MAIN AMPLIFIERS 



MAIN AMPLIFIERS 



Main amplifiers for electrostatic cathode ray tube deflection 



A capacitor-coupled main amplifier having a time constant of 2 seconds 

 which is in every way typical is given by Dickinson^^ and is reproduced in 

 Figure 39.22. The amplifier accepts a balanced input and delivers a balanced 

 output to the deflector plates. Coarse (stepped potential divider) and fine 

 (variable negative feedback) gain controls are provided, and Dickinson 

 points out that it is vital that the resistors used in the coarse control divider 

 chain be accurately matched lest an in-phase signal applied to the input 

 develop an out-of-phase component and appear on the tube base line as an 

 artefact. A direct-coupled counterpart due to the author is shown in Figure 

 39.23. Despite its simplicity the gain is such that with a 6 in. cathode ray 

 tube at an EHT of 2 kV, the spot is deflected from top to bottom of the 

 screen by 100 mV. The frequency response could be improved by adding 

 compensating capacitors to the interstage potential dividers, but the perfor- 

 mance is satisfactory for examining the shape of an amphibian action 

 potential. It must be admitted that the form of Y shift control used here 

 acts also as a differential gain control and can theoretically spoil the anti- 

 in-phase rejection ratio of the recording chain. In practice, the effect is not 

 serious, since the control follows 3 (if the pre-amplifier is included) stages of 

 accurate differential amplification. By the time signals reach it, the effect 

 of in-phase components has been substantially suppressed. 



Main amplifiers for magnetic cathode ray tube deflection 



In order to demonstrate electrophysiological phenomena to students an 

 oscilloscope possessing a large screen is a valuable asset. Large-screen 

 electrostatic cathode ray tubes can be made to special order but naturally 

 they are expensive and in addition they are rather long, leading to an instru- 

 ment of unsatisfactory dimensions. Television tubes are quite satisfactory 

 for low-frequency (0-10,000 c/s) oscilloscopy, are cheap and, because a wide 

 deflection angle is possible with the magnetic system, physically short. 



Magnetic deflection circuits are quite straightforward, but consume 

 rather a lot of current. It is necessary that the back e.m.f. occurring across 

 the deflector coils should never be so great as to carry the anodes of the 

 output valves outside their working region; thus the coil self-inductance 

 must be restricted. The self inductance depends on the square of the number 

 of deflector coil turns, whereas the deflecting capability of the coil is propor- 

 tional to the number of turns and, of course, the deflector coil current. 

 Therefore to scan completely the tube face at a high frequency requires 

 coils with a rather small number of turns, and a large current. Magnetic 

 cathode ray tubes and deflector coils have been discussed in Part III, and 

 the circuit of a simple practical amplifier devised by the writer given (Figure 

 32.39). The output valves operate in class AB push-pull. The standing anode 

 current for the four is adjusted to 60 m A total in the absence of any input, and 

 the trace centred. Upon the arrival of a direct input signal the current in 

 one or other pair rises to a maximum of 200 mA, at which point the spot is 

 at the top or bottom edge of the screen. Damage through overloading is 



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