Diod Transistors
![](https://www.malabdali.com/wp-content/uploads/2020/05/diod-transistor-1024x463.png)
Transistor as a Zener
![](https://venerabelia.co.uk/____impro/1/onewebmedia/TZener.jpg?etag=%22bf93-52bc0525%22&sourceContentType=image%2Fjpeg&ignoreAspectRatio&resize=594%2B270&extract=NaN%2BNaN%2B594%2B265&quality=85)
![](https://www.malabdali.com/wp-content/uploads/2020/05/zener-transistor-1024x536.png)
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.
![](https://venerabelia.co.uk/____impro/1/onewebmedia/zener%20diode.jpg?etag=%22W%2F%22%20%2296ad-52bc1c14%22&sourceContentType=image%2Fjpeg&quality=85)
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
![](https://venerabelia.co.uk/____impro/1/onewebmedia/zenercirc01.jpg?etag=%22W%2F%22%20%22d89a-52c02fba%22&sourceContentType=image%2Fjpeg&quality=85)
The above circuit Fig 4 could form the basis of a low power regulated power supply.
Vbe Multiplayer
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
![](https://venerabelia.co.uk/____impro/1/onewebmedia/2vbe2.jpg?etag=%22W%2F%22%20%22e1a2-52c6b2f3%22&sourceContentType=image%2Fjpeg&ignoreAspectRatio&resize=347%2B268&extract=NaN%2BNaN%2B347%2B265&quality=85)
Other more circuits
![](https://www.malabdali.com/wp-content/uploads/2020/05/more-volt-o7.png)
![](https://www.malabdali.com/wp-content/uploads/2020/05/stable-voltage-1024x441.png)
![](https://www.malabdali.com/wp-content/uploads/2020/05/vbe-multi-player-1024x386.png)