DIRECT-COUPLED PRE-AMPLIFIERS 



the amplifier is not less than 10,000 and the drift is equivalent to a signal of 

 less than ±20 ^V in half an hour. With the input terminal shorted with a 

 10 kQ resistor, the noise is less than 10 [xV peak-to-peak [sic]. The input 

 resistance is 1 MQ. The high-frequency performance is stated in terms of 

 the rise time for a step function at the input, and is given as 100 milliseconds. 

 The vibrating capacitor amplifier — The possible upper limit of frequency 

 response of chopper amplifiers is set by the upper frequency at which it is 

 possible to get relays to chop satisfactory. At present chopper relay designers 

 seem to be aiming at 50 or 60 cycles operation, as the mains can then be used 



R ^2 



o — vV^A^-^ II — ^ 



Input q 



„^ To a.c. ampl'ifier 



Figure 39.21 



to drive them. The vibrating capacitor amplifier promises much faster opera- 

 tion because the mechanical part is merely a metal reed oscillating near a fixed 

 plate. The principle of the method is shown in Figure 39.21. Q is the vibrating 

 capacitor and Cg is for coupling. 7? is a high resistance such that RC^ greatly 

 exceeds the period of one vibration. Then if the input is a relatively slow- 

 changing voltage, the charge on Q is also slow-changing and proportional to 

 it, and the voltage across Q is proportional to the reciprocal of the capaci- 

 tance and to the charge. Therefore the voltage across Q contains an alter- 

 nating component at the vibration frequency proportional to the input. 



The performance of amplifiers employing mechanical modulators has been 

 reviewed by Chance^ who concludes that chopper amplifiers are more 

 satisfactory with low impedance inputs and vibrating capacitor types with 

 high. It is thus not surprising that designs which have appeared for the latter 

 have been rather for the measurement of extremely small currents from, e.g. 

 ionization gauges^'^". Nevertheless it is to be hoped that before long the 

 vibrating capacitor amplifier will be tried for biological amplification. The 

 amplifier of Thomas and Finch is particularly interesting as the vibration 

 frequency is 550 c/s ; there is also a novel type of phase-sensitive rectifier, 

 using a double triode valve. 



Amplifiers with electronic modulation — The great advantage here is that 

 the carrier frequency can be sufficiently high for the signal pass-band to be as 

 wide as one could wish. Thus an amplifier described by Lampitt", by 

 employing a carrier of 50 kc/s, could handle signals from zero to 5,000 c/s. 

 The gain was 100,000 but the equipment was designed to measure signals 

 of the order of one mV, so there is no guarantee that noise level was not 

 rather high. The rectification of Lampitt's amplifier was not phase sensitive, 

 so that the output could only be of one polarity, which is somewhat disad- 

 vantageous. In addition, no component values were given, so that the pro- 

 duction of a similar amplifier would require development almost from scratch. 



Galvanometer amplifiers — For applications requiring superlative perfor- 

 mance at low frequencies and moderate input impedance, the reader is 

 referred to Preston^^, HilP^, and Frankenhaeuser and Macdonald^*. 



633 



