(06) clippers and clampers-1

*Clippers and Clampers-1*Clippers and Clampers

Clippers and Clampers-1

*Clippers and Clampers-1*ClippersA circuit that clips off or removes a portion of the input signal. A clipper can process any type of signal.


*Clippers and Clampers-1*There can be single clippers : Positive NegativeTwo categories :Series Diode and load in series.Parallel Diode and load in parallel.


*Clippers and Clampers-1*Series negative clipperThe circuitThe transfer characteristics


*Clippers and Clampers-1*What happens if the diode in a half-wave rectifier is reversed ?Ans. It still remains a half-wave rectifier, but it now works as a positive clipper.InputOutput ?


*Clippers and Clampers-1*Series Biased Clippers Bias means applying a dc voltage to change the dc level of a circuit.


*Clippers and Clampers-1*Guidelines to SolveDetermine the transition level at which the diode turns ON.With diode ON, find relation between vo and vi.Draw the transfer characteristic of the clipper.Plot the waveshape of vo for given input.


*Clippers and Clampers-1*We find thatDiode is ON for vi > VB.Therefore, vi(tr) = VB.When diode is ON, vo = vi VB.When diode is OFF, vo = 0.Plot the transfer characteristic of the clipper.


*Clippers and Clampers-1*Characteristic (Graph between input and output)Draw the output wave corresponding to a sinusoidal input with Vm > VB


*Clippers and Clampers-1*Now, draw the output.


*Clippers and Clampers-1*Example 1Determine the output waveform for the clipper circuit, if the input is a sinusoidal wave of peak value 15 V.


*Clippers and Clampers-1*Solution The direction of the diode suggests that it will be ON for positive values of vi.At transition level,


*Clippers and Clampers-1*After diode is ON,


*Clippers and Clampers-1*Transfer characteristic Draw the output.


*Clippers and Clampers-1*Example 2Determine the output waveform for the clipper circuit of Example 1, if its input is as follows


*Clippers and Clampers-1*SolutionProblem is simpler. Only two levels :vi = +15 V and vi = -5 V


*Clippers and Clampers-1*Now, you can draw the output.


*Clippers and Clampers-1*Note that Total swing of vi is 15 (-5) = 20 V.Total swing of vo is 18 0 = 18 V.Clipper circuit clipped off 2 V, and raised the dc level by 3 V.


*Clippers and Clampers-2*Parallel ClipperThe diode is put in parallel with the load. This too is the familiar half-wave rectifier circuit.Input wave.The circuit.Output wave.


*Clippers and Clampers-2*Practical Aspects of Parallel Clipper.If we take into account the threshold voltage,VT = 0.7 V, the clipping level is not zero, but 0.7 V.


*Clippers and Clampers-2*Defining Conditions.Small-signal diodes have small junction area, because they are optimized to work at high frequencies.They have more bulk resistance, RB.For example, a forward current of 10 mA at 1 V.For above specifications, determine RB.


*Clippers and Clampers-2*Clipper does not work properly, unless RB

*Clippers and Clampers-2*Simple Parallel Positive ClipperThe Circuit.The Input.The Output.Draw the output.


*Clippers and Clampers-2*Example 3Determine the output of the parallel biased clipper for the given input.


*Clippers and Clampers-2*Solution


*Clippers and Clampers-2*Now, draw the output wave, and get credit.Now, draw the transfer characteristic


*Clippers and Clampers-2*Example 4Repeat Example 3, taking a silicon diode with VT = 0.7 V, instead of an ideal diode.

Solution :To determine transition level, we use the condition,


*Clippers and Clampers-2*Applying KVL,For inputs less than 3.3 V (including negative values), the diode is ON, and vo = 3.3 VFor inputs greater than 3.3 V, the diode is OFF, and vo = vias shown in figure.


*Clippers and Clampers-2*Draw the output, and get credit.Note that VT reduces Vi(tr) to 3.3 V from 4 V.


*Clippers and Clampers-2*Design ProblemYou are givenPN-Junction diodes (assumed ideal),Resistors,Two batteries of 1.5 V each, andA sinusoidal voltage source.Design a circuit that gives an output as


*Clippers and Clampers-2*Solution D1 clips off positive parts above the positive bias level. D2 clips off below negative level. This circuit is called a combination clipper.


*Clippers and Clampers-2*Applications of Combination ClippersIf the input voltage is very large compared to the bias level, the output signal is a SQUARE WAVE.Thus, the circuit can be used for wave-shaping. It can also be used in a completely different way, as a limiter used to protect a sensitive circuit (e.g., OP-AMP, Galvanometer). The diodes conduct only when something abnormal happens.Sensitive Circuit


*Clippers and Clampers-3*Clampers (Electronic Circuits)Also called DC Restorers.It clamps (or holds, or ties) either the positive or the negative peak of a signal to a definite level.The circuit has a capacitor, a diode, and a resistor. In addition, it may have a dc supply to introduce additional shift.Time constant = RC is made much larger than T (time period) of the signal.The capacitor does not discharge when diode is not conducting.


*Clippers and Clampers-3*Positive Clamper(a) The input.(b) The circuit. On first negative cycle, the diode turns ON. The capacitor starts charging. At negative peak, the circuit is as shown in (c). The capacitor charges to Vm.


*Clippers and Clampers-3*(c)(d)Slightly beyond negative peak, the diode turns OFF. Capacitor does not discharge much because of high RC. At positive peak, the circuit is as shown in (d). The net output is shown in (e).


*Clippers and Clampers-3*(e) The output.The charged capacitor acts like a battery of Vm. This is the dc voltage that is added to the signal, as seen in (e). The output sits on 0 V level. The output is shifted in positive direction.


*Clippers and Clampers-3*Negative Clamper(a) The circuit.(b) The output. The diode is turned around. The capacitor voltage reverses, and the circuit becomes negative clamper. It is clamped to zero-volt level, but always remaining below 0 V. Memory Aid : The diode points in the direction of shift.


*Clippers and Clampers-3*Note :The total swing of the output is the same as that of the input.A clamper can also have an added dc voltage. It is then called biased clamper.Start the analysis of the circuit for that part of input, for which diode is ON. Assume that the capacitor charges to voltage level decided by the circuit.Assume that when diode is OFF, the capacitor does not discharge.


*Clippers and Clampers-3*Example 1Determine the output for the input of 1 kHz shown.(a) The input.(b) The circuit. Is it a positive or negative clamper ? Ans. : Biased positive clamper.


*Clippers and Clampers-3*Solution :The time constant of the circuit,The time period of the input signal, Thus, the capacitor holds the charge when the diode is OFF. We begin analysis with the period from T/2 to T; the diode is ON.The circuit is as shown in (a)


*Clippers and Clampers-3*(a) The output is across R, but it is also across 3-V battery. Hence, vo = 3 V, during this period. Applying KVL, -15 + VC -3 = 0; VC = 18 V. For the period from T to 3T/2, the circuit is as shown in (b).


*Clippers and Clampers-3*(b) Applying KVL, vo = 5 + 18 = 23 V. Thus, the output is as shown in (c). Note that the output swing is also 20 V.(c)


(06) clippers and clampers-1

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