# Is a diode's forward voltage stable enough to be used as a reference voltage?

Assuming a resistor is in series with a diode (say, a 1N4001) so that the diode has around 1mA of current, will its forward voltage be usable as a voltage reference to within 1% (relative to itself over time and with ambient temperature changing by 10F max)? The voltage doesn't matter, as long as it doesn't change to a noticeable degree.

• The datasheet of the diode will tell. – PlasmaHH Jan 24 '18 at 10:08
• Change with regard to what? Temperature? Generally no, it is a lousy voltage reference. If you have a constant current you are better off putting this over a resistor. – RoyC Jan 24 '18 at 10:09
• 10% sure, 1% not really. for 1% you're better off with something like TL431 – Jasen Jan 24 '18 at 10:16
• it'd take fairly exterme temperature variations to get 10% change, but 30 degrees will get you 1% of 600mV – Jasen Jan 24 '18 at 10:37
• If cost is an issue get a TL431 shunt reference. If you need to salvage it,you can find them in "almost" any switching power supply, connected to an optoisolator on the output side. – Spehro Pefhany Jan 25 '18 at 1:39

Is a diode's forward voltage stable enough to be used as a reference voltage?

No, it will change with temperature: -

Picture taken from here

At 25 degC and a bias of 5.1 mA (for example) the voltage will typically (that's typical and not always) be about 627 mV. At 30 degC the voltage will have dropped to about 618 mV - that's a change of 1.4% for a minor difference in temperature. In voltage reference terms that's a stability of 2878 ppm/degC. Compare this to just fairly cheap voltage references having a drift of 50 ppm/degC.

Assuming a constant, preferably low current (1mA) is applied to a diode

If you have a constant current source then apply it to a resistor and you'll get pretty much stable results compared to the diode.

• but if it's a slightly variable source the diode voltage wavers less than the resistor voltage would. – Jasen Jan 24 '18 at 10:19
• @Jasen The op said "constant current source". – Andy aka Jan 24 '18 at 10:27
• sure, but this is the real world. I mean a diode rejects fluctuations in the current source better than a resistor does, but it also makes a better thermometer. – Jasen Jan 24 '18 at 10:34
• @Jasen A real world constant current source has what type of spec would you say? – Andy aka Jan 24 '18 at 10:40
• @Andyaka I did not. I likely used confusing terminology, but I meant having the diode in series with an unchanging resistance. I.E, not having it power a load that changes. As mentioned, if I had a constant current source, I'd use that as my reference voltage. – Daffy Jan 24 '18 at 10:42

I have seen some instrument schematics, that use LED diode as power indicator and voltage source. Could the LED be a better alternative? Search for current, voltage source schematics examples, perhaps you will the answer.

P.S: The "instrument" is the LeCroy scope differential probe, not a toy.

• in this circit the temperature curve of the led and the transistor cancel each other's effect, I wonder to what extent this device is sensitive to bright lights. – Jasen Jan 24 '18 at 19:32

You can design BandGap voltage references, using multiple diodes running at different currents (thus different current densities and thus difference temperature coefficients. Some opamp circuit is needed to extract the CTAT. You combine the PTAT and a proper amount of the CTAT, and extract a very stable voltage reference. If you do this, please honor Paul Brokaw for the invention.