TRIANGULAR WAVE GENERATORS 



On the fastest sweep speed matters are rather different, /is 50 milliamps in 

 1 millisecond = 50 amps/sec and the back e.m.f. is therefore 3 X 50 = 1 50 V. 

 This back e.m.f. becomes active as soon as the forward stroke begins, 

 sweeping the working point 150 V to the left to A'. Thereafter it moves to 



HT=300 



Figure 16.30 



B' along a path parallel to AB, as the voltage drop across the resistance 

 increases. At the end of the sweep the total deflector coil voltage is Lt -\- 

 imsixR = 200 V, leaving an anode voltage for the valve of 100 V. 

 From the foregoing the points to watch in design emerge; they are: 



(1) B must not move outside the maximum power hyperbola; 



(2) B' must not move outside the working region at the knees of the anode 

 characteristics; 



(3) neither B nor B' must go above the maximum anode current line. 

 The parallelogram ABB' A' is a property of the particular deflector coil used 

 and the upper and lower sweep speeds. It may be drawn on tracing paper 

 and juggled about on the anode characteristics for various valves until one 

 is found which it will neatly fit. The quiescent bias and HT+ voltage 

 required are then determined. If the parallelogram cannot be made to fit 

 because Pmax or /a_niax for the valve is infringed, consider using two valves 

 in parallel. This halves the current each has to supply and also, incidentally, 

 since g„^ has been doubled, halves the amplitude of input wave required at 

 the grid. If it cannot be made to fit because no valve will stand the amount 

 of HT required, a new deflector coil having fewer turns is indicated. For 

 example, if we halve the number of turns, the inductance L will be reduced to 

 one quarter, /will have to be doubled, so the back e.m.f., Li, will be halved. 

 The input wave ampHtude will also in general require doubling. The extra 

 current required by the new coil may require two valves in parallel in place 

 of one, in which case the input required remains the same. 



Negative current feedback 



The linearity of the output current depends on the constant-current genera- 

 tor living up to its name. The pentode is entitled to be regarded as a constant- 

 current generator in virtue of its high internal resistance /-„, of the order of a 



249 



