AF117 HFE

[Danndad]

This af117 was removed from a fidelity 208 earlier, looking at the soldering around it the transistor looks like it has already been removed maybe for whisker trouble.

There are no shorts to screen so I have put it on my tester and the results show the hfe is high, are they usually this high??

   

Regards
Danndad

[PerdioPal]

I think that is about right for DC hfe, the data sheet says 150 +/- quite a lot is what one would expect for an early transistor. +/- a fair bit is what we get now.

[ppppenguin]

While that is unusually high for a germanium device it's not impossible. Does make me wonder if there is something being fooled by the test methods or misinterpreted by the clever software. A common example of this is leaky capacitors apparently reading high on simple testers. That is simply due to them taking longer to charge due to leakage. Since the tester measures time taken to charge as a proxy for capacitance the answer is inevitably wrong.

[Danndad]

Thanks for the replies.

I have got other af117s here that seem to be free from whiskers but none show hfe like this 1, thats why i asked.
I'm getting some whistles and chirps on this 208 and as the transistor has already been disturbed by the look of it, I had to remove it.
It will be replaced by a gt322.

[boater sam]

I have found testing older Germanium transistors that the results are often not what I expected. 
AF117s seem to be affected in odd ways. I wonder if it is all down to the whisker problems.
Don't have that problem with silicon devices.

[BusyBee]

I had a modern tester that was totally incapable on old germanium transistors, giving all manner of strange results. Luckily I do have a tester from the days of germanium and it works a treat. There is a warning in it's instructions about results and these indicated that a high hfe reading with high leakage was a problem.

To quote: "Low duty transistors which simultaneously have a great back current (Icb > 5 uA) and great gain factor (B > 150) in most cases cannot be used for practical purposes."

Whether or how this may apply here I don't know.

Tracy

[ppppenguin]

It isn't difficult to check the current gain (hfe) of a transistor with a couple of power supplies and meters. Crudely speaking, you vary the base current by a measured amount and see how much the collector current changes.

I'm glossing over the differences between small signal gain and large signal gain but don't think this matters here. At high frequencies, it's almost irrelevant whether hfe is high or low. Any well designed circuit should be able to cope with wide variations in hfe since it's highly variable.

As always, the ultimate test is how well the device works in circuit.

[Refugee]

I have got an old home made tester with five collector current settings and a crude leakage setting.
It uses a fixed Vce with a metal cased 741 to sense Vce at 5 volts and a pot and meter on the base.
HFE was found using a pencil and paper and later a pocket calculator.
Last time I used it I fed the collector current from a bench power supply with the DUT on a heat sink going much higher than it was from the start.
The original aim was to start making my own data sheets for unknown transistors that used to be on sale in bulk bags and finding matched pairs.