Transistor beta tester

[carbonzit]

(And no, I don't mean the poor schmuck who has to test all those new models of transistors ...)

So let's say a guy wanted to build a simple beta tester. A couple ways to skin that cat:

• Apply a fixed base current and measure collector current
• Vary the base current until a certain collector current is reached

I'm liking the first method better at this point. The circuits I've looked at on the web are mostly pretty similar: some type of (more or less) fixed current source, and use an external meter (DMM or VOM) to measure Ic.

So what do y'all think about this variation?

**broken link removed**

The LM334 gives a rock-steady current (dependent on temperature, of course). 6.8K sets it 10 µA. Measure Ic, scale and divide and bingo! you've got your β number.

(This is for small-signal transistors only, no power trannies, Darlingtons, etc.)

 

[KeepItSimpleStupid]

No issues. Just need another slightly different one for PNP. Might put a resistor in series with the base.

 

[carbonzit]

Might put a resistor in series with the base.

Why? Because other circuits have one? What purpose does that serve? All I need to do is feed a small fixed current through the base. The 334 limits the current.

I have to say, KISS, sometimes I think you ought to change your name ...

 

[ericgibbs]

hi cz,
This link is helpful as background info on Beta's.
[more bed time reading]

The only time I have bothered to select Beta's is when requiring a matched pair.
The circuit components supporting the transistor should be chosen so that the Beta is not dominant in the circuit. This means should the transistor 'die' a replacement same type could be used without the need for tweaking the surrounding components.

Many bench test meters do have a transistor check option




Quick Transistor Tutorial


Extract:
Beta also depends on the collector current, and the temperature. Typically the smaller the collector current, the smaller the beta. To see the effects of temperature it suffices to touch the transistor and a small change in collector current will occur. As a result when designing predictable BJT amplifiers the effects of beta, and temperature variations have to be taken under consideration.