Diode Characteristic Curve AND Finally Diode Resistance PDF

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sir khurram saleem gives these notes about diode behaiviour....

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DIODE CHARACTERISTIC CURVE AND FINALLY DIODE RESISTANCE :

When the forward voltage exceeds the diodes P-N junctions internal barrier voltage, which for silicon is about 0.7 volts, avalanche occurs and the forward current increases rapidly for a very small increase in voltage producing a nonlinear curve. The “knee” point on the forward curve. Likewise, when the diode is reversed biased, cathode positive with respect to the anode, the diode blocks current except for an extremely small leakage current, and operates in the lower left quadrant of its I-V characteristic curves. The diode continues to block current flow through it until the reverse voltage across the diode becomes greater than its breakdown voltage point resulting in a sudden increase in reverse current producing a fairly straight line downward curve as the voltage losses control. This reverse breakdown voltage point is used to good effect with zener diodes. Then we can see that the I-V Characteristic Curves for a silicon diode are nonlinear and very different to that of the previous resistors linear I-V curves as their electrical characteristics are different. Current-Voltage characteristics curves can be used to plot the operation of any electrical or electronic component from resistors, to amplifiers, to semiconductors and solar cells. The current-voltage characteristics of an electronic component tells us much about its operation and can be a very useful tool in determining the operating characteristics of a particular device or component by showing its possible combinations of current and voltage, and as a graphical aid can help visually understand better what is happening within a circuit.

GRAPH AND CODE :

Code: close all; t=0:0.001:2*pi; x=cos(t); y=sin(t); plot(x,y);

in matlab after simulate the above code the two waves are combining and the circle were made.if we change the (0.001) distance ,the circle will tilt like a plate and after some points there will be a straight line combining by two waves. As following simulation explains:

Code: close all; t=0:0.001:2*pi; x=cos(t); y=sin(t); plot(x,y); %% phase change x=cos(t); y=sin(t+pi/3); figure;plot(x,y)

as the phase is shifted pi/3 so the circle is tiled and after an appropriate phase angle will sooner be a straight line.

If we chane the angular frequency of waves then they will cut the line in a ratio according to their angular frequency.

Code : close all; t=0:0.001:2*pi; x=cos(t); y=sin(t); plot(x,y); %% phase change x=cos(t);

y=sin(t+pi/2); figure;plot(x,y)

Code: close all; t=0:0.001:2*pi; x=cos(t); y=sin(t); plot(x,y); %% phase change x=cos(t); y=sin(t+pi/2); figure;plot(x,y)

%% different frequency; x2=cos(3.*t) y2=sin(8.*t); figure;plot(x2,y2); as in above graph and code,the line intersects 3 times on horizontal axis and 8 times on vertical axis.so the ratio is 3:8 of intersecting waves.if we chane the angular frequency to 3 and 10.then the lines will intersect 3 times on horizontal axis and 10 times on vertical axis.so the graph and code is:

ANALYSIS OF CURRENT WAVE AND SEQUENCE OF SINE WAVE:

Code: close all Is=10e-12; nVt=25e-3; Vd=[0.5:0.01:.7]; Id=Is.*(exp(Vd./nVt)-1); plot(Vd, Id); %% sinewave testing in the middle t=0:0.001:(2.*pi); Vdsin=0.35+0.05.*sin(t); Idsin=Is.*(exp(Vdsin./nVt)-1); figure;plot(t,Vdsin);

figure;plot(t,Idsin); %% sinewave testing on right side of Vd Vdsin=0.65+0.02.*sin(t); Idsin=Is.*(exp(Vdsin./nVt)-1); figure;plot(t,Vdsin); figure;plot(t,Idsin);

As shown from the figures the current wave is not symmetric ,the upper part is more deep and lower part is more sequencing then upper part.by changing the factor multiplied by sin the sequence of current wave will be small.we use 0.65 dc offset in wave so that the wave starts from 0.65 and on the origion.

As in above graph the current wave is more squeezed in lower part than in the following graph the wave is less squeezed and more symmetric and we can make wave more symmetric.

DIODE RESISTANCE: As in diode, in forward biasing the diode resistance is very low like very few ohms but in reverse biasing the diode voltage is very much high,almost infinity : In forward biasing there are two types of diode resistance: 1) DC or stead resistance 2) AC or dynamic resistance DC RESISTANCE : plication of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time. The resistance of the diode at the operating point can be found simply by finding the corresponding levels of VD and ID

applying the following equation: RD =VD/ID The dc resistance levels at the knee and below will be greater than the resistance levels obtained for the vertical rise section of the characteristics. The resistance levels in the reverse-bias region will naturally be quite high. Since ohmmeters typically employ a relatively constant-current source, the resistance determined will be at a preset current level (typically, a few milliamperes). AC RESISTANCE: Ac resistance further divided into two parts: 1) When small votage change.(like 0.2v ) 2) When large voltage change.(like 0.8v) When small voltage change: In diode the current increases exponentially and on every point of I-v curve the ac resistance of diode is changing ,so to find a resistance at a particular point we take a small change in voltage and took their respective current values and find the ac resistance,according to following formula: rD=change in voltage/change in current when large voltage change: when we take large intervals in voltage like 0.8v or more then the resistance is called average ac resistance and denoted by rD(avg). the difference in large voltage change and small voltage change is in large voltage change the slope of resistance includes the exponential

curve of diode current and in small voltage change its exclude the exponential curve .(in graph of I-V of diode). Formula of large change in voltage resistance is: rD(avg)=change in voltage/change in current

ELECTRONICS 01 ASSIGHNMENT # 01 NAME: ZAIN ISLAM REG NO: FA16-BET/031 CALSS: BET-3A SUBMITTED TO: SIR KHURRAM SALEEM ALIMGEER DATE: 28/09/2017...