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DIODE CLIPPING and CLAMPING CIRCUITS

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					                              Electronic Circuits I Laboratory



2 DIODE CLIPPING and CLAMPING CIRCUITS

2.1 Objectives

   •    Understanding the operating principle of diode clipping circuit
   •    Understanding the operating principle of clamping circuit
   •    Understanding the waveform change of diode clipping and clamping circuits
        when the bias is applied.



2.2 Basic Description
       As you know, diodes can be used as switches depending on the biasing type,
reverse of forward. The clipping circuit, also referred to as clipper, clips off some of
the portions of the input signal and uses the clipped signal as the output signal. The
clamping circuit or clamper keeps the amplitude of the output signal same as that of
the input signal except that the D.C. level (offset) has been changed. The clamper
through which the input waveform shifts to positive direction is called positive
clamper, otherwise, is called negative clamper.




                       Fig. 2.1 – Ideal Diode – Switch Terminalogy

2.2.a Clipper Circuits
        There are two types of clipper circuits, the series and parallel diode clipping
circuits.

       2.2.a.1 Series Diode Clipping Circuit
       In these type of circuits, the diode is connected between the input and output
voltage terminals (Fig 2.2)




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                                       Fig. 2.2

       As Fig.2.2 reveals, the negative cycle of the input voltage can be clipped of by
this type of series clippers. Reverse of the diode pins yields to a positive cycle
clipping circuit as shown in Fig. 2.3.




                                         Fig. 2.3
       Previous circuits clip the values larger or smaller than zero voltage. This
voltage, technically called “threshold voltage” and can be changed to a desired value
by inserting a D.C. voltage source. This is achieved in two different ways.
      In the first type, the voltage source of Em ( positive or negative) is connected
through output terminals as in Fig. 2.4. Depending on the diode connection (normal


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or reverse), the values smaller (Fig.2.4.a) or greater (Fig.2.4.b) than Em is clipped
and assigned as Em. .




                                           a




                                           b

                                       Fig. 2.4.
     Note that if Em is negative, ( where the voltage source is reversely connected)
again the values smaller or larger than this negative value is clipped, do not get
confused.

       In the second type of thresholded series clipping, the voltage source is
applied between the input and output terminals, series with the diode. This time, the
clipped values are assigned to zero and the net output voltage equals to the
difference between the input and threshold values.(If Em is negative, then E0 = E – Em
= E + |Em|)




                                           a




                                           b

                                        Fig. 2.5




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     2.2.a.2 Parallel Diode Clipping Circuit
       In this type of clippers, the diode is connected between output terminals. The
on/off state of diode directly affects the output voltage. These type of clippers may
also have a non-zero threshold voltage by addition of a voltage series with diode.
Following figures illustrate the clipping process.




                                           a




                                           b

                     Fig. 2.6 – Zero Threshold Parallel Clippers




                                           a




                                           b

                       Fig. 2.7 – Thresholded Parallel Clippers




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2.2.b Clamper Circuits
       Clamper Circuits, or briefly clampers are used to change the D.C. level of a
signal to a desired value.( Fig 2.8 ).




                                        Fig 2.8
       Being different from clippers, clamping circuits uses a capacitor and a diode
connection. When diode is in its on state, the output voltage equals to diode drop
voltage (ideally zero) plus the voltage source, if any. Now let us examine the
clamping process for the circuit in Fig. 2.9.




                            Fig 2.9 – Typical Clamping Circuit
      As you know, this circuit, in fact, is a series R-C circuit. The resistance of diode
( several ohms above its drop voltage) and the small capacitance yield to a small
time-constant for this circuit. This means that the capacitor will rapidly be charged if
any input voltage, that is enough to swtich on the diode, is applied. The diode will
conduct during the positive cycle of the input signal (Fig. 2.10) and output voltage will
be ideally zero ( in practice this voltage equals ~0.6 V).




                       Fig 2.10. Diode conducts during positive cycle
        Note that during positive cycle the capacitor is rapidly charged in inverse
polarity with the input voltage. After transition to negative cycle, the diode becomes to
its off state. In this case, the output voltage equals to the sum of the input voltage
and the voltage across the terminals of the capacitor which have the same polarity
with each other.(Fig 2.11)


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E0 = - ( |Ei |+ |Ec | )




                 Fig. 2.11. Diode is switched off during negative cycle


The resulting signal after a complete cycle is shown below.




