NPN Transistor As Dual Diode
Category: Basic Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is designed and simulated in Proteus 7.10. Previous circuits shows how a transistor is used as a diode. Working of this circuit is given below. 1. Transistor is a very common device in electronics and variety of transistors are easily available in the market. 2. Sometimes components get short while making electronic circuits as a hobby. 3. If diode is not available but transistor is present and functionality of diode is needed. Then transistor can be used as a diode. 4. There are more than one combination / possibilities for using transistor as a diode as shown in previous circuits. 5. In this circuit both base collector junction and base emitter junction of npn transistor is used as a diode, or we can say transistor is used as dual diode. but resistance of both junction is different because of different concentration of doping. 6. JP1 is a switch, when it is closed, current starts flowing through the circuit. 7. When current is flowing through the circuit, voltage will drop across the component according to the resistance and property of a component. 8. Left most volt meter shows voltage (VR1) drop across the resistor R1. 9. Voltage drop across the base emitter junction (VBE = 0.57V), base collector (VBC = 0.56V) junction of transistor Q1 is also shown using volt meter. 10. Voltage (VR2) drop across the resistors R2 and R3 (load resistors) are also shown, which is same 11.2V (behaving as parallel diode). 11. The battery voltage is the sum of all the voltages. 12. VR2 = VB1 - VR1 - VBE and VR3 = VB1 - VR1 - VBC 14. The voltage VR2 and VR3 is not dependent on the resistance and voltage drop is 11.2V. Application: 1. This property of a circuit can be used dual diode using transistor. 2. This property can also be used to connect or switch load at emitter or collector. 3. This circuit can also be used as a diode. 4. If one junction of transistor is damaged or not working, then other junction can be used as diode.