TRIANGULAR WAVE GENERATORS 



Bhinilein integrator 

 Consider the circuit in Figure 16.14, and suppose that the gain between 



HT+ HT+ 



Figure 16.14 Figure 16.15 



grid and anode is —A. Then we have, assuming no grid current and that 

 therefore all the current / flows into C, 



also dV^=-AdV, 



dV 

 i=C(A + l)^' 



This equation describes what would happen if C were replaced by C{A + 1) 



between grid and earth as in Figure 16.15, which is in effect a re-statement of 



the Miller effect, and the reason why the Blumlein integrator is often called a 



Miller integrator. We can now write down by inspection that the equation to 



nis 



t 



E(\ -e «c(4+i)) 



t 



and therefore that V^ = -AE(\ - e RG{a+i)^ 



That is, it is as if a capacitance (A + 1) times bigger than C were being 

 charged by a voltage A times bigger than E. If A is 100 and E is made the 

 full HT voltage, perhaps 350 V, then the virtual charging voltage is 35,000, 

 which means that V„ will fall in a manner highly hnear with time (called the 

 Miller run-down). Moreover, since the virtual capacitance is 101 times the 

 actual one, slow time bases can be generated with quite moderate values 

 of C. 



The 'Miller run-down' proceeds* in a linear manner at the anode until 

 the anode voltage has fallen so low that A begins to fall. In order to secure 

 a large amplitude of run-down it is customary to exploit the ability of a 

 pentode to operate satisfactorily at low anode voltages, and if this is done 

 the run-down is conveniently controlled at the suppressor grid. When the 

 circuit is quiescent, the suppressor is biased sufficiently negative to cut off 

 the anode current. To initiate the run-down the suppressor bias is lifted by 



* Notice the run-down is preceded by an abrupt change of potential at grid and anode. 

 This is necessary to carry the grid cathode potential from the grid current region to the 

 amplifying region. 'Vertical fall' at the begirming of the stroke is characteristic of Miller 

 time bases. 



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