TRIGGERED PULSE GENERATORS 



megohm. The internal resistance can be made even higher by the use of 

 negative current feedback. In physical terms this means feeding back a 

 voltage proportional to the output current, rather than to the output voltage. 

 It will be remembered that the output resistance of the cathode follower, 

 which represents negative voltage feedback par excellence, is very low. 

 With negative current feedback the output impedance is very high. A simple 





!/.•■/? 



T— ^■OWT^-vW- 



4^0^ 



HT+ 



Figure 16.31 



Figure 16.32 



way of achieving negative current feedback is to include some resistance in 

 series with the valve cathode (Figure 16.31). Then we have 





V 11 



Combining 



Zl + Rk + ra 



I = 



and 



V = Kin - iRi 



HV 



va. 



Zl + ^a + (,U + \)Rk 



Comparing this with the expression for a stage without feedback having 

 the same load and a valve of internal resistance /•„ 



we see that the effect of the feedback is to increase the internal resistance from 

 r„ to r^ + (^ + \)Ric. This is seen better if we call it 



^a(l + ^^ Rl^ N /■„(! + gn.RK) 



Typically g„j is 10 niA/V and r^ 1,000 ohms, giving an 11-fold increase in 

 the internal resistance of our constant-current generator. It must not be 

 forgotten that the improvement is not obtainable without a loss of gain. 

 The input has to be increased by a factor 



^L-r ^a-r J<K ^a 



i.e. approximately by the same factor as the pentode internal resistance is 

 increased. 



Flyback — During the forward stroke energy is stored in the magnetic 

 field linked with the deflector coil, and when the anode current is suddenly 



250 



