LOAD LINE ANALYSIS
The straight line in the above graph is called the LOAD LINE because the intersection of the vertical axis is defined by the applied load R. the analysis to follow is therefore called LOAD LINE ANALYSIS.
The load line plots all possible combinations of diode current (Id) and voltage (Vd) for a given circuit. The maximum Id equals Vdd/R, and the maximum Vd equals Vdd.
The point where the load line and the characteristic curve intersect is the Q-point, which identifies Id and Vd for a particular diode in a given circuit.Q-point (quiescent point) decides the solution for the network and defines the current and voltage levels for the network.
Since the current going through the three elements in series must be the same, and the voltage at the terminals of the diode must be the same, the operating point of the circuit will be at the intersection of the curve with the load line.
The intersection of the two curves can be determined by applying KVL equation to the circuit (given in the fig. above),we get:
Vdd = Vd + Id*R;
Substituting Vd and Id equal to zero(one by one) we will get two points, one on x-axis and the other on y-axis respectively. When we join this two points we will get a straight line intersecting the characteristic curve of the device. This is how we get the load line.
Changing the value of R, the point of intersection on y-axis will also change (Id=Vdd/R).Thus changing the slope of the curve and point of intersection with the characteristic curve, hence changing the operating point of the device.
VOLTAGE MULTIPLIER
A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically by means of a network of capacitors and diodes.
Voltage multipliers can be used to generate bias voltages ranging from a few volts for electronic appliances to millions of volts for purposes such as high-energy physics experiments and lightning safety testing.
VOLTAGE DOUBLER
The network shown above is the circuit diagram for half wave voltage doubler circuit.
During the positive half cycle of the input signal is given to the circuit the diode D1 conducts(and diode D2 is cut off), this leads to the charging of the capacitor C1 to Vi volts with the positive charge getting created on the plates close to the input signal and negative charge being formed on the plates further from the input.
During the negative half cycle of the input signal diode, D1 is cut off and diode D2 is conducting charging the capacitor C2.
Since diode D2 acts as a short during the negative half cycle(and D1 acts as open), we can sum the two voltages around the outside loop:
-Vi – Vc1 + Vc2 = 0
-Vi – Vi + Vc2 = 0 (as the capacitor C1 starts acting as a battery, as it could not discharge)
Therefore, capacitor C2 charges up to 2Vi.
On the next positive half cycle, diode D2 is non-conducting and capacitor C2 will discharge through the load. If no load is connected across the capacitor C2, both the capacitors stay charged — C1 to Vi and C2 to 2Vi.
If, as would be expected, there is a load connected to the output, so the voltage across C2 will drop during the positive half cycle(at input) and will recharge up to 2Vi during the negative half cycle.
The output waveform across C2 is that of the capacitor filtered half wave signal. The peak inverse voltage across each diode is 2Vi.
FULL WAVE VOLTAGE DOUBLER :
The above circuit shows a basic symmetrical voltage multiplier circuit made up from two half-wave rectifier circuits. By adding a second diode and capacitor to the output of a standard half-wave rectifier, we can increase its output voltage by a set amount. This type of voltage multiplier configuration is known as a Full Wave Series Multiplier because one of the diodes is conducting in each half cycle, the same as for a full wave rectifier circuit.
When the sinusoidal input voltage is positive, capacitor C1 charges up through diodeD1 and when the sinusoidal voltage is negative, capacitor C2 charges up through the diode, D2. The output voltage 2VP is taken across the two series connected capacitors.
High voltage diodes have a higher VF than the ‘ideal’ model of 0.7V, so that many of the traditional ways of checking to make sure a diode is not failed shorted, or to verify the polarity of the device, will not work with high voltage diodes.
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