Earth Day from a Super M@ssive Point of View

On Sun, 22 Apr 2007 13:36:46 -0700, John Larkin Gave us:

Spec the diode again, not your sad design situation.

No go, I see. We all knew you wouldn't try. You always squirm away form any serious electronics or any trivial math.

John

On Sun, 22 Apr 2007 21:47:35 -0700, John Larkin Gave us:

Start small, johnny. You wanted a diode suggested. Spec it out.

Answer my original question. Is 0.8 volts the junction threshold potential? You didn't make a very clear initial query.

Diodes don't have a junction threshold potential. At low currents, voltage drop is proportional to the log of current, plus an ohmic term that sraightens out the curve at high currents. The math thing again.

I want a very small junction so I can get 0.7 or better 0.8 volts drop at, say, 250 uA, which is all the current I can spare. And it's got to fit into an 0603 footprint.

You'd be better off if you quit boasting about stuff you don't understand. You get called on it every time.

John

Looks good: 0.77 volts typ at 1 mA, around 5 cents:

John

But NXP has carried on the time-honored traditions of Philips Semiconductor: you can't get the parts.

Central has an equivalent.

John

Central Semi parts can also be hard to get. How about soomething like the BAS16 from Fairchild?

That's a "high conductance" diode, with forward drop around 0.7 at 10 mA. We want a teeny "low conductance" junction. The fictitious NXP parts run about 0.75 volts at 250 uA.

I tested a Skyworks super-low-capacitance varicap and got 0.78 at 500 uA (my Fluke tests diodes at 500 uA.) Nearly ditto a Skyworks 0.2 pF PIN diode, but pins can behave strangely so I'd rather avoid them. We could use the varicaps, but they're like 50 cents each, and the plain old diodes are under 10 cents. We'll probably have to buy a reel of

John

Si and Ge in series?

Bye. Jasen

Not in a single 0603 footprint! Does anybody make surface mount Ge diodes?

We just got a reel of Central Semiconductor "low leakage" silicon diodes in SC-79 packages, a perfect fit for an 0603. They're running about 0.73 volts at 500 uA, almost as good as the expensive varactor diodes.

John