Diode Clipping Circuits Good s264d

Diode Clipping Circuits The Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit that takes an input waveform and clips or cuts off its top half, bottom half or both halves together to produce an output waveform that resembles a flattened version of the input. There are two basic types of clippers: series clippers and shunt/parallel clippers. The series clipper contains a diode that is in series with the load. The shunt clipper contains a diode that is in parallel with the load. Although the input voltage to a diode clipping circuit can have any waveform shape, we will assume here that the input voltage is sinusoidal. Consider the circuits below. Positive Diode Clipping Circuit

In this diode clipping circuit, the diode is forward biased during the positive half cycle of the sinusoidal input waveform. For the diode to become forward biased, it must have the input voltage magnitude greater than +0.7 volts (0.3 volts for a germanium diode). When this happens the diodes begins to conduct and holds the voltage across itself constant at 0.7V until the sinusoidal waveform falls below this value. Thus the output voltage which is taken across the diode can never exceed 0.7 volts during the positive half cycle. During the negative half cycle, the diode is reverse biased blocking current flow through itself and as a result has no effect on the negative half of the sinusoidal voltage which es to the load unaltered. Then the diode limits the positive half of the input waveform and is known as a positive clipper circuit. Negative Diode Clipping Circuit

Here the reverse is true. The diode is forward biased during the negative half cycle of the sinusoidal waveform and limits or clips it to -0.7 volts while allowing the positive half cycle to unaltered when reverse biased. As the diode limits the negative half cycle of the input voltage it is therefore called a negative clipper circuit. Clipping of Both Half Cycles

If we connected two diodes in inverse parallel as shown, then both the positive and negative half cycles would be clipped as diode D1 clips the positive half cycle of the sinusoidal input waveform while diode D2 clips the negative half cycle. For ideal diodes the output waveform above would be zero. However, due to the forward bias voltage drop across the diodes the actual clipping point occurs at +0.7 volts and -0.7 volts respectively. But we can increase this ±0.7V threshold to any value we want up to the maximum value, (VPEAK) of the sinusoidal waveform either by connecting together more diodes in series creating multiples of 0.7 volts, or by adding a voltage bias to the diodes. Biased Diode Clipping Circuits To produce diode clipping of the voltage waveform at different levels, a bias voltage, VBIAS is added in series with the diode as shown. The voltage across the series combination must be greater than VBIAS + 0.7V before the diode becomes sufficiently forward biased to conduct. For example, if the VBIAS level is set at 4.0 volts, then the sinusoidal voltage at the diode’s anode terminal must be greater than 4.0 + 0.7 = 4.7 volts for it to become forward biased. Any anode voltage levels above this bias point are clipped off. Positive Bias Diode Clipping

Likewise, by reversing the diode and the battery bias voltage, when a diode conducts the negative half cycle of the output waveform is held to a level -VBIAS - 0.7V as shown. Negative Bias Diode Clipping

A variable diode clipping or diode limiting level can be achieved by varying the bias voltage of the diodes. If both the positive and the negative half cycles are to be clipped, then two biased clipping diodes are used. But for both positive and negative diode clipping, the bias voltage need not be the same. The positive bias voltage could be at one level, for example 4 volts, and the negative bias voltage at another, for example 6 volts as shown. Diode Clipping of Different Bias levels

When the voltage of the positive half cycle reaches +4.7 V, diode D1 conducts and limits the waveform at +4.7 V. Diode D2 does not conduct until the voltage reaches –6.7 V. Therefore, all positive voltages above +4.7 V and negative voltages below –6.7 V are automatically clipped. The advantage of biased diode clipping is that it prevents the output signal from exceeding preset voltage limits for both half cycles of the input waveform, which could be an input from a noisy sensor or the positive and negative supply rails of a power supply. If the diode clipping levels are set too low or the input waveform is too great then the elimination of both waveform peaks could end up with a square-wave shaped waveform. Clampers: A clamp circuit adds the positive or negative dc component to the input signal so as to push it either on the positive side, as illustrated in figure (a) or on the negative side, as illustrated in figure (b). The circuit will be called a positive clamper, when the signal is pushed upward by the circuit. When the signal moves upward, as shown in figure (a), the negative peak of the signal coincides with the zero level. The circuit will be called a negative clamper, when the signal is pushed downward by the circuit. When the signal is pushed on the negative side, as shown in figure (b), the positive peak of the input signal coincides with the zero level.

Clamping circuits are often used in television receivers as dc restorers. The signal that is sent to the TV receiver may lose the dc components after being ed through capacitively coupled amplifiers. Thus the signal loses its black and white reference levels and the blanking level. Before ing these signals to the picture tube, these reference levels have to be restored. This is done by using clamper circuits. They also find applications in storage counters, analog frequency meter, capacitance meter, divider and stair-case waveform generator. Consider a negative clamping circuit, a circuit that shifts the original signal in a vertical downward direction, as shown in the figure below. The diode D will be forward biased and the capacitor C is charged with the polarity shown, when an input signal is applied. During the positive half cycle of input, the output voltage will be equal

to the barrier potential of the diode, V0 and the capacitor is charged to (V – VQ). During the negative half cycle, the diode becomes reverse-biased and acts as an open-circuit. Thus, there will be no effect on the capacitor voltage. The resistance R, being of very high value, cannot discharge C a lot during the negative portion of the input waveform. Thus during negative input, the output voltage will be the sum of the input voltage and the capacitor voltage and is equal to – V – (V — V0) or – (2 V – V0). The value of the peak-to-peak output will be the difference of the negative and positive peak voltage levels is equal to V0-[-(2V-V0)] or 2 V. The figure shown below can be modified into a positive clamping circuit by reconnecting the diode with reversed polarity. The positive clamping circuit moves the original signal in a vertical upward direction. A positive clamping circuit is shown in the figure below. It contains a diode D and a capacitor C as are contained in a negative clamper. The only difference in the circuit is that the polarity of the diode is reversed. The remaining explanation regarding the working of the circuit is the same as it is explained for the negative clamper. For a clamping circuit at least three components — a diode, a capacitor and a resistor are required. Sometimes an independent dc supply is also required to cause an additional shift. The important points regarding clamping circuits are: (i) The shape of the waveform will be the same, but its level is shifted either upward or downward, (ii) There will be no change in the peak-to-peak or rms value of the waveform due to the clamping circuit. Thus, the input waveform and output waveform will have the same peak-to-peak value that is, 2Vmax. This is shown in the figure above. It must also be noted that same readings will be obtained in the ac voltmeter for the input voltage and the clamped output voltage. (iii) There will be a change in the peak and average values of the waveform. In the figure shown above, the input waveform has a peak value of Vmax and average value over a complete cycle is zero. The clamped output varies from 2 Vmax and 0 (or 0 and -2Vmax). Thus ths peak value of the clamped output is 2Vmax and average value is Vmax. (iv) The values of the resistor R and capacitor C affect the waveform. (v) The values for the resistor R and capacitor C should be determined from the time constant equation of the circuit, t = RC. The values must be large enough to make sure that the voltage across the capacitor C does not change significantly during the time interval the diode is non-conducting. In a good clamper circuit, the circuit time constant t = RC should be at least ten times the time period of the input signal voltage.