Jawaharlal Nehru Technological University Hyderabad B.Tech III Year BASIC ELECTRONICS Previous Question Paper (October - 2020 - R13)

The V-I characteristics of a P-N junction diode represent the relationship between current (I) and voltage (V). In the forward bias region, the anode is connected to the positive terminal and the cathode to the negative terminal. As the forward voltage increases, the depletion region narrows, and current increases exponentially once the barrier potential (approximately 0.7V for Silicon, 0.3V for Germanium) is overcome.

The dynamic resistance (or AC resistance) is defined as the reciprocal of the slope of the V-I characteristic curve at a specific operating point. It is calculated as the ratio of a small change in voltage to the corresponding small change in current:

where η is the ideality factor (1 for Ge, 2 for Si), V_T is the thermal voltage (approx. 26mV at room temperature), and I_D is the DC bias current. This resistance decreases as the forward bias current increases.

A π-filter consists of an input capacitor (C1) in parallel with the load, followed by a series inductor (L), and another capacitor (C2) in parallel with the load. It acts as a low-pass filter to smooth the output of a rectifier.

Working: The input capacitor C1 reduces the ripple by charging and discharging. The series inductor L offers high impedance to the AC ripple component while allowing DC to pass. The second capacitor C2 further shunts the remaining AC components to ground, resulting in a very smooth DC output.

The ripple factor (r) for a π-filter is derived by considering the attenuation of the ripple frequency (2f) through the filter stages. The expression is given by:

where ω is the angular frequency of the input supply, L is the inductance, C1 and C2 are the filter capacitances, and R_L is the load resistance. This shows that the ripple factor is inversely proportional to the product of L, C1, and C2, making it highly effective for filtering.