[nikongod]

Hello Boards

I have been on quite the CCS trip recently. Spiffy little circuits.

Is there are way to estimate the MINIMUM current that a given transistor will work with in a simple 1 transistor CCS? I would like to build something to regulate 0.15mA, and am concerned that there may be some hidden pitfalls compared to a similar circuit running higher currents.

 

[cetoole]

By 1 transistor CCS, do you mean a LED biased BJT, or single JFET/depletion mode MOSFET, or what? The BJT current sources are the easiest to figure out current for, but with the FET based ones, best you can do without measuring and trimming is look at the IDSS range.

 

[nikongod]

LED & BJT

Calculating the resistor values is easy enough, but Im concerned that I'l wind up below some cut off point that I dont know to look for.

Edited: I just realized that Hfe could really suck at such low currents, and I may be better off just using a big old resistor or forced to use 2 transistors (no big deal, they are cheap...) More advice is welcome though.

 

[cetoole]

Yeah, HFE is what to watch out for, but that is easy enough, as most bjt datasheets show curves of HFE v. IC, so just pick one which has a high HFE at the relevant current. Of course, no reason not to add a second (cascode) transistor. Doesnt even have to be another BJT.

 

[00940]

The Vbe will go down with Ic. With only 0.15ma, don't assume the Vbe is 0.65 when determining your resistor values in the leg of the ccs.

I wouldn't be concerned with hFE.. you're only drawing 0.15ma in collector current after all. According to datasheets, the bc557c hFE is around the same at 0.1ma and 1ma.

 

[error401]

I know you're aiming for a BJT CCS here, which I really can't help you with, but thought I'd mention Vishay AN103, useful information for JFET CCS.

 

[nikongod]

Thanks for the help guys. More looking at the data-sheets says this should not be as hard as I had feared.

A FET based CCS looks cool, maybe I'l do that instead. more reading first.

 

[majkel]

Try building the Widlar CCS, as it's dedicated for low currents. You'd be happy with the BC550 I guess, it's a low noise transistor. The only inconvenience is to calculate (or simulate) the resulting current sinking into the transistor, as it's a logarythmic equation. On the other hand, it's the main advantage of the Widlar CCS.