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transistor beta matching

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bioborg

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:?: is there a way to read a bjt transistors beta without a beta tester or the ohms way (in ms or ns).
 
Measuring Beta

Sure.

Set up a circuit with the emitter of the transistor under test grounded, a sensitive ammeter and a current-limiting resistor between a variable power supply and the base, and a higher current ammeter between the collector and a power supply about equal to the collector voltage in your application.

Set the variable supply to zero and apply power to the collector (via the ammeter). CAREFULLY and gradually increase the voltage supplying the base (via the resistor and sensitive ammeter) until you get the collector current you expect in your application. Beta is the ratio of the collector current to the base current.

Now, this simplest of setups will allow you to fry your ammeters if you are not careful. You can get some protection by anticipating the maximum base current you expect and using a fairly high variable power supply voltage (say, 10 or 20 volts) and a high value current limiting resistor that will give you about 150% of the expected base current at maximum voltage. That way, you are less likely to inadvertently get excessive collector current (because you applied too much base current by a careless twist of the knob).

You can also eliminate the meters, use appropriate resistors in the base and collector circuits, measure the voltage across the resistors with sensitive DVMs, and calculate the base and collector currents. You are less likely to burn out a meter this way, since most DVMs are pretty well protected from input overvoltages, but you have to know the resistor values fairly accurately.

If it would be useful in your application, you can also measure the base-emitter voltage with a DVM while you have the design currents flowing.

Be sure your currents and power dissipation are within ratings for the transistor (as mounted) during your test.

Have fun.
awright
 
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