What is a Transistor: – Transistor is a three terminal electronic device which is used as a switch or amplifies the signal. The three terminals are emitter, base and collector. The transistor is available either in NPN or PNP configuration.

 Need of a Transistor: – In 1930’s when radio transmitters required high power most of the radio receivers needed to amplify the radio signals so a device is needed which amplifies the signals. In various electronic circuitry power  loss occurs due to various reasons, that why we require the  electronic devices  which amplifies the low voltage signal to the particular desirable level.

Transistor symbols are shown as below.


Testing of Transistor: – As we know transistor is a combination of two diodes. So transistor can be tested by diode testing method. A good condition transistor checks by following method.

  1. Put the multimeter in diode testing mode and find out the base terminal.
  2. Base terminal is identifed by checking the reading of the both remaining terminals.
  3. If common terminal is connected to the base then it is NPN transistor otherwise it is  PNP transistor.
  4. Check the  transistor in forward mode and note the resistance, the terminal  which gives more reading is emitter and remaining is collector.
  5. If transistor  is  short circuited then it gives continuous beep and shows very low resistance.
  6. Another case is when transistor is  open circuit then it gives infinite resistance in forward biased.

Thus we find out that transistor is in good condition or not.

Numeric code of Transistor

Transistor codes beginning with B (or A)

  1. A is for germanium (rarely used now).
  2. The first letter B is for silicon,

The second letter indicates the type

  1. C means low power audio frequency
  2. D means high power audio frequency
  3. F means low power high frequency.

Here is some common transistor which mostly used in circuits

Rating Symbol BC546 BC547 BC548 Unit (DC)
Collector-Emitter Voltage VCEO 65 45 30 Volts
Collector-Base Voltage VCBO 80 50 30 Volts
Emitter-Base Voltage VEBO 6 Volts
Collector Current IC 100 M Amp

Transistor Leads: – Transistors are package in different package which shown below