. Electronic apparatus for biological research . HT- HT+ Figure 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 p
. Electronic apparatus for biological research . HT- HT+ Figure 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 sett
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