The purpose of clippers is to protect a circuit against too high or too low voltages. The most simple circuit contains one diode (Figure 2.31). As long as the input voltage is positive, the diode will not conduct. What remains, is a voltage divider. This is also the case for small negative input voltages. From the moment that the output voltage might exceed the diode’s knee voltage, the diode will conduct. The output voltage then equals the knee voltage (with a minus sign in the circuit below). In other words: from the moment the output voltage is tending to drop below the knee voltage, it will be limited to this value. This results in the input-output characteristic shown in Figure 2.32. An example wave form is shown in Figure 2.33. The function of the resistor R in the circuit is to protect the diode against too high currents.
Figure 2.31 A clipper circuit contains one or more diodes, and protects a certain load (RL) against too large (positive or negative) voltages.
Figure 2.32 The input-output characteristic of the clipper circuit above. Voltages below the knee voltage are not possible across the load.
Figure 2.33 Example waveforms for the clipper circuit.
If the diode is replaced by a Zener diode, the output voltage can also be limited at its positive peak (Figure 2.34). If the output voltage is tending to exceed the breakdown voltage of the Zener, the latter will break down, and the output voltage will be bounded to the breakdown voltage (Figure 2.35 and Figure 2.36).
Figure 2.34 A clipper circuit with a Zener diode.
Figure 2.35 Input-output characteristic of a clipper circuit with a Zener diode.
Figure 2.36 Example waveforms of a clipper circuit with a Zener diode.
More flexibility is obtained by placing two Zener diodes in series (Figure 2.37). The range of the output voltage is between VBD1 + VK2 (breakdown at positive input voltage) and –VBD2 – VK1 (at negative input voltage) (Figure 2.38).
Figure 2.37 A clipper circuit with two Zener diodes.
Figure 2.38 I/O characteristic of a clipper circuit with two Zener diodes.