Diode Clipper (Part 1)

There are a variety of diode network called clippers that have the ability to “clip” off a portion of the input signal without distorting the remaining part of the alternating waveform. The half wave rectifier is an example of the simplest form of diode clipper one resistor and diode. Depending on the orientation of the diode, the positive or negative region of the input signal is “clipped” off. There are two general categories of clippers: series and parallel. The series configuration is defined as one where the diode is in series with the load, while the parallel variety has the diode in a branch parallel to the load.

Series clipper:

The response of the series configuration of Fig. 2.67a to a variety of alternating waveforms is provided in Fig. 2.67b. Although first introduced as a half-wave rectifier (for sinusoidal waveforms), there are no boundaries on the type of signals that can be applied to a clipper. The addition of a dc supply such as shown in Fig. 2.68 can have a pronounced effect on the output of a clipper. Our initial discussion will be limited to ideal diodes, with the effect of V_T reserved for a concluding example.

Fig 2.67: Series clipper

Fig: 2.68 Series clipper with a dc supply.

There is no general procedure for analyzing networks such as the type in Fig. 2.68, but there are a few thoughts to keep in mind as you work toward a solution.

1. Make a mental sketch of the response of the network based on the direction of the diode and the applied voltage levels.

For the network of Fig. 2.68, the direction of the diode suggests that the signal must be positive to turn it on. The dc supply further requires that the voltage be greater than V volts to turn the diode on. The negative region of the input signal is “pressuring” the diode into the “off” state, supported further by the dc supply. In general, therefore, we can be quite sure that the diode is an open circuit (“off” state) for the negative region of the input signal.

2. Determine the applied voltage (transition voltage) that will cause a change in state for the diode.

For the ideal diode the transition between states will occur at the point on the characteristics where  V_d =0 V and   I_d=0 A. Applying the condition I_d=0 A at  V_d =0 V to the network of Fig. 2.68 will result in the configuration of Fig. 2.69, where it is recognized that the level of vi that will cause a transition in state is For an input voltage greater than V volts the diode is in the short-circuit state, while for input voltages less than V volts it is in the open-circuit or “off” state.

Fig: 2.69 Determining the transition level for the circuit of Fig. 2.68

3. Be continually aware of the defined terminals and polarity of V_o.When the diode is in the short-circuit state, such as shown in Fig. 2.70, the output voltage  V_o can be determined by applying Kirchhoff’s voltage law in the clockwise direction V_i – V – V_o (CW direction)

Fig: 2.70 Determining Vo.