Definition: Clamper circuits are the electronic circuits that shift the dc level of the AC signal. Clampers are also known as DC voltage restorers or level shifter. Clampers are basically classified as positive and negative that includes both biased and unbiased conditions individually.
These circuits are used to clamp an input signal to a different dc level. It basically adds dc component to the applied input signal in order to push the signal to either the positive or negative side. Clamper circuit is a combination of a resistor along with a diode and capacitor. It sometimes also employs dc battery so as to have an additional shift in the signal level.
Clamper circuits are constructed in a similar manner as that of clipper circuits. However, clamper includes an extra charging element that is the capacitor in its circuitry. The combination of resistor and capacitor in the clamper circuit is used to maintain different dc level at the output of the clamper.
Operating principle of Clamper circuits
As we have already discussed a clamper consist of the capacitor and a diode in shunt connection with the load.
The working of clamper circuits depends on the variation in the time constant of the capacitor. This variation is the outcome of changing the current path of the diode with the change in input signal polarity.
Here, the magnitude of the time constant is
this is chosen large enough in order to assure that voltage across the capacitor does not discharge consequently at the non-conducting interval of the diode. But, such a discharge takes place only when the load resistance is very large. This permits the capacitor to take larger discharge time. Conversely, a smaller value of the capacitor is chosen so that it will charge rapidly at the time of conduction of the diode.
Classification of Clamper Circuits
Clampers are classified in the following groups:
Positive Clamper circuit
The figure below shows the circuit of a positive clamper-
As we can see here, the diode is in parallel connection with the load. So we can say reverse biasing of the diode will provide the output at the load.
Initially, the positive half of the applied input signal reverse biases the diode but the capacitor is not still charged. So, at this period of time output will not be considered.
For, the negative half of the AC signal, the capacitor now gets fully charged up to the peak of the AC signal but with inverse polarity. This negative half forward biases the diode that results in the flow of the forward current through the diode. The next positive half then reverse biases the diode due to which signal will appear at the output.
At the beginning of the positive half of the AC signal, the diode is in the non-conducting state that results in discharging of capacitor charge. So, at the output, we will have the summation of the voltage stored across the capacitor and applied the AC input signal. This is given by
Vo = Vm + Vm = 2Vm
Here as we can in the output waveform shown above, the signal level is shifted upward or positive side. Hence it is named as positive clamper.
Negative Clamper circuit
Let’s have a look at the figure shown below of negative clamper in order to understand the detailed operation-
At the time when positive half of the AC input is applied, the diode comes to forward bias condition that results in no-load current at the output. However, a forward current flows through the diode that charges the capacitor to the peak of the ac signal but again with inverse polarity. The capacitor here is charged up to the forward biased condition of the diode.
When negative half of the AC signal is applied, the diode now becomes reverse biased. This allows load current to appear at the output of the circuit. Now, this non-conducting state of the diode discharges the capacitor. So, at the output, a summation of capacitor voltage along with the input voltage is achieved.
Hence at the output, we have,
Vo = – Vm – Vm = -2Vm
This results in the downward shift of the signal. Therefore, it is termed as negative clamper circuit.
Positive clamper circuit with biasing
This is basically done to introduce an additional shift in the level of the signal. Here, the biasing provided to the circuit is of two types. It can be a positively or negatively biased circuit. So, we will discuss both the cases separately.
1. Case of positive biasing
The working is almost similar to the positive unbiased case but here an additional voltage is provided so as to have an additional shift in the level of the signal.
When positive half of the input signal is applied, the diode is reverse biased due to ac input but is forward biased due to battery voltage. So, until the voltage of a battery is greater than the ac input the diode conducts. This forward current through the diode charges the capacitor but with the battery voltage. As the ac input surpasses battery voltage, the diode now gets reverse biased and hence conduction through the diode stops.
On the application of the negative half of the input signal, the diode is now forward biased due to both ac input and battery voltage and starts conducting. This charges the capacitor with voltage summation of ac input along with battery voltage. Hence such an output voltage level is achieved.
2. Case of negative biasing
At the time of positive half of the AC signal, the diode gets reverse biased by both ac input and battery voltage. Due to this current flows through the load and combinedly maintain the voltage level.
At the time of the negative half, the diode is in the forward biased condition due to ac input but is in reverse biased condition due to battery voltage. So, the diode conducts only when the AC input dominates the battery voltage. This charges the capacitor hence we get a shifted signal at the output.
Negative Clamper circuit with biasing
In a similar way as previous, positive and negative biasing is provided to the negative clamper circuit. Let’s now move further and discuss both the case separately.
1. Case of positive biasing
As we have already discussed that a negative clamper shifts the signal downward. However, in the case of the positively biased negative clamper, the signal is somewhat raised to a positive level due to the positively applied battery voltage. When positive half of the AC signal is applied, the diode is in the forward biased state due to ac supply but is reverse biased because of battery voltage. So, the diode conducts when ac supply surpasses battery voltage.
Moving further during the negative half, the diode is now in reverse biased state by cause of both the AC supply and battery voltage. This non-conducting state of the diode discharges the capacitor. Thus, the voltage across the capacitor appears at the output.
2. Case of negative biasing
At the time of positive half of ac input, the diode gets forward biased by the cause of ac input and battery voltage. This starts conduction through the diode. Resultantly charges the capacitor.
At the time of the negative half, the diode gets reverse biased but will still conduct due to forward biased condition applied by the battery. The diode current flows until the battery voltage is more than the ac input supply. The time when ac input surpasses battery voltage, the diode gets reverse biased and the capacitor discharges. Thus the voltage across capacitor appears at the load.
Applications of Clamper Circuits
- Clampers are used to identify the polarity of the circuits.
- These circuits are used as voltage doublers and help in eliminating distortions.
- Reverse recovery time can be improved using Clampers.
Clippers and clampers have the almost same constructional feature but the introduction of a new element in clamper circuits distinguishes the working of the two.