Bipolar junction Transistor – BJT

Emitter is moderate size and heavily doped. Collector is large in size and moderately doped. Base is thin and lightly doped. When collector supply is 5V then collector cut off voltage is 5V. Zero signal condition is when transistor see dc load.

Cut-off region – both junctions are reverse biased, Saturation region – both junctions are forward biased, Active region – Emitter base is forward biased and collector base is reverse biased. Transistor can work as a switch in saturation and cut off mode. Transistor work as an amplifier in active mode. Maximum power dissipation occurs in active mode. Inverted mode is not used because of very law current gain. In active mode collector current depends on base current and independent to collector emitter voltage

Transistor has high output resistance because of reverse biased collector junction. Maximum operating frequency of a transistor is 50% of the frequency at cut off. Silicon transistor works in high temperature (exceeds 185f). Heat dissipation is important feature of power transistor. Transistor used in video amplifier has low frequency response. Parasitic oscillations are unwanted oscillations in electronic components and circuits

Q point (operating point) parameters in transistor biasing is Ic and Vce. Current mirror is used for temperature stabilised biasing. Potential divider bias is used to stabilise collector current. Operating point is maintained by a factor called stability factor (S), which indicates the change in operating point with temperature change

Forward bias voltage – 0.5 to 1 V, Reverse bias voltage – 5 to 15 V

Common collector is used as buffer, voltage follower, emitter follower and used for impedance matching. Class A, AB and B are common collector. Common base is used as RF voltage amplifier, Common emitter is used as AF voltage amplifier, low noise amplifier. Leakage current in transistor is Icbo (cbo – collector to base with open emitter), Icbo is also called Ico

Leakage current is highly temperature dependent, for Ge leakage current doubles for every 10 degree temperature rise, for Si leakage current doubles for every 6 degree rise, To avoid thermal runaway Vce ≤ Vcc/2. Reason for thermal runaway is change in reverse collector saturation current due to temperature rise. Collector current of a CE transistor can be increased by changing biasing resistances as result conductance increases

AC load line is steeper than DC load line. Two type of transistor break downs are there – avalanche breakdown and reach through/punch through. Reach through occurs when voltage across collector base is increased, collector junction width is increased, in this increase of width more junction is penetrated into the base because of lightly doped base this process is called early effect or base-width modulation

Short circuit current gain – bandwidth product of a transistor changes with change in the operating collector current. Emitter follower is widely used in electronics instrumentation because it has high input impedance and low output impedance. Emitter follower has 100 % negative feedback. Common emitter is also called grounded emitter.