AP 3302 Pt. 3
Section 2
CHAPTER 4
Limiting Circuits
Limiting in a Pentode Amplifier
In a normal valve amplifier circuit with a resistive anode load, if Vgk rises, Ia rises and the increased voltage drop across the load causes Va to fall. Fig 12a shows a family of typical pentode Ia-Va. curves. Let us see the effect of using a very large value resistor as the anode load RL. We shall assume that RL is so large that when Vgk is zero (curve B) the anode current is such that the voltage drop across RL brings Va down to only 50V. From Fig 12a we can see that when Vgk is zero and Va is 50V, Ia is l0mA. However the coincident values of Ia = l0mA and Va = 50V apply to other curves also, i.e. curve C (Vgk = -2V) and curve A (Vgk = + 2V).
