Clipping and Clamping Circuits

Clipping and Clamping Circuits

• Waveforms are shaped using diodes, resistors, and capacitors.
• Clipping circuits are used to limit or clip parts of a signal.
• Clamping circuits shift the DC voltage level of a signal.
• These circuits are also known as wave shaping circuits.
• Applications include communications and control/instrumentation circuits.

Clippers Using Ideal Diodes

• An ideal diode has a threshold voltage of zero and zero forward resistance.
• One such circuit contains an ideal diode, a resistor R, and a battery VB connected to the N side of the diode.
• Output voltage V0(t) for a given input Vi(t) is determined by first establishing when the diode conducts.
• Conduction happens when the voltage at the P side (Vi(t)) exceeds the voltage at the N side (VB).
• When Vi(t) < VB, the diode is off, behaving as an open circuit.
• Therefore, when the diode is not conducting, V0(t) = Vi(t).
• Diode conducts when Vi(t) > VB.
• When the diode conducts, V0(t) = VB because the ideal diode drop is zero.
• The input voltage waveform's upper portion above VB is clipped off.

Clippers using Constant Voltage Drop Diodes

• A real diode has a voltage drop VDO, 0.7V for silicon.
• rD (resistance of the diode) is ignored in comparison to resistance R.
• Replace the diode by its equivalent model: a battery showing the drop VDO along with an ideal diode.
• Diode conducts when Vi > VB + VDO.
• When the diode conducts, V0(t) = VB + VDO.
• When Vi < VB + VDO, the diode does not conduct, therefore V0(t) = Vi(t).
• The upper portion of the input voltage waveform above VB + VDO is clipped off.
• A diode drop results in more of the upper portion being clipped.

Transfer Characteristics

• Transfer characteristic is a plot showing output voltage (Vo) versus input voltage (Vi).
• Output and input voltages are equal until Vi < VB + VDO, showing a linear relationship with a slope of 1 passing through the origin.
• When Vi > VB + VDO, the output voltage is constant at VB + VDO.
• Describes conducting and non-conducting states.

Example Circuit with Triangular Input

• Circuit uses a 4V battery, connected with its positive terminal to the P side of the diode, and a triangular input voltage (Vi) of 16V peak value.
• VDO is 0.7V.
• Condition for conduction is determined by comparing the voltage at the P and N sides of the diode.
• The diode conducts when Vi is less than 4 - VDO (3.3V).
• When the diode conducts, the output voltage is clipped at 3.3V.
• When Vi > 3.3V, the diode does not conduct, and V0 = Vi.
• Lower portion of the waveform is clipped.

Circuit with Two Diodes and Two Batteries

• Circuit uses two diodes (D1, D2) and two batteries (VB1, VB2) connected with certain polarities.
• Input voltage (Vi) is sinusoidal.
• Constant voltage drop model is used for the diodes.
• The diodes are replaced by their constant voltage drop models.
• VB1 and VB2 initially reverse biasing the diodes.
• Diode D1 conducts only during the input voltage's positive half cycle; diode D2 conducts only during the negative half cycle.
• D1 conducts when Vi > VB1 + VDO.
• In the diode's positive half cycle, D2 does not conduct.
• In the negative half cycle, magnitude of Vi > magnitude of VB2 + VDO.
• The output voltage waveform is determined by analyzing when each diode conducts.

Clipper Circuits

• Portions of a waveform are clipped away by clipper circuits, both upper and lower.
• The waveform may not be symmetrical, depending on the magnitudes of VB1 and VB2 (voltage sources).
• The waveform will be symmetrical if VB1 and VB2 are equal.
• The waveform will be asymmetrical if VB1 is higher than VB2, or vice versa.
• The diode conducts when Vi is greater than VB1 + VDO, clamping the voltage to VB1 + VDO.
• When Vi is less than VB1 + VDO, the diode does not conduct, and Vo equals Vi.
• In the negative half cycle, when Vi is greater than VB2 + VDO, diode D2 conducts, and the voltage is VB2 + VDO.
• Output voltage plot (Vo) shows the clipped waveform.