                                        Fig. 2.12
       By this process, the input signal is shifted to negative D.C. value (its maximum
value is ideally zero) without any change in its amplitude ideally.


       There exist again modified versions of this circuit inwhich a threshold value is
inserted for clamping. Following figures illustrate these modifications and resulting
outputs.




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           Fig.2.13




          Fig. 2.14




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2.3 Experiment Equipments
   1. KL- 200 Linear Circuit Lab. Device
   2. Experiment Module: KL-23001
   3. Experiment Instruments: Oscilloscope, Multimeter
   4. Connection cables and short-circuit clips

2.4 Procedures

   Procedure 1: Series Diode Clipping Circuit
   (1)    Locate block b of module KL-23001

   (2)    Insert short-circuit clips refering to Fig. 2.15 a




                                     Fig. 2.15

   (3)    Using oscilloscope, adjust function generator to 10 V p-p ( 7.07 V rms)
      , 1kHz Sine wave. Make a good adjustment since this signal is the common
      input signal that will be used during whole experiment.


   (4)    Adjust oscilloscope divisions until you have a clear view of input signal.



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(5)   Disconnect function generator output from oscilloscope and apply this
  to TP2.

(6)    Examine the output signal by connecting the second channel of
  oscilloscope to TP3. Record the plot on Graph 2.1 a.

(7)    Turn off all active devices, and reconfigure the short-circuit clips refering
  to Fig. 2.15 b.

(8)   Only change the generator output from TP2 to TP1.

(9)  Again examine the output waveform and plot output waveform on
  Graph 2.1.b




              a                                                        b

                                Graph 2.1




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     Procedure 2: Thresholded Series Diode Clipping Circuit
           In this procedure, a threshold value is added to the previous circuit layout.
This is achieved by substitution of lower short-circuit clip with a voltage source.

     (1)     Turn off all active devices, and connect short-circuit clip and voltage
         source ( + 5 V DC) by refering to Fig.2.16-a.




                                       Fig. 2.16

     (2)     Record the plot of output voltage on Graph 2.2.a (on Pg. 25)

     (3)     Turn off all active devices, and reconfigure short-circuit clips refering to
         Fig.2.16-b.

     (4)     Plot output waveform on Graph 2.2.b

         Turn off all active devices. Now you will apply negative threshold value to
     the clipper circuit.

     (5)     Refering to Fig.2.17-a, reconfigure short circuit clips and substitute +5V
         DC source with -5 V DC Source. Do not directly reverse the terminals of
         + 5 V source, this will cause a short circuit !!!




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                              Fig. 2.17
(6)   Observe and plot the output waveform on Graph 2.2.c

(7)    Turn off all active devices again and rearrange short circuit clip refering
  to Fig.2.17.b

(8)   Observe and plot output voltage on Graph 2.2.d




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            a                                                 b




            c                                                 d

                                Graph 2.2

Procedure 3: Parallel Diode Clipping Circuit


(1)    Locate block c of module KL-23001

(2)    Refer to Fig.2.18-a and arrange short circuit clips.




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                                      a




                                      b

                                  Fig. 2.18
(3)    Observe output voltage on TP2 and plot on Graph 2.3.a
(4)    Turn off all active devices and rearrange short circuit clips by refering to
  Fig. 2.18 b.
(5)    Observe output voltage on TP2 and plot on Graph 2.3.b




               a                                                 b

                                 Graph 2.3




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Procedure 4 : Diode Clamping Circuit


(1)    Now locate block e.

(2)    Arrange clips according to Fig.2.19-a.




                                     a




                                     b

                                 Fig. 2.19


(3)    Observe and plot OUT voltage on Graph 2.4.a
(4)    Turn off all active devices and rearrange circuit in Fig. 2.19-b
(5)    Again plot OUT voltage on Graph 2.4.b.




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                  a                                                      b

                                        Graph 2.4

2.5 Conclusion
       In this experiment you have learned two main types of diode circuits.

          •   Clippers
          •   Clampers

        Although the clippers are also classified into series and parallel clipping
circuits, both circuits are based on the same principle. In other words, the design of
both circuits utilizes the characteristics of diode that will be forward turned on and will
reversely turned off.

       Clampers can be used to change the DC level of an input signal.




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