreatone@example.com> wrote:
On Thu, 04 Nov 2004 19:37:19 -0500, John Popelish <jpopelish@rica.net
wrote:

Jim Thompson wrote:

Anyone have experience at creating a CMOS current mirror with really
high output Z?

I'm trying attain about 1G-ohm, at 100Hz to 5KHz.

I can use cascodes and feedback loops.

Just thought I'd ask before I plunge in and shoot myself in the foot
;-)

Doesn't 1G-ohm at 5KHz imply no more than .03pf of parallel
capacitance?

That's a lot of capacitance in the CMOS world... 30fF ;-)
Click to expand...
Take a look at these active-feedback cascoded mirrors - they
manage about 16fF output capacitance:

Electronics Letters , Volume: 33 , Issue: 12 , 5 June 1997
pages 1042 - 1043.


--
Rick
 
In sci.electronics.cad Jim Thompson <thegreatone@example.com> wrote:
: Anyone have experience at creating a CMOS current mirror with really
: high output Z?

: I'm trying attain about 1G-ohm, at 100Hz to 5KHz.

: I can use cascodes and feedback loops.

: Just thought I'd ask before I plunge in and shoot myself in the foot
: ;-)

1GOhm at 5kHz is about 30 fF. Good luck in getting down the capacities...

Bye
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top