- In this page we discuss what are series clippers and how they work.
- Series clippers can be biased as well as unbiased. Both biased as well as unbiased series clippers are discussed in detail.
- Series clippers can clip positive or negative portions of the input signal. We shall see both positive and negative clippers in detail.
Negative series clippers
The figure below shows a series (negative) clipper.
In the above figure, take a look at the input signal and output signal. The input signal is sinusoidal and the output signal has its negative cycle clipped off. Hence the name negative clipper. The clipping of the signal occurs due to the property of diode to conduct current in only one direction and block the current in the other direction.
We shall discuss in detail how the negative portion of the signal is clipped off when the diode is connected with the load as shown in the above figure.
Refer the figure shown above which explains in detail how the series negative clipper works. It consists of three sections.
- Section (a) shows how the diode is connected with the load. It also shows the input and output waveform. The output signal is highlighted for two different intervals – one for the positive half cycle and the other for the zero output signal.
- Consider section (b) which explains why the output is exact replica of the input for the positive half cycle. Here we assume that the diode is ideal. This simply means that the turn-on voltage of the diode is any voltage greater than 0 V, and the diode resistance is 0 Ω. When the signal voltage is positive, the diode is turned on and it acts as closed switch. Hence no voltage drops across the diode. All the input voltage appears across the load, and hence the ouptut signal voltage is same as input voltage.
- Section (c) indicates the condition of diode when the input voltage is negative. During negative half cycle of the input voltage, the diode is reverse biased and acts open switch. In such a case, no current flows through the diode and the load, and hence the output voltage across the load is zero.
Biased series negative clippers : In the above example, we saw that the clipping of the signal takes place as soon as the input signal goes negative. If we want to change/adjust the clipping level of AC voltage, then external biasing voltage must be used. The figure given below shows a biased (series) clipper.
A biasing voltage is connected in series with the diode as shown in the above figure. Clippers which include biasing voltage must be analysed carefully in order to determine the exact output voltage. Keep in mind that the biasing voltage can aid or work against the signal voltage. Hence care must be taken while analyzing the network to see when the diode changes its state from “on” to “off” or vice-versa.
Analysis of biased series (negative) clippers : Let us discuss how the biased clipper shown in the above figure can be analyzed. Although here we discuss how to analyse the clipper with configuration as shown in the above figure, the basic idea remains quite similar for other configurations too.
- 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 see how we can analyze the biased clipper shown in the above figure. When the input signal is positive, it tries to establish the current through the diode. However, the biasing voltage V opposes the flow of current and tries to keep the diode in “off” state. As soon as the signal voltage rises above voltage V, the diode starts to conduct and the current flows through the load. The output appears across the load as soon as the current flows though it. So we split the analysis into two parts. In the first part, we analyse the network before the signal voltage reaches V volts, where the diode is in “off” state. In the second section, we analyse the network for input signal voltages greater than V volts, where the diode is in “on” state.
The above figure shows the network when the signal voltage is less than voltage V. The input signal is also highlighted in the above figure for time interval “t”. Diode is replaced with its equivalent circuit (open switch). During this time interval, the output voltage remains zero. This is also indicated in the output. Now consider the figure below for signal voltages greater than voltage V.
As you can see in the above figure, diode is replaced with closed switch. Hence current flows through the diode and load resistor R. The output voltage appears across the load. Maximum value of output voltage is Vm – V . The output voltage for one complete cycle of input voltage is shown below.
Positive series clippers
Having learned about negative clippers, it is quite obvious what positive clippers are. Positive clippers are used to clip positive portions of the input signal and allow the negative portions of the signal to pass through. The design of positive clipper is quite similar to the design of negative clippers. In order to design a series positive clipper, just flip the diode symbol in the network of series negative clipper and its done. The figure below shows a series positive clipper along with the explanation of its working.
The discussion for positive clippers is quite similar to the way it was done for negative clippers. Again the figure is divided into three section. Let us discuss each section separately.
- Section (a) shows the input and output signal along with the positive clipper. The positive cycle is completely clipped off by the clipper. In order to understand why the positive portion is completely clipped off by the clipper, we refer to section (b) and (c).
- Section (b) shows the same clipper configuration as it is shown in (a), but the diode is replaced with its equivalent circuit model. When the input signal goes positive, the cathode terminal of diode attains a higher potential than anode. This makes the diode reverse biased. Here we assume the diode to be ideal. The diode in reverse biased configuration can be modelled as an open switch. This is indicated in the figure. As the diode acts as an open switch, no current flows through the load and hence no output appears across the load. This is the reason why the positive cycle is completely clipped off.
- When the input signal goes negative, the anode attains higher potential than cathode and the diode is forward biased. An ideal diode in forward bias configuration can be modelled as closed switch. This is indicated in figure (c). As the diode acts as closed switch, current flows through the load and all the voltage appears across the terminals of the load.
Biased series positive clippers : As discussed in the section on negative clippers, we apply biasing to the clippers to change/adjust the clipping level of AC voltage. Same discussion can also be applied to positive clippers. Let us look at the circuit of biased positive clipper along with the input and output signal.
Keep in mind all the points that we discussed in the section on negative clippers about how to analyse the biased clipper. (Refer it once again, if you want, just for quick revision). Do not worry about how the output signal is obtained as shown in the above figure. We shall analyse the above clipper configuration in detail.
With that being said, let us first discuss when the diode gets turned “on” and when it gets turned “off”. Then we shall replace the diode with its equivalent circuit model and determine the output.
- Consider the instant when the signal voltage is 0 and instant t=0. At this instant the battery voltage reverse biases the diode. This is because the cathode of the diode is at a higher potential than anode. As the diode is reverse biased, it is considered to be in “off” state and it acts open switch. As the signal voltage goes positive, it aids the battery voltage in reverse biasing the diode and consequently the reverse bias voltage across the diode increases. For the complete positive half cycle, the diode stays in reverse bias and acts as open switch. Now consider the instant when the signal voltage goes negative. As soon as the signal voltage goes negative, it starts to oppose the battery voltage and tries to forward bias the diode. However at this point, the signal voltage is not sufficient to forward bias the diode. Once the signal voltage is greater than the battery voltage (we assume Vm to be greater than battery voltage V) it forward biases the diode. The diode is now in the “on” state. So we conclude that any signal voltage greater than -V will reverse bias the diode. For such values of signal voltages, the diode can be considered open switch.
- When the signal voltage is less than -V, the potential at anode is more than that of cathode. Hence the diode is forward biased and it acts as closed switch. In such a condition, the output follows the input. This is shown in the figure below.