Diodes are used to perform various operations on the signal. In the previous section, we saw how diodes can be used to clip positive or negative portions of the signal. The goal of this section is to understand the concept of diode clampers in detail. We shall also see the design of biased clampers.
As you can see from the figure above, a battery of 5 V is connected in series with the signal source. Observe carefully the magnitude and direction of the battery as it determines the resultant output voltage. For the circuit shown above, the output signal shifts upwards along the y-axis by 5 V. If we reverse the polarity of the connected battery, then the signal shifts downwards along the y-axis. Thus we can add the required DC level by connecting the battery in series with the signal source BUT there are some practical limitations which prevents us from using the above circuit as a clamper.
- Battery is usually bulky as compared to other electronic components. This makes the overall circuit bulkier. Obviously you do not want the electronic equipment to be bulky. Do you?
- Battery requires frequent replacement, which again limits its use.
- Battery is also costly and you certainly would not want to spend more money on electronic equipment.
With all the limitations in mind, we now design a clamper circuit which uses cheaper as well as compact electronic components. Such a clamper circuit involves the usage of diodes and capacitors. We now define clampers as a network consisting of diode, resistors and capacitors which adds a DC offset to the signal. A clamper consisting of diode, resistor and a capacitor is shown in the figure below.The diode is connected in parallel with the load. Observe carefully the direction in which diode is connected, because the direction of diode determines the direction (or polarity) of the DC offset that is added to the signal.
Negative clampers :
The circuit shown in the above figure works as a negative clamper (clamper which adds a negative DC offset to the signal). We shall analyse the clamper circuit in detail and understand how it works. We start with the analysis at time t=0 when the capacitor is uncharged. Initially when the input signal goes from zero to peak value, the diode gets forward biased. If we consider the diode to be ideal, then it acts as closed switch. This allows the capacitor to charge to its peak value in a very short span of time.The charging time constant is Rd * C, where Rd is the forward resistance of the diode which is only a few ohms. To simplify the discussion, we have considered the diode to be ideal having zero forward resistance which is a good approximation. Since the actual time constant Rd * C is very less, the capacitor quickly charges to the peak value. This is shown graphically in the figure below.
The output is zero during the time interval when the signal initially starts from zero to peak value. Now consider the situation when the amplitude of signal starts decreasing from its peak value. As soon as the signal starts decreasing in amplitude from the peak value, the diode gets reverse biased. This is because the cathode of the diode is at higher potential than the anode. The diode in such a condition acts as an open switch. In order to understand the concept clearly, let us first visualize how the circuit would work, and then we shall discuss how clamping action is achieved.
As we discussed initially, the capacitor charges to the peak value during the positive half cycle of signal. So when the signal starts decreasing from its peak value and the diode gets reverse biased, the capacitor voltage essentially acts as a battery in series with the signal voltage. Of course, the capacitor would discharge through the load resistance RL. The discharge time constant is RL * C. The discharge time constant is set in such a way that capacitor holds the charge for the entire cycle. For good clamping action, the time constant RL * C should be atleast ten times the period of applied signal frequency.
Positive clamper :
The circuit shown in the above figure works as a positive clamper. The input and output signal is also shown along with the circuit. The working of positive clamper is similar to the working of negative clamper we discussed first. We leave the analysis to you, so that you may develop necessary skills to analyze a circuit. For any doubts, don’t hesitate to leave a comment below and we would be glad to assist you.
Biased clampers :
So far in our discussion on clampers, you might have observed that the “level” at which DC offset was added to the signal was equal to the peak value of signal. Yes, observe the output signal again if you want. If we want to add a DC clamp to the signal other than the fixed value of Vp (peak signal voltage), then we use biased clampers. The circuit diagram of positive and negative biased clamper are shown below. A potentiometer is connected in series with the diode. Potentiometer is used practically to provide desired level of biasing by varying the knob. Any desired offset can be achieved between approximately 0 V and the supply voltage V.