- Basically clippers can be divided into two categories – series clippers and shunt clippers. In the previous section, we saw in detail what are series clippers and how they work. So in this section, let us discuss what are shunt clippers.
- We shall also discuss how it can be used to clip positive or negative portions of the input signal.
- Finally we shall discuss the effect of biasing voltage on clippers. Such clippers are called biased clippers.
Positive shunt clippers
The figure given below shows a typical positive shunt clipper.
As it is indicated in the above figure, the diode is connected in parallel with the load. Hence the name – shunt clipper. It consists of two resistors- R1 and RL where RL is the load resistance. The output is taken across the terminals of RL. The resistance R1 is used to limit the current through the diode when the signal goes positive. Here the diode is assumed to be ideal. So when the input signal goes positive, the current passes through R1 and the diode. As the positive signal forward biases the diode, it acts as a closed switch hence providing zero resistance path for the current to flow. In such a case, no current flows through the load resistance and hence output voltage is zero. This is shown in the figure below.
The property of diode to conduct current in only one direction is responsible for such clipping of signal. When the input signal goes positive, the diode acts as a closed switch and the signal gets shorted across the terminals of the load, resulting in zero output. Now consider the situation when the signal voltage goes negative. The diode gets reverse biased when the input signal goes negative. We have assumed the diode to be ideal. Hence the reverse biased diode will act as an open switch. Consequently no current flows through the diode. The current only flows through the series combination of R1 and RL. In such a case, the output voltage can be determined by the voltage divider rule.
As indicated in the above figure, the minimum value of the output signal is given by
- (Vm * RL)/(R1 + RL).
Here we conclude that the output of positive shunt clipper as given in the figure is a signal with positive half-cycle completely clipped-off. For the negative half cycle, the output can be determined by applying voltage divider rule.
Biased positive shunt clipper
The clipper circuit we discussed in the above section completely clips-off the positive half-cycle. If we want to change/adjust the clipping level of AC signal, then biasing voltage must be used. Let us understand how the output signal changes when a biasing voltage is applied in series with the diode. The configuration we shall see in detail is shown in the figure below. Observe carefully how the terminals of the battery are connected with the diode.
Analysis of biased positive shunt clipper : Let us discuss how the biased positive shunt clipper can be analysed. Although we shall discuss in detail how to analyse biased shunt positive clipper with the above configuration, same steps can be followed for other configurations too. Similar type of discussion is done in the section on series clippers. Keep the following points in mind and the analysis should get easy for you.
- Determine the voltage levels at which the diode turns from “on” state to “off” state and vice-versa : It is very important to know when the diode gets turned “on” or “off” with respect to input signal voltage. The bias voltage can work against the signal voltage or it can even aid the signal voltage. So we must find out when the diode is on or off. Then we can split the analysis into sections, based on the condition of diode being “on” or “off”.
- Replace the diode with its equivalent circuit model : If the diode is considered to be ideal, replace the diode with open switch when it is in “off” state and replace the diode with closed switch when it is in “on” state. This will help in determining the output signal easily.
With the above points in mind, let us analyse the biased shunt clipper as shown in the figure above. Initially when the signal starts to rise from zero voltage and gradually attains positive voltage, it tries to push the current through two parallel branches. One branch consists of load resistor RL,while the second branch consists of battery in series with the diode. The current can easily flow through the load resistor RL, however the branch consisting of series combination of battery and the diode must be analysed carefully.Observe carefully the terminals of the battery. For positive values, the battery opposes the signal. If the signal voltage is below battery voltage V, the diode is reverse biased. As a result, the diode acts as an open switch . Consequently no current flows through the branch consisting of battery and the diode. The figure given below shows the input and output signal for the signal voltages below V volts.
As the signal voltage gradually increases above the battery voltage, the diode gets forward biased. As the forward biased diode acts as a closed switch, the current starts to flow the battery. In such a case, the values of output signal no longer increases above the battery voltage.The signal gets clipped-off above the battery voltage V. Such a condition is shown in the figure below.
For the values of signal less than the battery voltage, again the diode acts as an open switch. The output voltage can be found by the voltage divider rule.
Negative shunt clippers
Having learned about positive shunt clippers uptil now, let us now understand how negative shunt clippers works. Negative shunt clippers, as the name suggests, are shunt clippers which clips negative portions of the input signals. There are many configurations of negative shunt clippers possible, one such configuration is shown below.
The input and output signals are also shown along with the clipper configuration. The negative portion of the signal is completely clipped-off when an AC signal is subjected to the clipper configuration shown above. Also notice the difference in the peak value of input and output signals. In order to simplify the analysis, we split the analysis into two sections. First we shall analyse the circuit for positive values of input signal, then we shall analyse the circuit for negative values of input signal.
Take a look at the figure below which shows how the clippers works when the input signal is positive.
As the input signal rises from zero to positive values, the diode gets reverse biased. The ideal reverse biased diode do not allow the current to flow through, and hence acts as open switch. Now only two resistors come into picture – R1 and the load resistor RL (refer the above figure). The output voltage across RL can be found by voltage divider rule, which is shown in the above figure.
For negative values of input signal, the diode gets forward biased and it acts as closed switch. As a result, the signal effectively gets shorted across the terminals of the load, resulting in zero output voltage. This is shown in the figure below.
Biased negative shunt clipper : To adjust/change the clipping level of AC signal, we use biasing voltage. Biased negative shunt clipper along with the input and output signal is shown below. We leave the analysis to you, so that you may develop necessary reasoning and logic to analyse a circuit. If you face any problems, feel free to leave a comment and we would be glad to help you.