Measuring Gain on NPN?

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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LooseCannon
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Post by LooseCannon »

Hey guys,

I'm looking for a way to measure hfe on silicon NPN transistors (I don't have an HFE function on my DMM).

I've searched all over the web but only found ways to measure Germaniums which includes voltage leakage.

I did find this page: http://sound.westhost.com/project31.htm

Will the circuit on figure 1 work? He doesn't state the value of the resistor and what power supply should be used.

Any ideas?

Thanks

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DrNomis
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Post by DrNomis »

LooseCannon wrote:Hey guys,

I'm looking for a way to measure hfe on silicon NPN transistors (I don't have an HFE function on my DMM).

I've searched all over the web but only found ways to measure Germaniums which includes voltage leakage.

I did find this page: http://sound.westhost.com/project31.htm

Will the circuit on figure 1 work? He doesn't state the value of the resistor and what power supply should be used.

Any ideas?

Thanks


You should be able to use the circuit on R.G. Keen's website to measure the gains of NPN Silicon Transistors, since the circuit should work equally well because Ge and Si Transistors work in the same basic way, except that Si Transistors have leakages in nano-amps whereas Ge Transistors have leakages in micro-amps, which is about 1000 times more than Si Transistors, your gain readings will be higher though, I just checked out the link and there is a schematic for a suitable power supply, I wouldn't mind building the circuit up cause it looks like a useful bit of test equipment....... :thumbsup


It looks like the circuit in fig 1 should work fine..... :thumbsup


What these Transistor Tester circuits are doing is feeding a set amoumt of current into the base of the Transistor being tested, and they measure the amount of current flowing through the collector and compare the difference, if you want to get a bit more mathematical the current-gain (Hfe) of a Transistor, sometimes called the "Beta", is equal to the Collector-current divided by the Base-current, this is irrespective of what semiconductor material is used to make the transistor....... :thumbsup


Hope that helps..... :thumbsup
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Post by LooseCannon »

Thanks for the info DrNomis!

So again regarding the figure 1 schematic, what resistor value should I use?

I've seen the schematic for the power supply but there's no way i'm gonna build that for this little tester ;)

Would a 9v battery work as a power supply?

Thanks

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Post by DrNomis »

LooseCannon wrote:Thanks for the info DrNomis!

So again regarding the figure 1 schematic, what resistor value should I use?

I've seen the schematic for the power supply but there's no way i'm gonna build that for this little tester ;)

Would a 9v battery work as a power supply?

Thanks


Maybe try using a 1M resistor for the base resistor, you should then be fine with using a 9V battery as the power supply, note that the accuracy depends on the accuracy of the 1M resistor, a 1% metal film resistor should do the job, still, the accuracy should be good enough for a ballpark figure...... :thumbsup
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Post by LooseCannon »

OK I built it and I get 0 current measurement when using my DMM on the Collector and +9v.

Any ideas?

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Post by LooseCannon »

This is a new cheap DMM I got and I didn't use it to test current yet, I guess it's possinle that something is wrong with it? Is there an easy simple way to verify this?

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Post by LooseCannon »

OK I replaced a fuse in my DMM and now it's ok.

So when testing with a 1M resistor I get a mA reading of 00.15 for a sample transistor, and with a 2.2M resistor I get a reading of 00.39.

So which resistor would give me the right reading?

Thanks

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Post by DrNomis »

LooseCannon wrote:OK I replaced a fuse in my DMM and now it's ok.

So when testing with a 1M resistor I get a mA reading of 00.15 for a sample transistor, and with a 2.2M resistor I get a reading of 00.39.

So which resistor would give me the right reading?

Thanks


Either resistor should give you valid readings, what you need to do first is some maths, you need to find out how much current the base resistor is allowing to flow into the transistor's base, you can work out the current by dividing the supply voltage by the value of the base resistor, in other words:


9V/1M= Base current


Have a look at R.G. Keen's article on picking transistors for Fuzz Face:

http://www.geofex.com/
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Post by LooseCannon »

http://www.geofex.com/Article_Folders/ffselect.htm

So according to R.G. Keen, for Germanium testing you measure the leakage with a resistor from the collector to ground, then subtract it from the reading of the 2M2 resistor connected to base, then multiply by 100 to get actual gain.

So for my purpose should I just measure the 2M2 connected to base in mV, multiply by 100, and it would give me the actual gain? Or is it a different measurment without the 2.472k resistor from collector to ground?

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