DIY Transistor tester Vs. My DMM what's the diff?

[Hiwatt25]

Will building one of the various transistor testers/matchers available give me any information that I can't get from the transistor tester on my DMM?  Is there an advantage to building one when it comes to testing/matching?

Thanks

[brett]

Hi
RG Keen's tester is designed to measure leakage, which most DMM testers ignore because modern silicon transistors leak very very little.
So if you plan to use old Si transistors or any Ge transistors, you'll need a tester like RG's at geofex.com
cheers

PS most testers are a little limited in that they test at one amount of base current (Ib) and usually measure collector current (Ic) (often indirectly via voltage).  Because different circuits set diffent Ib and Ic, the conditions of the test and the conditions of use will be different, sometimes by a small amount, but sometimes by a very large amount.  Because there are different relationships between hFE (aka "gain") and Ib and Ic for different devices, selecting for a match at one level of Ib or Ic will only give an approximate match at other Ib and Ic.

[Paul Perry (Frostwave)]

I'm suprised that nobody (so far as I know) makes a transistor tester that allows one to bias up the transistor and then apply a small audio test signal and see what it is amplified to. Depending on the circuit, one could then work back to the gain figure, without any complications of leakage.

it is certainly theoretically possible to make a circuit that would read out leakage AND small signal AC gain... anyone?

[brett]

Hi
A simple sine wave generator like the two transistor one at geofex.com could be used as a signal source.  IIRC it generates about 1V p-p from a 9V source.  This output could be trimmed to 100mV by a 1k trimmer (the 100 mV output is conveniently measured by your DMM*).  Then divide the signal down to approx 1mV (100 ohms + 10k divider resistors).  Plug that into the base of the test transistor (biased into action).  The output in mV is equal to the AC gain. 
Because its a sine wave, even a cheap DMM will get the results right. 
All done for about 20 parts and $3.

* If your DMM doesn't have an AC scale that goes as low as mV readings, you can use an "ideal rectifier" to rectify the signal and use the DC V scale.  An ideal rectifier is simply an op-amp with a rectifying diode in the feedback loop.  Search the archives for details.