CLIPPING AND CLAMPING CIRCUITS
AIM: To Study characteristics of clipping and clamping circuits.
APPARATUS: Audio Frequency oscillator. (RO, dual trace),
Multimeter, DC power supply.
THEORY: Clipping Circuits
The circuit with which the waveform is shaped by removing (or clipping) a portion of the applied wave is known as a clipping circuit.
Clippers find extensive use in radar, digital and other electronic systems. Although several clipping circuits have been developed to change the wave shape, we shall confine our attention to diode clippers. These clippers can remove signal voltages above or below a specified level. The important diode clippers are (i) Positive clipper, (ii) Biased clipper, (iii) Combination clipper.
(i) Positive clipper: - A positive clipper is that which removes the positive half cycles of the input voltage. Fig. (1) shows the typical circuit of a positive clipper using a diode. As shown, the output voltage has the positive half cycles removed or clippers off.
Fig. (1)
During the positive half cycle if the input voltage, the diode is forward biased and conducts heavily. Therefore, the voltage across the diode (which behaves as a short) and hence across the load RL is zero. Hence output voltage during positive half cycles is zero.
During the negative half cycle of the input voltage, the diode is reverse biased behaves as an open.
Output voltage = - RL Vm
R + RL
Generally, RL is much greater than R.
? Output voltage = - Vm
It may be noted that if it is desired to remove the negative half cycle of the input, the only thing is to done is to reverse the polarities of the diode in the circuit shown in fig. (1). Such a clipper is then called a negative clipper.
(ii) Biased Clipper :- The diode will conduct heavily so long as input voltage is greater than +V. When input voltage is greater than +V, the diode behaves as short and the output equals +V. The output will stay at +V so long as the input voltage is greater than +V.
During the period the input voltage is less than +V, the diode is reversed biased and behaves as an open. Therefore, most of the input voltage appears across the output. In this way, the biased positive clippers remove input voltage above +V.
During the negative half cycle of the input voltage, the diode remains reverse biased. Therefore, almost entire negative half cycle appears across the load.
Fig. (2)
If it is desired to clip a portion of negative half cycle of input voltage, the only thing to be done is to reverse the polarities of diode or battery. Such a circuit is then called a biased negative clipper.
(iii) Combination Clipper: - It is combination of biased positive and negative clippers. With a combination clipper, a portion of both positive and negative half cycle of input voltage can be removed or clipped as shown in Fig. (3)
Fig. (3)
When positive input voltage is greater than +V1, diode D1 conducts heavily while diode D2 remains reverse biased. Therefore, a voltage +V1 appears across the load. This output stays at +V1 so long as the input voltage exceeds +V1. On the other hand, during the negative half cycle, the diode D2 will conduct heavily and the output stays at �V2 so long as the input voltage is greater than �V2. Note that +V1 and �V2 are less than +Vm and �Vm respectively.
Between +V1 and �V2 neither diode is on. Therefore, in this condition, most of the input voltage appears across the load. It is interesting to note that this clipping circuit can give square wave output
If Vm is much greater than the clipping levels.
Clamping Circuits
A circuit that places either the positive or negative peak of a signal at a desired DC level is known as a clamping circuit.
Fig. (1)
A clamping circuit (or a clamper) essentially adds a DC component to the signal. Fig. (1) shows the key idea behind clamping.
It may be seen that the shape of the original signal. Has not changed; only there is vertical shift in the signal. Such a clamper is called a positive clamper.
The negative clamper does not reverse i.e. it pushes the signal downwards so that the positive peaks fall on the zero level.
The following points may be noted carefully: -
(i) The clamping circuit does not change the peak-to-peak or r.m.s.
value of the waveform. Thus referring to Fig. (1) above, the input waveform and clamped output have the same peak-to-peak value i.e. 10v in this case. If you measure the input voltage and clamped output with an a.c. voltmeter, the readings will be same.
(ii) A clamping circuit changes the peak and average value of a waveform. This point needs explanation, thus in the above circuit, it is easy to see that input waveform has a peak value of 5v and average value over a cycle is zero. The clamped output varies between10v and 0v. Therefore the peak value of clamped output is 10v and average value is 5v. Hence we arrive at a very important conclusion that a clamper changes the peak value as well as the average of a waveform
Basic idea of a clamper
Fig. (2)
The operation of a clamper is based on a principle that charging time of a capacitor is made very small as compared to its discharging time. Thus referring to Fig. (2),
? Charging time constant, T = Rf. c = (10?10-6 ) = 10?s
Total charging time, Tc = 5Rf. c = 5?10 = 50?s
? Discharging time constant, T = RLC
= (10?103 ) ? ( 1?10-6 )
= 10ms.
Total discharging time = T = 5RLC
= 5?10
= 50ms
It may be noted that the charging time (i.e., 50 ms) is very small as compared to the discharging time. (i.e., 50 ms) This is the basis of clamper circuit operation. In a practical clamping circuit, the values of C and RL are so chosen that discharging time is very large.
Procedure: -
Clipping Circuit
1) Set up the clipper circuits as shown in Fig. (1)
2) Apply a sinusoidal input of 5v at 1KHz frequency observe the output waveform using with CRO. Sketch the wave shape and label the amplitude. Indicate the type of clipping in each case.
3) Set up the clipper circuit as shown in Fig. (2) and repeat step 2.
4) Set up the clipper circuit as shown in Fig. (3), repeat above measurement.
5) Set up the clipper circuit as shown in Fig. (4), (5), repeat above measurement.
6) Obtain the transfer characteristics (Vo,Vs, Vi), of all the biased clipper (CKTS 3,4,5) by feeding a D.C. voltage at the input in both positive and negative directions and measuring the D.C. output voltage.
Clamping Circuit
1) Set up clamping circuits (1).
2) Feed sinusoidal input of 3v. 1KHz at the input and measure the output D.C. or HD A.C. voltages. Indicate type of clamping in each case.
3) Set up circuits (1), (2), (4) and repeat the measurement of step (2).
Conclusion: -
As per experiment we can conclude that any in sinusoidal wave we can clips out same portion at sine wave that is as we wants current flow through tit or voltage through the CKT.