Simple Transistor Hfe Tester Circuit Diagram Using IC 741

Thursday, October 2, 2014

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P1 is used to set a reference voltage derived from voltage UXY UD1 (or D2 for a PNP transistor). This means that l the setting of the potentiometer is directly proportional to the hpp; of the transistor under test and is independent of supply voltage.
The voltage across R2 and the voltage set with P1 are compared by lC1 which is connected as a comparator. Potentiometer P1 is now set so that the LED at the output of the op-amp just lights or is just dimmed.
This hfe tester is interesting because of its simplicity and because it enables the B of both PNP and NPN transistors to be measured.
At this setting the voltage across the potentiometer is equal to the voltage across R2. Switch S1 is used to switch from NPN to PNP (or vice versa) by reversing the polarity of voltage UXY.
LEDs D3 and D4 in the supply lines ensure that the input voltages to be measured are within the common mode range of the opamp used.
Furthermore the measurement is independent of the supply voltage of the tester. As the diagram shows, the base current of the transistor under test travels via R1. Its base current IB is thus equal to The voltage drop across the collector resistor is hfe x IB x R2.