This is a circuit can b e used to check the hfe of transistors. Both PNP and NPN transistors can be checked using this circuit. Hfe as high as 1000 can be measured by using this circuit. This circuit is based on two constant current sources build around transistors Q1 and Q2. Q1 is a PNP transistor and the constant current flows in the emitter lead. The value of constant current can be given by the equation; (V D1 -0.6)/ (R2+R4).The POT R4 can be adjusted to get a constant current of 10uA.
Q2 is an NPN transistor and the constant current flows into the collector lead. The value of this constant current can be given by the equation; (VD2-0.6)/(R3+R5).The POT R5 can be adjusted to get a constant current of 10uA.This constant current provided by the Q1 circuit if the transistor under test is an NPN transistor and by Q2 circuit if the transistor under test is a PNP transistor is fed to the base of transistor under test. This current multiplied by the hfe flows in the collector of the transistor and it will be indicated by the meter. The meter can be directly calibrated to read the hfe of the transistor.
- D1,D2: Diode Zener 5.6volt 1/2 watt
- Q1: 2N2907 PNP transistor
- Q2: 2N222 NPN transistor
- R2,R3: 470K Resistor
- R4,R5: 100K Variable Resistot (Potensiometer)
- M1: Miliampere meter Analog (10mA)
- J1,J2: transistor sockets