Ring of two current source

I think Bill Sloman or someone posted a .asc file of a "ring of two" current source a while back. I have an application in mind where I need a very temperature stable current source and would like to play around with it, but I can't find the message.

Could it be posted again? Thanks.

--
----Android NewsGroup Reader---- 
http://usenet.sinaapp.com/
 Click to see the full signature
Reply to
bitrex
Loading thread data ...

I won't post the long goolge link search for "AoE3 query re current sources" From piglet.

George H.

Reply to
George Herold

Here:

There are better solutions if the temperature dependence is critical; TL431 and a transistor,

or negative power supply and transistor, for that matter.

Reply to
whit3rd

formatting link

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

Neither of those will be super stable.

This is pretty good. It will typically be dominated by the TC of the source resistor.

formatting link

The bandgap could be replaced by a really good 3-terminal voltage reference. This is a sink, so flip things over for a source.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

Is that a good book Phil? (with ~800 pages of AoE3 still to go I'm not really looking for another EE text.)

George H.

Reply to
George Herold

Its 3$ and shipping on Abe Books...

Might be worth a look at 2.5% of the cost of AoEIII

Steve

Reply to
sroberts6328

Crap, I just ordered it used on Amazon for 30 bucks :-(

Cheers

Klaus

Reply to
Klaus Kragelund

Hey, someone stashed one of my links "for future reference" :-)

FWIW, I'd suggest the current mirror* here:

formatting link

*Pun not intended, but gladly accepted ;)

Not that I've updated that side of my website in years.

Anyway, the "ring of two" Bill was talking about, I think, replaced the two diodes with a voltage reference zener, so that its tempco cancels with the BJT tempco. Note that the tempco must be from the other side of the hump (>6V), otherwise you need at least one diode to cancel the Vbe tempco.

The dropout is quite large (~6V) of course, but if you need a good current source, you'll be willing to tolerate the extra supply voltage requirement.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
 Click to see the full signature
Reply to
Tim Williams

What do you mean? It's hardly different from the above. ;-)

Of course, you're welcome to use an enhancement mode FET in the earlier circuit; or a Darlington.

I'm guessing the "1.2V" reference is intended to be something with a better tempco than TL431, but not a three terminal type.

You could use something like an LM4041, which is better than TL431, but operation is reversed (VREF is referenced to +V, not -V). Which would make it perfect for a current *source*.

I forget if there are other devices with convenient "offset op-amp" type behavior, in the same (or better) accuracy class, and with +V or -V referenced inputs.

I think the very accurate (four terminal) types are usually ground referenced, so you'd be looking more at sinks than sources, without using a more complicated circuit of course.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
 Click to see the full signature
Reply to
Tim Williams

No idea. It had a page on the actual ring-of-two circuit, that's all.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

On Friday, 1 May 2015 04:36:31 UTC+10, bitrex wrote:

Version 4 SHEET 1 880 680 WIRE -96 -464 -320 -464 WIRE 336 -464 -96 -464 WIRE -96 -400 -96 -464 WIRE 336 -368 336 -464 WIRE -96 -288 -96 -320 WIRE -96 -176 -96 -208 WIRE 336 -176 336 -304 WIRE 176 -128 -32 -128 WIRE 272 -128 176 -128 WIRE 176 -48 176 -128 WIRE 336 -48 336 -80 WIRE 336 -48 176 -48 WIRE -96 48 -96 -80 WIRE 80 48 -96 48 WIRE -96 80 -96 48 WIRE 336 80 336 -48 WIRE -320 112 -320 -464 WIRE 80 128 80 48 WIRE 80 128 -32 128 WIRE 272 128 80 128 WIRE 336 208 336 176 WIRE -96 272 -96 176 WIRE 336 320 336 288 WIRE -320 448 -320 192 WIRE -144 448 -320 448 WIRE -96 448 -96 336 WIRE -96 448 -144 448 WIRE 336 448 336 400 WIRE 336 448 -96 448 WIRE -144 512 -144 448 FLAG -144 512 0 SYMBOL zener -80 336 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value 1N829 SYMBOL zener 352 -304 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMATTR Value 1N829 SYMBOL res -112 -416 R0 SYMATTR InstName R1 SYMATTR Value 768R SYMATTR SpiceLine tol=0.1% SYMBOL Misc\\xvaristor -112 -304 R0 SYMATTR InstName U1 SYMATTR Value 0R to 500R SYMBOL res 320 304 R0 SYMATTR InstName R2 SYMATTR Value 768R SYMATTR SpiceLine tol=0.1% SYMBOL Misc\\xvaristor 320 192 R0 SYMATTR InstName U2 SYMATTR Value 0R to 500R SYMBOL voltage -320 96 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 12V SYMBOL npn 272 80 R0 SYMATTR InstName Q1B SYMATTR Value BCV61C-2 SYMBOL npn -32 80 M0 SYMATTR InstName Q1A SYMATTR Value BCV61C-1 SYMBOL pnp -32 -80 R180 SYMATTR InstName Q2A SYMATTR Value BCV62C-1 SYMBOL pnp 272 -80 M180 SYMATTR InstName Q2B SYMATTR Value BCV62C-2

