Clamp circuit composed of diodes and capacitors

Clamp circuit composed of diodes and capacitors

A diode limiting circuit can cut off a portion of the signal, but it will not affect the remaining signal. Today, we will learn another diode based circuit that can lift or lower the input waveform above or below a certain DC reference level without changing the shape of the input signal waveform. This circuit is called a diode "clamp" circuit. The clamp circuit is also known as a DC restorer.

The clamp circuit can be divided into two categories as a whole:

Positive Clampers Circuit

Negative Clampers Circuit

Forward clamp circuit: This type of clamp circuit moves the input signal waveform forward, resulting in the waveform being above the DC reference level.

Negative clamp circuit: This type of clamp circuit moves the input signal waveform in the negative direction, resulting in the waveform being below the DC reference level.

The direction of the diode in the clamp circuit determines the type of clamp circuit.

When designing a clamp circuit, it is necessary to choose appropriate capacitance and load resistance values to ensure that τ=RC (time constant) is large enough to ensure that the voltage across the capacitor does not discharge significantly during the diode cutoff period. In this article, we assume that the time constant τ ≥ 5T (T represents the period of the input signal).

Assuming that the input signal is in the positive half cycle when powered on, the output signal is basically equal to the input signal because the capacitor is slowly charging through the load resistor RL with a larger resistance value. The voltage drop across the capacitor is 0, and the voltage drop of the entire circuit is entirely on RL.

Once the input waveform signal switches to the negative half cycle, the diode is forward biased, equivalent to a section of wire. As a result, due to the small on resistance of the diode, the capacitor quickly charges to the input signal voltage, which we call Vc. During the negative half cycle of the input signal, the diode is in a conducting state

The output during this period is 0V, as the diode closing is equivalent to a short circuit. Additionally, please note that during this period, the capacitor will quickly charge to a value close to the input signal "V", as the time constant RC will be very small due to the small effective resistance when the diode is conducting. Please pay attention to the polarity of the capacitor when fully charged.

Once the input signal switches to the positive half cycle, the diode will be reverse biased. During this period, the diode will be in an off state, equivalent to an open circuit

During the positive half cycle, a capacitor that is fully charged but discharges slowly is equivalent to a battery, and the input signal is also equivalent to a battery. The power supply part of the entire circuit is equivalent to two batteries connected in series