1N34A germanium diode. This datasheet says Vf is 1.0V, but germanium diodes Vf are 0.25V to0.3V, What am I misunderstanding?

 

According to this, it seems that Vf is quite dependent on If:

 

The data sheet doesn't have graphs, only 1V for Vf. So that didn't really answer my question.

Follow the graph for the1N34A diode, 0.3V to 0.5V Vf assuming you can see it in the spaghetti.

 

The data sheet doesn't have graphs, only 1V for Vf. So that didn't really answer my question..

Oh, but it was an answer. You have a point and from the diode equation you can find other points. Note that the diode you have from NTE may or may not correspond to a real 1N34A or even any other diode. It may be a species unto itself.

 

Your datasheet specifies a Vf of 1.0v at a particular If of 5mA. The various 1N34 diodes in the graph have a Vf of 1 volt at around 7 or so mA, which is not too dissimilar from your datasheet.

Here for example is the Vf for a 1N4148, a silicon diode with a Vf ostensibly about 0.65 to 0.75 volt. But note that this is only at the specified If measurements:




A more complete picture shows that the Vf of a 1N4148 lies between a quarter and three quarters of a volt, depending on forward current, quite a wide variation:

 

What you are seeing is using If @ 5.0mA for the testing results that yeilds, at maximum, 1Vf. Not the actual Vf break point. Test it and see... And not all have the identical Vf break point. There is some variation in lots.

1N34A pdf, 1N34A Description, 1N34A Datasheet, 1N34A view ::: ALLDATASHEET :::

 

It is the maximum rating. So, given the large variability of germanium devices, they were probably just making sure that they would all pass.

 

Here's the LTspice sim for the 1N34 model I have:

 

That is the excellent linearity the datasheet mentioned. Still confusing when you're used to things of a fairly stable voltage drop as I am.

 

1N34A germanium diode. This datasheet says Vf is 1.0V, but germanium diodes Vf are 0.25V to0.3V, What am I misunderstanding?

The 1.0 V spec is the MAX Vf that you might see at a forward current of 5 mA (at a junction temperature of 25°C). It is unlikely that you would ever see an actual diode that would be very close to that.

Your 0.25 V to 0.3 V values are the typical knee voltages, which are at sub-milliamp currents.

If you follow the graphs for the 1N34A variants in the plot provided by boostbuck, they seem to fall in the 0.7 V to 0.8 V range at 5 mA. These curves are almost certainly for typical devices, so each curve is basically at the center of a spread of actual performance, and thus you would expect them to be somewhat below the max specification since the upper edge of the actual performance envelope has to fall below that max spec.

 

That is the excellent linearity the datasheet mentioned. Still confusing when you're used to things of a fairly stable voltage drop as I am.

Linear in a log plot, which means exponential.

 

That is the excellent linearity the datasheet mentioned. Still confusing when you're used to things of a fairly stable voltage drop as I am.

Most diodes show such a straight when plotted with a log current scale (below for 1N4148):

The voltage is not really "stable", but follows the usual log function relation between voltage and current (top trace).
We often just use 0.6V-0.7V since that's in the "knee" appearing voltage for a typical silicon junction-diode operating current of a few mA when plotted with a linear voltage/current scale (bottom trace).