A transistor is a device which can switch on or off depending on the base current. A transistor can be called a switch because in case of a switch we use mechanical force to turn on or off the switch and a transistor can also be turned on or off but by using electrical signals. It also has an intermediate stage between on and off called the active region which has a linear output with respect to the input at base. the off state of transistor is cutoff region and the on switch state is called as the saturation region.
lets take an example of a water dam to understand the working of a transistor. In case of a water dam there is lots of water stored on one side and an empty area on the other side, to which water flows. the flow of water from one side to other is controlled using the dam gates.if the gates are opened slightly then water flows in less quantity and if gates are fully opened then water rushes.
The working of transistor is similar to this dam where the side with lots of water is the collector, the side to which water flows is the emitter and the dam gate is the base terminal.
the most used transistor is the NPN transistor which switches on with a positive base voltage while the other PNP transistor switches with a ground or negative base voltage.
SELECTING THE CORRECT TRANSISTOR
There are thousands of transistor models present to match our needs. Depending on our need we can choose the transistor based on various parameters.
Voltage : Usually the transistors we use are rated at 24v max but if in applications such as smps, HV circuits,etc. we need to choose transistors which can handle such higher voltages.
Current : In low power circuits we can use transistors such as BC547, BC548 which handle a maximum current of 100mA but in high power circuits such as Led drivers, Amplifiers, Inverters etc we need to use transistors which can handle higher current. below are some examples of transistors and their current ratings.
2N3904-200mA
BC337-500mA
2N2222- 800mA
BD139,BD140-1.5A
TIP31,TIP32-3A
2N3055-15A
Power : power choice is also taken with respect to both the voltage and the cuurent ratings of a transistor.
Frequency : In circuits such as FM transmitters, High frequency oscillators, High speed switchs, RF circuits etc we cannot use normal transistors, we should choose transistors which can operate at such high frequency to satisfy the speed and also the output.
here are some examples of Frequency values of transistors
2N3904-300Mhz
2N3866-500Mhz
BFY90-1Ghz
2SC3358-7Ghz
Gain : For applications such as inverters and switching circuits amplification is not needed so gain may not be considered in these cases but for amplifiers we need to consider the gain to know the number of stages of amplification needed to get the required amplitude from smaller signals. usually it is given as hfe. here are gains of few transistors.
BC547-110-800
2N3055-20
TIP142- >1000
BD139-40-250
Apart from these parameters we can also consider other parameters such as base current, package type, heat dissipation etc.
Package :
Package :
here are two tables with various parameters of few transistors. we can use them to select the appropriate tansistor.
