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Monday, 25 March 2013

Diodes Lecture Notes

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The Basic Diode Theory :

Diodes can be defined as non-linear devices. That is we can not apply superposition to circuits containing diodes. By Ohms law, when a voltage is applied across a resistor, the current flows in proportion to the voltage and the proportionality factor is constant. The voltage- current (VI) characteristic for a resistor is shown in Figure 1and is expressed analytically by V = I*R, I=V/R

Figure 1: Resistor circuit and its V-I characteristics

The V-I characteristics of a diode is shown in figure 2 and is described analytically by the equation .

Where Is is the reverse saturation current, a small ( ≈ 10-9 A  ) current will occur at the negative voltage region. q / KT  is a constant dependents on temperature ≈ 1/0.026V  at room temperature. VD  is the voltage occurred across the diode and
is the  dimensionless constant determined by the type of the diode (Silicon, Germanium, etc.)
Figure 2: Diode circuit and its V-I characteristics

For the non-linearity of a diode, consider the circuit shown in Figure 3
Figure 3: A diode circuit with 2 voltage sources.

When V1 and V2 are applied simultaneously, the voltage across D is -3 V and the resulting current is very close to -Is . If superposition is applied, we get  V1 produces 2 V across D and  I1 = 2190 ISV2  Produces -5 V across D and I2 = -Is . If we add these two currents, we will get I = 2189 Is.  which is an incorrect current. Since the superposition principle does not apply, we can say that a diode is a non-linear element.

Tuesday, 12 March 2013

The Transistor as a Switch

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Transistors can be considered as a type of switch in electronic circuits. They are used in a variety of circuits and all the circuit chips will contains at least one transistor. Transistors are the basic component of electronics and are of in two main types; NPN and PNP. Most circuits uses NPN type transistor. There are hundreds of transistors which work at different voltages. All transistors in the world can be classified into these two categories.

Transistors Lecture Notes : Transistors can be produced in different shapes but they have only three leads (legs).

The BASE  - The lead responsible for activation of the transistor.
The COLLECTOR  - The lead which is the positive part of transistor.
The EMITTER  -  The lead which is the negative part of transistor.

Figure 1 shows the symbol of an NPN transistor.

Figure 1 shows an NPN transistor which can be used as a type of switch. A small current or voltage at the base will yields for a larger voltage to flow through the other two leads ie,from the collector to the emitter.

Figure 2 is based on an NPN transistor. When the switch is in ON state, a current passes through the resistor to the base of the transistor. The transistor then permits the current to flow from the +9 volts to the 0vs, and the lamp glows.

The transistor needs to get a voltage at its ‘base’ portion and until this happens the lamp does not glows.

As transistors can be damaged easily by a high voltage / current, the resistor is used to protect the transistor. Transistors are the essential component in electronic circuits. Transistors can sometimes used to amplify a signal. For oscillators also transistor plays the key role.