OBJECTIVE:
To study the diode as a voltage limiter.
THEORY :
The diode limiter also called Clipper as it is used to limit the input voltage. A basic diode limiter circuit is composed of a diode and a resistor. Depending upon the circuit configuration and bias, the circuit may clip or eliminate all or part of an input waveform.
The biasing in diodes is the main reason for diode limiters. A forward-biased diode conducts, acting almost like a short circuit. It will have a very small voltage drop due to the barrier junction. If the diode is germanium, the drop will be .3 V. If it is a silicon diode, the drop will be .7 V. A reversed biased diode, being cut off, acts as an open circuit.
Depending upon how the input wave is to be altered, diode Limiting circuits are divided in to 5 types
- Series Positive Limiter
- Series Negative Limiter
- Parallel Positive Limiter
- Parallel Negative Limiter
- Parallel Positive and Negative Limiter.
Parallel Positive Limiter
The circuit shown in figure 1 illustrates a typical parallel positive limiter circuit.
The circuit is called parallel positive limiter circuit because the diode is in parallel with the output, and the circuit clips or alters the positive portion of the input waveform.
When the input signal goes positive, the diode is forward biased. As a forward biased diode conducts, the positive portion of the input signal is shunted to ground, preventing any signal from being passed out of the circuit with a very small amount of Voltage drop. A negative input signal reverse biases the diode so diode acts as an open circuit.
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Parallel Negative Limiter
The circuit shown in the figure 2 illustrates a typical parallel negative limiter circuit. It is called the parallel negative limiter circuit because the diode is in parallel with the output, and the circuit clips or alters the negative portion of the input waveform.
When the input signal goes negative, the diode is forward biased. As a forward biased diode conducts, the negative portion of the input signal is shunted to ground, preventing any signal from being passed out of the circuit output with a very small amount of Voltage drop. A positive input signal reverse biases the diode so diode acts as an open circuit.
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Parallel Positive and Negative Limiter
The circuit shown in the figure 3 illustrates a typical parallel positive and negative limiter circuit. It is called the parallel positive and negative limiter circuit because the diode is in parallel with the output, and the circuit clips or alters the positive and the negative portion of the input waveform.
The parallel positive and negative limiter is constructed from two diodes in parallel. During the positive alternation, one diode is forward biased. The other diode is forward biased during the negative alteration.
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