Test: BJT Construction & Operation - Electronics and Communication Engineering (ECE) MCQ

A BJT is a device with a layer of semiconductor sandwiched between 2 unlike types of semiconductors and hence, PPN is not a valid type of a BJT.

The emitter is the most heavily doped and contains the maximum amount of charge carriers. It is the emitter’s task to inject carriers into the base. These bases are thin and lightly doped. For npn B JT, emitter injects electrons, and for pnp, it injects holes.

As the base current is quite lower as compared to the collector and emitter current, it is usually in the order of microamperes.

Leakage current in BJT is represented by I_CO, which is due to the flow of minority carriers in the transistor. It consists of I_CBO and I_CEO. I_CO depends on temperature, doubling with 10° rise in temperature. It thus effects total collector current, I_C, and hence affects the power dissipation.

On applying KCL to the BJT, we get I_C + I_B = I_E.

As the base is lightly doped, the number of free charge carriers are less and hence the resistance is high and as the emitter is the most highly doped, its resistance is low.

ower Dissipation in a BJT is given by P=V_CE.I_C. This is in the form of k=xy which is the equation of a hyperbola.

There are multiple ways to test the BJT to be npn or pnp using a DMM based upon which terminals are connected to which lead of the DMM. If positive is to base and negative to the emitter of the BJT, and if a low reading is obtained and not a over limit, then the transistor is NPN.

Concept:

For an NMOS transistor If;

V_GS > V_T  and V_DS < (VGS - VT) → Device is in Triode region

VGS > VT  and VDS ≥  (VGS - VT) → Device is in the Saturation region

Analysis:

Given;

V_t = 0.7 V

V_S = 0 V

V_G = 1.5 V

V_D = 0.5 V

V_GS = V_G - V_S = 1.5 - 0 = 1.5V

V_DS = V_D - V_S =0.5 - 0 = 0.5 V

As, VDS < (VGS - VT)

The device will be in Triode region'

Common Base Configuration of BJT:

• In the Common base configuration, the base connection is common to both the input signal AND the output signal.
• The input signal is applied between the transistors base and the emitter terminals, while the corresponding output signal is taken from between the base and the collector terminals.
• The base terminal is grounded or can be connected to some fixed reference voltage point.

Characteristics of Common base BJT:

• Input Impedance - Low
• Output Impedance - Very High
• Phase Shift - 0^o
• Voltage Gain - High
• Current Gain - Low
• Power Gain - Low