Clipping Circuits

Clipping circuits are used as wave shaping circuits. Usually it is used to shape sine waves by cutting off some of its parts, but it can be used for other waves also.The idea will be clear if we familiar with the simplest member of clipper family. 

An example of a clipper circuit is Half wave rectifier. Circuit, Output voltage ('Vo' vs 't' -obtain using CRO) and Transfer characteristics  (' Vo' vs 'Vin' -obtain using CRO) are shown.

fig 1

Here i take two circuits -one in top and other in the bottom position, both give same output, hence same transfer chara. Though you know very well about the working, am just briefing it - for my convenience. Circuit in the top consists of a resistor(R) and a diode(D). Output takes across D ie. CRO probes are connect across two ends of D. 

For positive half of input sine wave, D is in forward bias - current is flowing without any resistance, so voltage zero(diode ON). //A voltage is developed only across a resistance, if resistance of a component is zero then voltage across it is also zero. But this an ideal situation only - not practical. Every component, even 'diode in forward bias' have a small resistance (ON resistance), therefore a small voltage across it ie. 0.7v //

For negative half cycle of sine wave, D is in reverse bias. No current flow through D. Diode act as 'Open circuit'. So whatever the voltage applied in the input side, is completely appeared across D - in other words, output follows sine wave's(input) negative half cycle. //When there is no current flow through a component, resistance of it is taken as infinity(resistance=voltage/current). Comparing to this infinite resistance, all other resistances in the concerned circuit are negligible(even though it has any value -K ohm/M ohm/..). We know V=RI, since resistance is infinite, voltage is very high(not infinite). Therefore whatever the voltage given as input to the circuit, that full voltage is dropped across this infinite resistance. ie. voltage across an 'open circuit' = input voltage --all other components in the concerned circuit have negligible voltage.// 

   We can easily draw transfer chara using input and output waves. Below given explanation will help you for it.

Transfer characteristics : It means input vs output.When both input and output are sine waves, transfer chara is 

fig a

When Vin increases +vely (in transfer chara, from (0,0) towards right), Vout also increases +vely(in transfer chara, from(0,0) towards top). Result is a straight line with 45 degree slope. Similarly for -ve half cycle also.   

Now let's see how transfer characteristics is obtained in fig 1. Take fig a, as reference. Consider transfer chara of fig 1. For +ve half of Vin, Vout is constant at 0.7v - so transfer characteristics is a straight line at 0.7v(output is constant at 0.7v irrespective of change in input). It is indicated both in output waveform and transfer chara. For -ve half of Vin, fig 1 is same as fig a.

Hope the above explanation is satisfactory. Now move to next clipper circuit.

fig 2

 Explanation for obtaining such an output is as follows: We are adding a +ve bias (+ve voltage) in the basic circuit(fig1) using a battery and it shifts the cut off voltage of diode from 0.7v to (Vr + 0.7v). Vr is the voltage of battery(we can use 5v or 7v or any other, but should be less than Vin). You can easily understand its transfer chara by comparing it with fig a.

Next clipper is obtained by adding a -ve bias to circuit in fig 1. It shifts the cut off voltage to (-Vr + 0.7v).

fig 3

 It is easy to remember the output of fig 2 and fig 3 by relating it to output of fig 1.

(i)adding Vr to output of fig1     (iii)output of fig2

(i)adding -Vr to output of fig1     (iii)output of fig3

No need to study theoretical explanation of how to obtain the output for a particular clipper circuit. Such essay questions are not yet asked for theory exams. But short answer questions may be asked and it will be like 'given a transfer chara. draw the corresponding circuit'. To answer such questions, first you draw the output wave form using given transfer chara and then the circuit.

Next look into next set of clipper circuits. Here circuit in fig 4 is taken as the basic circuit and is obtained by reversing the diode in fig 1.

fig 4

In fig 4 cut off is at (-0.7v) and is an example of a half wave rectifier. Now see what happens if a negative bias is added to circuit in fig4.

fig 5

A - ve bias of (-Vr) is added to fig 4 to obtain circuit in fig 5. Then cut off voltage changes from -0.7 to (-0.7-Vr). It is adopted in output waveform and transfer characteristics. Similarly if a +ve bias of Vr is added to circuit in fig4, we will get circuit in fig 6. Cut off voltage is shifted to (-0.7+Vr). //For example, battery voltage Vr =5v. Then cut off voltage is -0.7+5 = +4.3//

fig 6

These are the simple clipping circuits. Once you familiarized them, move to combination clippers