This isn't the modern way of making a precision current source.

There are now better - and easier to use - voltage references than the 1n82

  1. The Linear Technology LT6655-5 is one of the better ones. If you use an op amp to compare the voltage drop across an LTC6655-5 with the voltage drop a cross a 15ppm 0.1% resistor (or better),returned to the same negative rail as the LTC6655-5, and use the output of the op amp to drive the gate of a n

-channel FET or a small NMOSFET power transistor, the drain of the FET will be a pretty stable and predictable current sink.

If you want a current source, you can do much the same job with P-channel F ET or MOSFET. Biassing the LTC6655 becomes slightly trickier - it needs a p ower supply that's at least 0.5V above the reference output - slightly more for the the lower voltage versions (1.25V and 2.048V which need at least 3 .0V of supply voltage) - but nothing all that complicated.

formatting link

--
Bill Sloman, Sydney
Reply to
Bill Sloman

If you don't mind the base current error. Or the TL431 nasties.

Right. Some bandgaps have pretty good TCs. Sadly, there are few precision negative 3-terminal references.

We sometimes use a grounded positive reference to make a precision curent sink, then reflect that up into a resistor hanging on V+. The drop in that resistor becomes the reference for a positive current source. That's especially useful if you have a grounded DAC to fine-tune the current source.

Does that make sense?

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

I have +/- 12 volt rails available so shouldn't be a problem.

--
----Android NewsGroup Reader---- 
http://usenet.sinaapp.com/
 Click to see the full signature
Reply to
bitrex

The only cheap one, is to have another power rail. NIM electronics always had

+/- 12V and +/- 24V, regulated to a percent or better. So, an NPN with base at -12V, emitter to resistor to -24V, had collector current

Ic= (1-1/Hfe) * (24V - 12V -Vbe)/R

Notice, over a wide temperature range, Vbe goes from 0.5 to 0.7V, so makes less than a +/- 1% thermal variance; R would typically have less than 1% thermal variance; the Hfe term, for a 'typical' low noise transistor, would range from (1-1/200) to (1-1/400), again, less than one percent. Thus, a complex system with dozens of accurate current sinks only costs you one good resistor and a cheap transistor per sink.

If you want amazing accuracy, just go to a -200V well-regulated negative supply (but your resistors might self-heat some). Transistors with very high current gain are available (2SD2114KW current gain is well into four digits).

Reply to
whit3rd

Did you say how much current?

George H.

Reply to
George Herold

Gosh, NIM, blast from the past. But the supplies won't be super stable. A lot depends on what bitrex means by "very temperature stable."

--
John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
 Click to see the full signature
Reply to
John Larkin

No problemo for some -- Tek stuff in the 50s and 60s used hybrids throughout; 300V on a 30V transistor? No problem! Just better current sources/sinks that way!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
 Click to see the full signature
Reply to
Tim Williams

On Thursday, April 30, 2015 at 12:41:03 PM UTC-7, Phil Hobbs wrote: ...

...

That's the traditional ring-of-two but it relies on leakage to start up. A resistor between the bases can help but it compromises current stability with supply voltage changes.

kevin

Reply to
kevin93

Huh? NIM power supplies were spec'ed at 0.5% maximum drift, with only 3 mV of allowed ripple. You can do better, I suppose, but for something off-the-shelf, the old NIM spec was pretty good. Calibration was done to 0.1% accuracy on those power rails.

Reply to
whit3rd

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.