Transistor as a Zener

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Zener diodes connected as shown in Fig. 1 are carefully doped at manufacture such that they break down when a critical reverse bias voltage is applied and allows current to flow. The current is limited by the power ratings of the particular device employed.

Zeners are used to provide a voltage reference and any transistor can be employed to do the same job if connected as shown in Fig. 2.

Different transistors will produce different zener voltages and a series of tests can be done to establish what the typical zener voltage is for that device.


A suitable setup could be employed as shown below.


 

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Transistor as a Zener


  A BC548B gives a zener voltage Vz = 8.2V


  A BC549B gives a zener voltage Vz = 10.8V


  A BC547B gives a zener voltage Vz = 9.1V


  A BC184 gives a zener voltage Vz = 8.2V



Using the Configuration in Practice

The above circuit Fig 4 could form the basis of a low power regulated power supply.


Variable Reference Voltage Circuit

Fig. 5 shows a way of adjusting the voltage Vz to suit by adjusting the resistor values of R1 and R2.


Vz = Vbe((R1 + R2)/R2)


Better regulation can be achieved with a darlington configuration as shown below where it is used in a simple regulated supply, but now the voltage Vz becomes:-


Vz = 2Vbe((R1 + R2)/R2)


and the regulated output voltage becomes:-


Vo = Vz - Vbe

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