AP 3302 Pt. 3
Section 2
CHAPTER 4
Limiting Circuits
Parallel Limiters
In a parallel limiter the diode is connected in parallel with the component to which we apply the output waveform and the output voltage is taken across the diode.
If the input to the parallel positive limiter shown in Fig 5a is a sine wave which varies about earth the diode conducts only during the positive-going half-cycles and the voltage developed across it (the output voltage Vout) is then practically zero. When the diode is cut off on the negative-going half-cycles of input practically the whole of the input voltage is developed across the diode and applied to the output terminals. This circuit therefore clips the portion of the input waveform which goes positive with respect to earth.
If we wish to remove the negative-going portion of the input waveform we simply turn the diode round to give the parallel negative limiter shown in Fig 5b.
Limiting to Voltages other than Zero
The circuits we have just considered all limit to zero volts, i.e. they use earth as a reference and cut off the portion of the input waveform which rises above, or falls below, zero volts.
In practice we often need to clip the portion above or below some reference voltage other than zero. This may be done by using slightly modified versions of the basic limiting circuit (Fig 6).
