DIRECT-COUPLED PRE-AMPLIFIERS 



Having got a relay to chop properly — no mean feat* — it is tempting to get 

 all the necessary gain by a.c. amplification and subsequent phase-sensitive 

 rectification, i.e., to eliminate the subsequent 'straightforward' main ampli- 

 fier. In practice this is usually done, but if the input and output terminals of 

 a high gain amplifier are brought to within a few mm of one another at 

 the relay contacts, the consequent capacitive feedback will almost certainly 

 cause the amplifier to oscillate. 





Input ii^,^ 



50~ 

 a c. in 



Pentode 

 1 



Hh 



Pentode 

 2 



Hh 



Twinf-i 



Triode 



^V 



Cathode 

 follower 



50~ac. 



Phase - 



sensitive 



rect 



output 



i 



Gain control 



,N.FB. 



Figure 39.19 



There are two possible ways out of this. One is to use two relays, widely 

 separated, whose coils are fed from the same oscillator, one to short-circuit 

 the input and one to short-circuit the output. The second is to use a different 

 kind of phase-sensitive rectifier altogether. 



An amplifier using the second expedient has been described by Nielsen 

 and Rosenberg'^. Though developed for pH measurement there seems to be 

 no reason why this should not be used for biological studies of slow pheno- 

 mena; the chopper frequency is 50 c/s, so a frequency response of more 

 than a cycle or two per second would not be expected. The short-term 

 drift is 10 /^V and the long-term (12 hours) 30 //V. The maximum gain is not 

 stated but is probably well over 100,000. An extremely elaborate electroni- 

 cally stabilized power supply for both HT and heaters is also described. 

 Figure 39.19 shows the amplifier proper in schematic form. The chopper 

 relay is succeeded by two stages of pentode amplification, after which there 

 is a triode arranged as a 50 c/s tuned amplifier by a negative feedback loop 

 containing a twin T network. This restricts the amplifier band-width and 

 hence excludes from the output much of the noise generated at other fre- 

 quencies by the earlier stages. The phase-sensitive rectifier is of the full- 

 wave type comprising a ring of 4 diodes. Control of gain is by negative 

 feedback to the spare relay contact; as usual the use of negative feedback 

 in this way serves also to increase the input resistance of the device, the 

 expression for which is rather complicated. The low-pass filter in the input 

 circuit could be dispensed with for many applications. 



A two-relay chopper amplifier is shown in Figure 39.20. This apparatus 

 was developed for electro-oculography by E.M.L Electronics Ltd., to whom 

 I am grateful for allowing me to reproduce the circuit. The amplifier accepts 

 either single-sided or differential input. After chopping at 100 c/s, there are 

 3 stages of amplification, followed by a concertina phase-splitter which, 

 with the second relay, forms a full-wave phase-sensitive rectifier. Next comes 

 two stages of low-pass tapered RC filtering, then a fourth filter of the phase- 

 shift feedback type, and lastly a cathode-follower output stage. The gain of 



* There must be no 'contact bounce'. 



631 



