A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labelled circuit diagram she would use and explain how it works.

In order to convert alternating current into direct current, she can use a full-wave rectifier. A full-wave rectifier uses two diodes, which enables it to pass both the positive and the negative side of the alternating current input. The diodes are connected to the transformer. The full-wave rectifier requires that you use a centre-tapped transformer. The diodes are connected to the two outer taps, and the centre tap is used as a common ground for the rectified DC voltage. The full-wave rectifier converts both halves of the AC sine wave to positive-voltage direct current. During the positive half cycle of AC, end A becomes positive and D 1 becomes forward biased and D 2 is reverse biased, so conducts and D 2 doesn’t. So conventional current flows through D 1 , R L and upper half of secondary winding. Similarly, during the negative half cycle of AC, diode D 2 becomes forward biased and D 1 is reverse biased, current flows through D 2 , R L and the lower half of secondary winding. Thus, current flows in the same direction in both half cycles of input AC voltage.

Why cannot we use Si and Ge in fabrication of visible LEDs ?

The energy gap for Si is 1 . 1 eV and for Ge is 0 . 7 eV . Both of the semiconductor are having energy gap less than 1 . 8 eV . Thus, instead of producing photon of visible wavelength, they produce heat in the form of Infrared rays. Hence, Si and Ge are not used in fabrication of visible LEDs.

How does an increase in doping concentration affect the width of depletion layer of a p-n junction diode?

As we increase the doping concentration the holes in p-sides across the junction and electrons in n-sides across the junction increase. As a result there will be decrease in breadth of the depletion layer.

Draw the circuit diagram of a full wave rectifier and explain its working. Also, give the input and output wave forms.

For full-wave rectifier, we use two junction diodes as shown in the figure. During the first half cycle of input a.c. signal the terminal S 1 is positive relative to S and S 2 is negative, then diode D 1 is in forward biased and diode D 2 is reverse biased. Therefore, current flows in D 1 not in D 2 . In the next half cycle, S 1 is negative and S 2 is positive relative to S . Then D 1 is in reverse biased and D 2 is in forward biased. Therefore, current flows in D 2 not in D 1 . Thus, for input a.c. signal the output current is a continuous series of unidirectional pulse.

Draw the circuit arrangement for studying the output characteristics of n-p-n transistor in CE configuration. Explain how the output characteristics is obtained.

The CE configuration of the transistor is when the grounded emitter is kept as a common terminal. The base is input terminal and the collector is the output terminal. Here, I b is base current; V B E is base-emitter voltage; Ie is emitter current; V C E is collector-emitter voltage, and I c is the collector current. Here, B , E a n d C corresponds to base, emitter, and collector, respectively. The output characteristics of CE configuration represents the variation of collector current with respect to the collector–emitter voltage keeping base current fixed. To get the output characteristics of an n-p-n transistor in CE configuration, we need to carry out the following steps: • Adjust a suitable constant value of base current . • Apply various values of collector-emitter voltage and note that the corresponding values of collector current . • Repeat the above step for various constant base currents. • Plot a graph between collector-emitter voltage and collector current. • The current obtained are called output characteristics.

Describe briefly the functions of the three segments of n-p-n transistor.

The three segments of n-p-n transistor are emitter, base and collector. (i) Emitter: The function of emitter is to emit the majority carriers which are electrons in the case of n-p-n transistor. It is moderate in size and heavily doped. (ii) Base: The function of base is to provide proper interaction between emitter and collector. It is then and lightly doped . (iii) Collector: the function of collector is to collect the majority carries. It is large in size and moderately doped.