|
|
|
|
|
|
Datasheets : Transistor - Driver Unit
What does it do?
|
||||||||||
|
Transistor Driver circuit Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
The subsystem acts as an inverter; the output signal is the inverse of the input signal. The output subsystem is connected between the supply rail (+Vs) and the output signal. The output subsystem is sometimes called the load resistance. The transistor driver circuit uses an NPN transistor, which has three legs known as the base, emitter and collector.
|
|
When the voltage between the base and the emitter is at least 0.7V, a small current flowing into the base will cause a much larger current to flow from the collector to the emitter.
The transistor current gain can be calculated:
A typical transistor would have a current gain of about 100. The BC108 is widely used in school electronics but it is more expensive and less effective than modern transistors. The BC548B is a typical modern transistor. It can provide up to 100mA for a load and can directly replace the BC108 in circuits. The resistor R1 is used to limit the current from the previous subsystem. For use with a CMOS IC R1 should be about 22k. For use with a PIC R1 should be about 1k2. Full details on selecting this resistor are given on this web site. The resistor R2 is included to help with testing – it pulls the output signal up to Vs when the transistor is off. | |
|
Pins of the BC547B |
 How part of the PCB might look |
When using a transistor it is important to be clear about which pin is the collector (C), which is the base (B) and which is the emitter (E). The PCB diagram shows the connections for the BC547B.
Manufacturer’s data sheets usually show the pins viewed from underneath (unlike IC pins, where the view shown is from above).
Build and test the unit that will provide the input signal before building the Transistor Driver.
Make sure that the signal going out (on the green PCB track) is the inverse of the input signal (on the blue PCB track).
If there is a fault, check that the transistor is connected the right way round. Then check the resistor values. Check the tracks and solder joints.
