GAIN AND BALANCE CONTROLS 



are to be maintained. This is inconvenient, but can fortunately be overcome 

 by making R^ a pentode valve (Figure 12.25), for the pentode has a very high 

 incremental resistance (rj but quite a low d.c. resistance (VJIJ. The 



HT+ 



HT- 



HT+ 



Figure 12.25 



effective value of R^ will be the /•„ of the valve — possibly 1 megohm — yet this 

 may be achieved with an anode-cathode potential, and hence negative supply 

 of perhaps only 20 V. If the pentode anode current is 1 milliamp, the per- 

 formance achieved is the same as if a 1 megohm resistance had been used, and 

 a 1,000 V negative supply. 



This arrangement is undoubtedly capable of yielding extremely high 

 discrimination ratios but it is necessary to issue a warning. To begin with, 

 there is no point in having a splendid theoretically possible rejection ratio if 

 the stage balancing arrangements are not elaborate enough to secure it. 

 Again, it will be shown in the chapter on interference that the 'equal' in-phase 

 signals cannot be reUed upon to be all that equal. If this is so there is no point 

 in striving after enormous rejection ratios. A rejection of 10,000 is usually 

 considered ample and this is easily attained without employing prodigiously 

 negative supphes. In-so-far as an extra valve and its attendant bias, screen 

 and heater supphes are required, the author is of the opinion that, for general 

 purposes at least, the use of a pentode for the cathode resistance is unjustified. 



GAIN AND BALANCE CONTROLS FOR 

 DIFFERENTIAL VOLTAGE AMPLIFIERS 

 Gain 



The purpose of gain control is to maintain the output of an amphfier at a 

 convenient level despite wide variations in the magnitude of the input. In 

 biological amplifiers it is achieved either by varying the gain of one of the 

 stages of amplification by negative feedback or by incorporating a potentio- 

 meter in one of the interstage couphngs, or both. 



The point in the amphfying chain at which control of gain takes place 

 requires careful choice. If the point is in the early, low-level stages of the 

 amphfier, then when the gain control is turned down there is no reduction in 

 the noise generated by later stages; the signal-to-noise ratio is poor at low- 

 gain settings. If the control point is in the late, high-level stages, then valves 



185 



