What is BJT? BJT (bipolar junction transistor) are widely used an amplifier, oscillator, switch etc. It is a current-driven device (MOSFET is voltage driven), the output current is equal to the input current times a factor which is called Gain. A basic BJT has three pins: the Base, Collector, and Emitter.
The output characteristic curve is useful as it shows the variations in collector current, Ic for a given base current, Ib over a range of collector-emitter voltage, Vce. This gives us the modes of the BJT under different conditions. There are three modes in BJT – Forward-Active (Amplification), Saturation, and Cut-off.
Saturation: high current conduction from the emitter to the collector. This mode corresponds to a closed switch. This could be also used for resistors simulation in small circuits.
Cut-off: the biasing conditions is the opposite of saturation (both junctions reverse biased) which corresponds to an open switch. The cut-off and saturation can be used together to form a digital (1 or 0) type of circuit for computers.
Forward-active: This is the linear region of the curves (shown as amplification mode in the diagram). The collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations. BJT amplifiers uses the Forward-active characteristics.
You might wonder whether you should use BJT or MOSFET, check out the argument here.
How to use BJT?
As a switch
Current-limiting resistors are usually used between the Arduino (or other micro controllers) and BJT to prevent damage and over heating from large current. Protection diodes are also sometimes used in case of Back-EMF from an inductive load.
For NPN BJT, the emitter is always connected to either the negative voltage supply (i.e. GND) and the collector is always connected to the load. The base is used to activate the switch.
As an amplifier
When used as an amplifier, the biasing is arranged so that the transistor operates in the linear region ( shown above as almost horizontal sections). An amplifier will usually be biased to about half the supply voltage to allow for maximum output swing.
Type of BJT
There are two types of BJT transistors PNP and NPN based on the doping types of the three main terminal silicon layers.
- PNP: usually used as a high-side switch where the emitter of a PNP transistor connects to the voltage supply, the collector connects to the load. To turn this transistor off, we can connect the base to the emitter. Turning this transistor on is a little confusing because a negative current or a 0v (GND) signal needs to be applied to the base.
- NPN: usually used as a low-side switch, the emitter of an NPN transistor connects to the GND, the collector connects to the load. To turn this transistor off, the base must connect to the emitter (GND). This transistor is turned on by applying a positive current to the base.
2n2222a NPN transistor is a simple low-side amplifier switch. It can be fully switched on with only a few milliamps of current and it can switch nearly 1 ampere.
2n2907a is the PNP, and it can be used as a simple high-side switch.
How does BJT work in theory?
A BJT is formed of a three-layer sandwich of doped semiconductor materials, either PNP or NPN. Each layer has a specific name, i.e. collector, emitter and base.
The proper biasing of the junctions when operating is the functional difference between a PNP transistor and an NPN transistor. For any given state of operation the current directions and voltage polarities for each kind of transistor are opposite.
Bipolar junctions transistors is current controlled which means a smaller current at the base controls the main current at the collector and emitter. For PNP transistors the main current goes from collector to emitter and the small controlling current goes from emitter to base, while for NPN transistors the main current goes from emitter to collector and the controlling current goes from base to emitter.
For example in an NPN transistor, when positive bias is applied to the base, the equilibrium is disturbed between the thermally generated carriers and the repelling electric field of the n-doped emitter depletion region. This allows thermally excited electrons to inject from the emitter into the base.
These electrons diffuse through the base from the high concentration region near the emitter towards the low concentration region near the collector. The electrons is minority carriers and holes the majority carrier in the base, because the base is P-doped.