hFE tester

32 hFE tester This hFE tester is interesting because of its simplicity and because it enables the of both PNP and NPN transistors to be measured. Further- more 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 l is thusequaltoUXYR1UBE The voltage drop across the collector resistor is hFE x IB x R2. P1 is used to set a reference voltage derived from voltage UXY — UD 1 (or D2 for a PNP transistor). This means that the setting of the potentiometer is directly proportional to the hFE of irvwiwa 1 300 pÏ 500 D4 Al 200 Cl which is connected as a compara- tor. Potentiometer Pl is now set so that the LED at the output of the op-amp just lights or is just dimmed. At this setting the voltage across the potentiometer is equal to the voltage across R2. Switch Si is used to switch from NPN to PNP (or vice versa) by reversing the polarity of voltage UXy. LEDs D3 and D4 in t...