Applications of Diode - Types of Diode Clampers (Positive and Negative clampers, Biased and Unbiased Clampers) - Circuit Diagrams and Working, Waveforms and Comparison
Another popular diode-based circuit is the clamping circuit. A clamper circuit or network adds DC level to a given AC signal without any change in signal shape. A simple circuit is shown below, comprise of a capacitor, diode and resistor. Carefully choose the capacitance value. The time constant should be large enough to maintain the shape of the output signal. The time constant can be increased by using larger capacitance values. Throughout this article, I consider a practical diode with a drop of 0.7V.
V1 is the sinusoidal input
VP is the peak voltage
Vo is the output voltage
Outline:
In this article, I explain different types of clamper circuits, like biased clampers and unbiased clampers, positive and negative clampers.
Comparison between biased positive clampers and unbiased positive clampers.
Comparison between biased negative clampers and unbiased negative clampers.
Positive Clamper Circuit Diagram and Working:
It clamps the incoming signal to an upward direction. See the circuit below (figure 1). The diode arrow points upward. It means the signal is going to shift in an upward direction.
During the negative half-cycle, the diode turns on. As a result, the capacitor charges up to peak value (VP - 0.7V) that is 9.3V. (Considering practical diode)
During the positive half cycle, the diode opens (reverse bias). No path to flow current. To determine the output voltage that appears across the resistor, apply KVL to the loop.
Vo = V1 + VP
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| Figure 1 Positive Clampers, capacitance value should high to keep time constant large |
Negative Clamper Circuit Diagram and Working:
It clamps the incoming signal to a downward direction. See the circuit below (figure 2). The diode arrow points downward. It means the signal is going to shift in a downward direction. Each point on the sine wave is shifted to a downward direction by an amount of VP.
During the positive half cycle, the diode turns on. As a result, the capacitor charges up to peak value (VP) which is 9.3 V (considering diode drop).
During the negative half-cycle, the diode opens (reverse biased). To determine the output voltage that appears across the resistor, apply KVL to the loop.
Vo = - Vi - VP
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| Figure 2 Negative Clampers |
Positive Biased Clamper Circuit Working:
During the negative half-cycle, the diode is reverse biased for a small part of the input signal. This is because of DC source polarity (V2 = 2V). The anode is more positive than the cathode for a small part of the input waveform (V1 <V2). For higher values of V1 (V1> 2V) the diode turns on and capacitor charges with the polarity shown.
V1 - VC1 - 0.7 - 2 = 0
VC1 = 7.3V
It means the capacitor clamps the output signal to a voltage level of 7.3V.
During the positive half cycle, the diode remains to turn off. The capacitor holds its charge because of the larger capacitance value.
-V1 -VC1+Vo= 0
Vo = 17.3 V
Output voltage clamps and swings from 17.3 V to -2.7 V.
See in next section where I compare both positive clampers Vs positively biased clampers.
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| Figure 3 Positive Biased Clampers |
Negative Biased Clamper Clamper Circuit Working:
During the positive half cycle, the diode remains to turn off for a small part of the input waveform. This is because of the DC source (3V) at the anode. To make the diode forward biased, the anode should more positive than the cathode.
V1 > (3 + 0.7)V …. Turn on condition
Where
V1 = input voltage
0.7V is the forward voltage drop
As the diode turns on, the capacitor charges till the peak value.
-V1 + VC1+0.7+3 = 0
VC1 = 10 - 3.7
VC1 = 6.3 V
During the negative half-cycle, the diode remains off. To determine output voltage, apply KVL to the loop.
V1 + VC1 + Vo = 0
10 + 6.3 + Vo = 0
Vo = -16.3 V
Output voltage clamps and swings from -16.3 V to 3.7 V.
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| Figure 4 Negative biased clampers |
Compare the results:
Compare the unbiased clamper with the biased clamper circuit. What is the effect of adding a DC source in series with the diode?
Positive Clamper (Unbiased) Vs Positive Biased Clamper:
Compare the results obtained from biased and unbiased circuits.
Unbiased clampers give clamped waveforms. The clamping value is equal to the peak input voltage.
For example, if we have a peak input voltage equal to 10V (peak to peak), swings from +10V to -10V.
The unbiased clamper produces an output that is exactly similar to input but has added DC value. The output signal clamped at 9.3V. It swings from +20 to 0V.
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| Figure 5 Positive clampers Vs positive biased clampers |
Biased Clampers also give clamped waveforms. The difference is the clamping level. Capacitor charging describes the clamping level. In the above circuit positive biased clampers, the capacitor charges up to 7.3V.
We have peak input voltage equal to 10V (peak to peak), swings from +10V to -10V.
The biased clamper produces an output that is exactly similar to input but has added DC value. The output signal clamped at 7.3 V. It swings from +17.3 to -2.7V.
Negative Clamper (Unbiased) Vs Negative Biased Clamper:
Compare results obtained from negative clampers and negative biased clampers.
Unbiased Clampers give clamped waveforms. The clamping value is equal to the negative peak of the input current voltage.
We have input voltage equal to 10V (peak to peak), swings from +10V to -10V.
The negative clamper added a DC level. The output signal swings from 0.7V to -19.3V.
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| Figure 6 Negative Clampers Vs negative biased clampers |
Biased Clampers: In the above example of negatively biased clamper, we have a 3V DC voltage source. Due to this source, we have a different clamping level. In this case, the capacitor charges up to 6.3V.
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