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Schottky diode voltage drop vs. current, 60mV rule

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kell

Sun Aug 05 2007, 05:28AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

More than once in my reading I've encountered a claim that diodes drop 60mV per current decade change, so I thought I'd check it.
I measured shottkys, standard silicon diodes, and b/e junctions.
By my testing ONLY THE SHOTTKYs follow the 60mV rule.
MBR3035PT: 59.3 mV 1N5818: 60.4 mV
Ordinary silicon diodes 1N4148: 119 mV 1N4003: 102 mV
I tested the b/e junctions of half a dozen different kinds of TO-92
pnp transistors (collector open), and all measure around 75 mV per decade, give or take
a few millivolts.
I suppose this characteristic doesn't affect most circuits significantly, but I encountered a difficult (for me) design challenge where the solution I finally found actually uses this characteristic. Had to measure it myself to find out for sure.
Thought I'd put the results of my investigation out there.

Bored Chemist

Sun Aug 05 2007, 08:11AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Well, at least you haven't fallen into the trap of thinking that diodes always drop about 1.2V.
What range of currents did you measure the voltage drops over?

Sulaiman

Sun Aug 05 2007, 09:14AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

BC has given you a vital clue!

When texts refer to diode they mean the region where the 'ideal diode law' applies
which is typically up to 1000 times the reverse leakage current at low reverse voltage.

You are probably measuring at much higher currents
where the resistance of the diode materials becomes significant.

The schottky diodes measured well because one side of the junction is metal
and the other is highly doped (low resistance) silicon.

kell

Mon Aug 06 2007, 03:58PM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Sulaiman wrote ...

You are probably measuring at much higher currents
where the resistance of the diode materials becomes significant.

A clue? Really?
I went to a lot of trouble and graphed many data points for Vf versus logI for those diodes, and I could see where the line was straight and where it started to curve.
The numbers I gave you are totally solid. 60mV is an old wives' tale. The data don't lie.
Up to about 50mA the 1N5818 obeys lnI=38.122Vf-14.507 perfectly.
You can do the math, but the mV per decade is ln10/38.122 or 60.4 mV per decade.
Now, the 1N4148, which stays diodic (did I coin a new word?) at least to .7 Vf, follows
lnI=19.34Vf-19.08 at about .75 it starts to veer off. But below 6 or 7 mA it is quite well behaved and follows the ideal diode law admirably. ln10/19.34 comes to about 119 mV per decade.
b/e junctions fall somewhere between the schottkys and the conventional diodes.
Typing all this is actually pretty boring, but I get annoyed when people defend conventional wisdom reflexively. So I had to get all empirical on you.

Bored Chemist

Mon Aug 06 2007, 05:09PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I think that, in principle, I might be able to calculate the temperature of your workshop because I'd be suprised if that 60mV per decade isn't temperature dependent but I may be totally wrong.
Anyway, if the theory doesn't agree with reality there's no point trying to fix reality. As you say, "The data don't lie."

Steve Conner

Mon Aug 06 2007, 06:11PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Strictly speaking, I think the 60mV is the required difference in base-emitter voltage to produce a factor of 10 change in collector current of a transistor (or a 10:1 imbalance in a long-tailed pair) according to the Ebers-Moll equation.

If you measured the collector current on your transistors instead of the base current, I bet the answer would come out a lot nearer 60mV than 75.

The Art Of Electronics mentions that the Ebers-Moll law applies to diodes too, but the thermal voltage must be multiplied by a correction factor between 1 and 2. (So I guess it's 1 for the Schottky diodes you measured and nearer 2 for the 1N4148s.)

Finally, since the thermal voltage is kT/q, all the results will be proportional to absolute temperature. You used to see a diode drop quoted as 0.6V in American texts and 0.7V in European ones: maybe American labs were hotter in the 70s. Though if you got this far, you know what a gross simplification it is to say that Vbe is constant.

BTW, I saw a strikingly similar post on sci.electronics.misc:

and my answer would be along those lines too. Start ringing that bell! :P

Sulaiman

Mon Aug 06 2007, 06:36PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Well, I stand corrected!

Guess I'll have to do a few measurements.

EDIT:
OK I just did a few rough measurements - you were correct in chastising me!

1N5822 schottky 64mV/decade
UF5406 fast diode 84mV/decade
BA159 1kV diode 118 mV/decade

The measurements were quick/rough/limited but good enough for a quick check
Ambient temperature was 23 Celcius,
measuring current used was about 120uA and 1.2mA (varies a bit with diode type)

Hazmatt_(The Underdog)

Mon Aug 06 2007, 09:27PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

In our EE330 class we had to pay attention to Vt in case the question didn't specify 20*C (25mV). Why they chose 20*C insted of the ACS standard of 25*C is beyond me, but yes room temprature is going to change your threshold voltage.

kell

Tue Aug 14 2007, 07:58AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Steve Conner wrote ...

If you measured the collector current on your transistors instead of the base current, I bet the answer would come out a lot nearer 60mV than 75.

I decided to check out the collector current.
Not exactly a long-tailed pair, I know, but:
I connected the bases of two npn's together. I took them from the same tape. One of the transistors' emitters is connected to ground, and the emitter of the other transistor is connected through a voltage divider to the supply, a 12 volt battery; I put the divider there (20 and 4k7) to get a small offset in the b/e voltages.
I ran a 1 meg resistor from the connected bases to battery positive and ran a pair of matched resistors (988 ohm) from the collectors to battery positive.
After I had it all hooked up with collector current flowing, I measured an offset voltage across the emitters of 31mV.
I measured the voltages across the 988 ohm resistors and got a ratio of 4.45.
.031/log4.45=48mV
The temperature at the bench was 26 C.

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