Transistor As a Diode (Closed Collector)
Category: Basic Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is designed and simulated in Proteus 7.10. This circuit 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. 5. In this circuit base emitter junction of npn transistor is used as a diode, while collector is connected with base. 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 accross the component according to the resistance and property of a component. 8. VR1 shows the voltage drop across the resistor R1. 9. VBE shows the voltage drop across the base emitter junction (diode) of transistor Q1. 10. VR2 shows the voltage drop across the resistor R2 (load resistor), 11. The battery voltage is the sum of all the voltages. 12. VR2 = VB1 - VR1 - VBE 13. The voltage VR2 is not dependent on the resistance of R2. Its the property of a transistor that voltage at emitter is the voltage present at the base minus base emitter junction. Application: 1. This property of a circuit can be used in making a linear regulator with a transistor. 2. This property can also be used to connect or switch load at emitter. 3. This circuit can also be used as a diode.