AP 3302 Pt. 3
Section 2
CHAPTER 4
Limiting Circuits
Limiting in a cathode follower (Fig 15) is similar to that in a triode amplifier; negative limiting is achieved by grid cut-off and positive limiting by grid current flow through a grid stopper. However in a cathode follower the effective grid-cathode input is the difference between the input and output voltages. Hence to provide the limiting action a large amplitude input is required (typically 100V peak).
Transistor Limiting
A transistor may be used as a squarer by applying a large amplitude sine wave to its base. The basic circuit of a p-n-p transistor limiter is shown in Fig 16a and the resulting waveforms in Fig 16b.
It will be remembered that the bias conditions for a p-n-p transistor are opposite to those for a valve, i.e. the application of a positive bias to the base reduces the collector current and if the base voltage is sufficiently positive the transistor will cut off. Thus the positive-going half-cycles of the input sine wave are limited by transistor cut-off, the output voltage Vc at the collector then being equal to the supply voltage, in this case - 18V. On the negative-going half-cycles of the input, if the collector load is sufficiently large, e.g. l00kW, the transistor bottoms in much the same way as a pentode, because of the similarity of their characteristics. When this happens the output voltage Vc at the collector rises almost to earth potential (zero volts). An approximate square wave output then results. The symmetry of the square wave output may be adjusted by altering the bias applied to the base.
The purpose of the limiting resistor in the base is much the same as that of a grid stopper in a pentode, i.e. it limits the base current and prevents damage to the transistor.