Example Clipper Circuit with Ideal Diodes

• Diodes (D1 and D2) are ideal, therefore voltage drop is zero and resistances are ignored.
• The circuit includes two batteries: 2V and 4V, which reverse bias the diodes.
• Vi is 6 sin(ωt) with a peak value of 6V.
• From 0 to 2V, both D1 and D2 are OFF (not conducting).
• Output voltage (Vo) equals input voltage (Vi) when Vi is less than 2V.
• When Vi is greater than 2V, D1 conducts, and current flows in the circuit.

Calculating Voltage When D1 Conducts

• Voltage drop across two points is 2V plus i * R2.
• i = (Vi - 2) / (10k + 10k) = (Vi - 2) / 20k.
• When D1 turns ON, Vo = 2 + i1 * R2 = (Vi / 2) + 1.
• If Vi is 6V, then Vo = (6 / 2) + 1 = 4V.
• D2 does not conduct up to -4V in the negative half cycle.
• D2 conducts when the negative voltage is greater than 4V.
• Since D2 is ideal, the voltage is -4V when D2 is ON.

Combining Waveforms of the Clipper Circuit

• From 0 to 2V, input and output are the same.
• At input 6V, output is 4V.
• The circuit doesn't conduct until the negative half cycle is greater than 4V, then the voltage is -4V.
• Unlike constant voltage drop in earlier circuits, the output is clipped but with a slope.

Transfer Characteristic

• From 0 to 2V, output and input voltage are the same in a straight line with slope 1.
• Output is at 4V (the voltage at the output) when greater than 2V, from this point to Vi at 6V.
• Diode D2 is not conducting, so in the negative half cycle (again up to -4V), input and output voltage are the same.
• When crossing -4V, diode D2 conducts, and the output voltage (Vo) is -4V.

Clamper Circuits

• Clamper shifts the DC level of a waveform to a different value.
• A Sinusoidal waveform's average DC value of zero can be changed.
• A simple clamper includes a capacitor (C), a resistor (R), and a diode.
• The discharging time constant (RC) is very large compared to the time period of the input voltage.

Clamper Circuit Operation with Square Wave Input and Ideal Diode

• Square wave input with voltages V and -V.
• In the positive half cycle, the diode is forward biased.
• The capacitor charges to a voltage of V.
• Charging time constant is almost zero, and the capacitor has negligible resistance.
• In the negative half cycle, the diode is reverse biased by a voltage of twice V.
• Output voltage (Vo) is zero in the positive half cycle.
• In the negative half cycle, Vo is -2V.

Clamper Circuit with Asymmetrical Square Wave and Non-Ideal Diode

• Diode has a 0.7 voltage drop.
• Asymmetrical square wave input: 20V in the positive half cycle and -5V in the negative half cycle.
• The diode is reverse biased and does not conduct in the positive half cycle.
• There is no current, and the voltage drop across the resistance is zero.

Clamper Circuit Analysis

• 2V reverse biases the diode.
• Positive half cycle of 20V is applied, reverse biasing the diode and making it non-conducting.
• In the negative half cycle, +5V is forward biasing the diode.
• The N point has 2.7V total potential, while this point has +5V.
• Applying Kirchhoff’s voltage law: Vo - 0.7 - 2 + 5 = 0.
• Vo = -5 + 2.7 = -2.3V on the negative side.
• Zero voltage on the positive half cycle and clipped at -2.3V on the negative side.

Zener Diodes for Clamping

• Zener diodes are used for both clipping and clamping.
• Ideal Zener diodes have a diode drop (VDO) in the forward direction and a Zener voltage (Vz).
• One Zener diode is reverse biased when the input voltage is in the positive half cycle and the other acts like a normal diode.
• The Zener diode breaks down when the input voltage is greater than Vz + VDO.
• The voltage across the two points is Vz + VDO.
• In the negative half cycle, the first diode is reverse biased, and the next is forward biased.
• If the input voltage is greater than Vz + VDO in the reverse bias, there is conduction, and the is Vz + VDO.
• Output voltage is clipped at ±(Vz + VDO).
• Draw an approximate equivalent circuit with Zener voltage and diode drop for analysis.

Applications

• Clipper and clamper circuits have pratical uses.
• Clippers clip portions of the waveform.
• Clampers shift or clamp the DC voltage level of the waveform to some other value.