Friday, March 14, 2014

Expression for energy density of a parallel plate capacitor

As we know that the energy associated with any parallel plate capacitor is given by:

Modulation and its need in Communication System

Modulation is the process of varying one or more properties of a high frequency carrier signal, with respect to the message signal. Different types of modulation are: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).
The need for modulation can be summarized as follows:
(1) Message signal or voice signal have low frequencies, and hence cannot travel long distances, so in order to make this possible we use the technique called modulation
(2)  The antenna needed for transmitting signals should have size at least λ/4, where, λ is the wavelength. The information signal, also known as baseband signal is of low frequency (and therefore the wavelength is high). If we need to transmit such a signal directly, the size of the antenna will be very large and impossible to build. Hence direct transmission is not practical.
(3)   The radiated power by an antenna is inversely proportional to the square of the wavelength. So, if we use high frequency signals, the power radiated will be increased.
(4)  If we transmit the baseband signals directly, the signals from different transmitters will get mixed up and the information will be lost.
Demodulation is the process of extracting the original information - bearing signal from a modulated carrier wave.

Sunday, March 9, 2014

Solution to Doppler Effect Numerical Problems

Doppler Effect Formula List

 Numerical 13 is similar problem as 12 only the values has changed.

Wednesday, March 5, 2014

Tuesday, December 17, 2013

PhotoDiode

What is a photo-diode?

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.

The common, traditional solar cell used to generate electric solar power is a large area photodiode. Photodiodes are similar to regular semiconductor diodes except that are exposed (to detect UV or X-rays or visual light to reach the sensitive part of the device so that the electron excitation can take place from valence band to the conduction band, thereby developing required potential difference.

How photo-diodes are operated?

A photo-diode is designed to operate in reverse bias When photo diode is directly biased it acts like normal diode. But in reverse biased mode current through the diode depends on the brightness and you can use this relation between brightness and current for something you want your circuit to do.

Why photo-diodes are preferably used in reverse bias mode?

A photo diode is a diode and it do act as a barrier in reverse bias as in the case of diode, but in case of photo-diode as the light falls on the reverse biased PN junction there forms a discharge that discharge leads to formation of new electrons and holes, as we have electrons and holes in the reverse bias PN junction and due to the external voltage applied across a the diode leads to the reverse break down (i.e Zenner breakdown)and hence the (minority dimonated) current flows. The number of electrons and holes in reverse bias junction depends upon the discharge which in turn depends upon the light intensity and with the increase intensity of light the current through the diode in reverse biase increases. Thus in reverse biased mode current through the diode depends on the brightness and you can use this relation between brightness and current for something you want your circuit to do.





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