TRIGGERED PULSE GENERATORS 



Figure 16.28b from a constant-voltage generator. We take as an example a 

 deflector coil requiring 50 mA to sweep the cathode ray beam across the 

 full diameter of the screen face (assuming that by some external and un- 

 specified means the spot is caused to rest at one edge of the screen when the 



Slope : / 



Current 

 wave 



(a) 



Slope = IR 



VoUage 

 wave 



LI 



Time 



Figure 16.27 



(b) 



Figure 16.28 



deflector coil current is zero) and traverses to the diametral edge when the 

 current is 50 mA. Let the inductance be 3 H and the resistance 100 ohms, 

 and assume that the fastest time base required is 1 millisecond and the 

 slowest 1 second. 



Constant-current approach 



The relevant constant-current generator here is clearly an output pentode, 

 with the deflector coil in the anode circuit and a triangular voltage wave 



r-^MSlSLr 



R 



HT+ 





Figure 16.29 



applied to the grid {Figure 16.29). To secure economy in anode current the 

 input wave will be positive going, so that the anode current in the quiescent 

 state is small and determined by the grid bias obtained from the datum 

 potential of the input wave. The working point on the anode characteristic 

 will be A in Figure 16.30. To get an anode current of 50 mA we need to 

 swing the grid positive an amount 50 mA/^,„. 



Suppose we are using the slowest sweep speed. / is 50 mA in 1 second, or 

 0-05 amps/second and the back e.m.f. will be 3 X 0-05 = 0T5 V, quite neg- 

 ligible. We can thus forget the effect of the deflector coil inductance and say 

 that during the forward stroke of the circuit the working point will move 

 along a rather steep load line (corresponding to 1,000 ohms — rather small as 

 loads go) from A to B. 



248 



