hi guys,

i'm doing some HFE tester and i need to compare the GAIN of TRANSISToR with a referce value.

any ideas how cna i do it ?

regards,
crowinu

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AC hfe? DC Hfe?

At what current level(s)? What Vce level(s)?
The values you get will depend on these parameters.

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Ron

 

If your collector voltage is above the saturation region, the Hfe won't change much with collector voltage, so all you need is a resistor in the collector to measure the collector current and a resistor in the base circuit to measure the base current.

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it is DC HFE

my aim is to for example set a certian GAIN value of 200 and determin if it is greater than 200 or less thne 200

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you mean i get ib and ic and then with ic/ib i get the gain ?

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yes.

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ok i undestood that, but need to do it with a microcontroller maybe i didn't explain myself right :|

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So, you need 3 or 4 A/Ds in the microcontroller to measure the base voltage, the base drive voltage, the collector voltage, and if the VCC is not fixed, a 4th A/D to measure VCC. Alternativly, you could use one A/D and switch the inputs.

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what do you mean by base drive voltage ? and is it worth it to change VCC or just keep it fixed ?

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The classic way to measure gain is to measure only ONE parameter, the collector current. For your purposes it would be easier to measure the collector voltage, you can then easily calculate the current and hence the gain.

The technique is to feed the base from a constant current source, no need to measure it as you already know what it is.

So historically, you would feed the base via a constant current source, and have a milliammeter in the collector, by suitable selection of the constant base current and meter range, you can read the current gain directly off the analogue meter scale.

There are obviously far more complicated methods, but this is simple, easy to do, and should do all you need?.

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Base drive voltage is the voltage supplying current to the base. It could be VCC which would reduce the number of measurments you have to make. Fixed VCC will simplify matters because you don't have to measure it with the micro.

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yes i think its a goo idea. i dont want to see the gain trough an anlaouge meter cause i need to determine if the gain of the tested transistor is bigger of smaller than a certain value.

thanks nigel

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That was why I said to measure the collector voltage and calculate the current - a simple job for a micro-controller.

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You don't need a microcontroller. You could do this with a comparator, unless you are doing it as an assignment for a class.

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Ron

 

I was basing that on his earlier post!

Presumably it's a class project, or he wants to add more functionality?.

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