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Showing posts from October, 2011

O/p resistance of Mosfet

In analog applications such as current mirrors or active loads, it is important for the transistor to have a large output resistance. Such circuits emulate a current source or current sink, and the Norton resistance of such a circuit should be large for ideal behavior. The output resistance, usually denoted by rO, is a measure of how much drain-to-source voltage change is necessary to cause a given change in transistor output current when the transistor is in active mode. This resistance depends upon VGS, of course, because the channel conductivity depends upon the number of carriers within it, and that increases with gate voltage. However, rO also varies with VDS. The reason a change in drain bias changes the resistance is that the channel exists only when the oxide field is sufficient to form a channel. At the source itself the oxide field is dependent upon the voltage drop VGS, which in active mode is above the threshold voltage, and so a channel forms. However, near th…

Operation of MOSFET

A narrow, surface inversion layer of electrons forms at large enough positive gate voltages. The horizontal dashed line indicates the Fermi level, the energy of levels (should they exist) that are half-occupied at the selected temperature. The control of the channel by the gate is similar to the formation of an inversion layer in the MOS capacitor, which is only a two-terminal device (gate and body contacts). The case of a p-type semiconductor body in which mobile holes are introduced in the valence band by introducing acceptor impurities is described below. The acceptors suck electrons out of the valence band, becoming fixed negative ions, and leaving electron vacancies in the valence band that behave as positively charge mobile holes. The formation of this layer is understood by examining the behavior of the energy band edges under an applied field. The left-hand panels of the figure depict the lowest energy level of the conduction band of energies and the highest energy lev…

Mosfet

The Metal-oxide-semiconductor field-effect transistor (MOSFET) is a type of FET that consists of three layers: a metal top electrode (a conductor, called the gate), an oxide layer (working as an insulator separating the gate from the semiconductor layer), and a semiconductor layer (called the body). Its operation is based upon the modulation of the semiconductor conductivity by the electric field introduced in the body by the gate, the so-called field effect. This transistor was invented by Dawon Khang and Martin Atalla in 1960, at Bell Labs. There are four contacts altogether: in addition to the gate and body contacts already described, there are two contacts atop the body at opposite sides of the gate called source and drain. Because the transistor is symmetrical, they can swap their functions. They do not permit current flow to the body in normal operation, as they form reverse biased diodes with the body. They do allow current between source and drain upon formation (by t…

Limitations of real op-amps

Real op-amps can only approach this ideal: in addition to the practical limitations on slew rate, bandwidth, offset and so forth mentioned above, real op-amp parameters are subject to drift over time and with changes in temperature, input conditions, etc. Modern integrated FET or MOSFET op-amps approximate more closely the ideal op-amp than bipolar ICs where large signals must be handled at room temperature over a limited bandwidth; input impedance, in particular, is much higher, although the bipolar op-amps usually exhibit superior (i.e., lower) input offset drift and noise characteristics. Where the limitations of real devices can be ignored, an op-amp can be viewed as a black box with gain; circuit function and parameters are determined by feedback, usually negative. IC op-amps as implemented in practice are moderately complex integrated circuits; see the internal circuitry for the relatively simple 741 op-amp below, for example. DC imperfectionsOpen-loop gain is defin…

Operational Amplifiers

In electronics, an operational amplifier, or op-amp, is a DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output. Typically the output of the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation. High input impedance at the input terminals and low output impedance are typical characteristics. Op-amps are among the most widely used electronic devices, used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost over $100 US in small quantities. Modern designs are electronically more rugged than earlier implementations and some can sustain direct short-circuits on their outputs without damage.
Circ…

Integrated Stereo Amplifier

The type TDA1521 from Vallvo/Mullard is an Integrated Stereo amplifier designed for mains fed applications such as stereo TV.

Transistor Biasing

The basic function of transistor is amplification. The process of raising the strength of a weak signal without any change in its general shape is referred to as faithful amplification. For faithful amplification it is essential that                     1.Emitter-base junction is forward biased.                    2.Collector –base junction is reverse biased;                    3.Proper zero signal collector current.
For achieving faithful amplification, fulfillment of the following basic conditions is essential :
1.Proper Zero Signal Collector Current:- Zero signal collector current should be atleast  equal to the maximum collector current due to signal alone i.e  zero signal >or equal to maximum collector current. 2.Minimum Proper base-emitter Voltage  at Any Instant:- The base-emitter voltage Vbe  should not fall below  0.3V for germanium transistors and 0.7 V for silicon transistors at any instant. If the base emitter voltage  Vbe  falls below these values during any part of signal,…

P-N diode applications

An ideal P-N junction diode is of two terminal polarity sensitive device that has zero resistance ( i.e diode conducts)when it is forward biased and infinite resistance( i.e diode does not conduct) when reverse biased. Because of this property the diode finds use in many applications as enumerated below:
1.As rectifier in dc power supplies. 2.In demodulation or detector circuits. 3.In clamping networks employed as dc restorers in TV receivers  and  voltage multipliers. 4.In clipping circuits used as wave shaping circuits in computers, radars, radio and TV receivers. 5.As switches in digital logical circuits.
The same PN junction with different doping concentrations finds special applications as follows:
1.As zener  diodes  in voltage regulators, peak clippers, in switching operation. 2.As tunnel diodes as a relaxation oscillator at microwave frequencies. 3.As light emitting diodes(LEDs) in digital displays. 4.As LASER diodes in optical communications. 5.As varactor diodes in tuning  sections of …