Transistor gain checking...Not Ge

[m_charles]

Hi,

I was wondering if anyone could help a mathmatically challenged guy learn to check transistor gains on regular silicone transistors with only a cheapo ANALOG multi-meter. I read the GEO article about checking leakage and gains for Ge FF trannies, but it kinda lost me. To start, just for fun, I'd just like to be able to check the gain on a 2n3904 or something but have read the instructions on my AMM and can't find anything. Of course my cheapo AMM doesn't have a setting specifically for tranny gain checking.

Is there a "simple" way?

thank you,

chuck

[R.G.]

Yep. Silicon does not leak like germanium does. That makes it easy.

The simplest thing to do is to buy a cheap meter with Hfe checking. This can be under $20.

The next simplest thing is to put a few microamps of current into the base and measure the collector current, then divide.

Here's how:
1) set up a test circuit with a 9V battery. For NPNs, hook a 1M resistor from the base to +9, the emitter to ground. Hook a 1.0K resistor from +9 to the collector.

2) Measure the voltage across the base resistor and the collector resistor. The base resistor voltage will be battery voltage minus about 0.6V. The voltage across the 1.0K will vary.

3) The ratio of the currents in the resistors is the gain. Conveniently, the voltage across the resistors is the current times the resistance. So now we do the math:
Ib = (Vbat-Vbe)/Rb      Ic = (Vbat-Vcollector)/Rc

Since your can probe both ends of the two resistors, you don't have to do the subtractions.

Then hfe = Ic/Ib

This is the DC hfe, not the AC hfe, but it's a useful indicator, especially since hfe varies with lots of things.

Example:
Vbattery = 8.7V
Rb = 1.0M, Rc = 1.0k
Vrb = 8.1V
Vrc = 1.0287V

Ib = 8.1V/1.0M = 8.1uA
Ic = 1.0287V/1.0K = 1.0287 ma.

hfe = 1.0287/0.0081 = 127


Some caveats:
1. The precision of your resistors determines the precision of the measurement and so the maximum precision of the answer. This is usually not a problem since the hfe varies with operating current and temperature anyway.
2. If you find a voltage on the collector resistor of nearly the battery voltage, you may have a high gain transistor that is saturating and so you need to use a smaller collector resistor to let an accurate current flow.
3. If the voltage across the collector resistor is so small that you have trouble reading it, you need to use a larger collector resistor to let you measure better.

[m_charles]

Wow, thanks RG!

Apprieciate the time you spent.

Can you help me remember my algebra symbols from decades past?

is it: anything in () means multiply and / means division (I realize I'm oversimplifing)?

thanks again RG,

chuck

[slacker]

The brackets mean you do that bit of the equation as though it was a separate calculation.
So for example in this case   Ib = (Vbat-Vbe)/Rb you do Vbat-Vbe then divide that answer by Rb. Or just put it into your calculator as it's written and you should get the right answer.

[m_charles]

sad that I'm struggling with 8th grade level math...

thanks for the info guys.

[m_charles]

wait, whats the difference between the AC and DC hfe?

[antojado]

AC and DC mean 'alternating current' and 'direct current' respectively. Batteries provide dc voltage which is why the setup above provides the dc hfe.