Limiter

tion is

own.

precisely,

of mV

comparator in

that's

ched set,

better.

ED.JPG

rc_d...

rc_b...

bout 4000

A little difficult. I'm in Christchurch, New Zealand, at the moment, and my electronic gear - such as it is - is out of reach.

You were interested eniugh to measure the temperature stability of the circuits you'd put together. Why weren't you interested enough to work out where the temperature variation was coming from?

(PDT),BillSloman

is

precisely,

comparator in

that's

set,

better.

Because I was designing a real product, a fast laser driver, and the LED current source was plenty good enough. 100 PPM/K is down where the power supply, the resistors, the capacitors, and beta variations all matter. If I wanted a more precise current source, which I often do, it would be closed-loop. I didn't have need or room for that on this little board, and the LED thing was fun. But not fun enough to spend a full day on it.

Here's the board:

(PDT),BillSloman

is

precisely,

comparator in

that's

set,

better.

Larkin can't handle the math >:-} ...Jim Thompson

(PDT),BillSloman

saturation is

own.

precisely,

mV

comparator in

that's

set,

better.

about 4000

It's plain that this circuit will never be a really good current source, for several reasons, so why bother doing a lot of math? Some of us don't need to do pages of algebra or a Spice sim to see the obvious.

Power supply rejection can be greatly improved, and I bet you can't tell us how, without looking up my old posts.

Design some *real* electronics, you obsolete old geezer:

turation is

my own.

ore precisely,

or comparator in

ay, that's

matched set,

do better.

rc_LED.JPG

D_Isrc_d...

D_Isrc_b...

the

ply

of about 4000

for

to do

It can't be perfect, but it could have been better than you achieved with fairly minor modifications, none of which demand pages of algebra or a Spice sim - though it's often quicker (if less reliable) to run a Spice sim than it is to solder in the extra/differemt components.

us how,

The obvious technique is to make it a "ring of two" circuit, and use the LED voltage to set up a current source to bias the LED at a more nearly constant current. The traditional way would add three more transistors - two of them in a current mirror.

ive

amplifier

tios. The

pre- and

up using a

rt of like

real

er 100 GHz

ting,

board

But ground loops put noise on the power supply rails.

You called it oscillation, rather than electromagnetic interference, so the implication is that the extra feedback paths through the supply rails (including the ground connection) were creating the instability.