. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. HT-t (a) (b) Figure Arrangements for maintaining the level of oscillation have to be more elaborate than the simple self-biasing device which is adequate for LC oscillators. This is because the RC frequency-determining networks have of themselves only a very small Q value; one could not possibly tolerate the pulsatory currents which are permissible in LC oscillators because there is no device with a g of several hundred to re-create a pure sine waveform. Thus the valves must amphfy i
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. HT-t (a) (b) Figure Arrangements for maintaining the level of oscillation have to be more elaborate than the simple self-biasing device which is adequate for LC oscillators. This is because the RC frequency-determining networks have of themselves only a very small Q value; one could not possibly tolerate the pulsatory currents which are permissible in LC oscillators because there is no device with a g of several hundred to re-create a pure sine waveform. Thus the valves must amphfy in class A, without distortion, and some other means must be found of regulating the gain. D'Argidmbeau's method—There is a variety of pentode known as 'variable fi' {Figure ). As a result of grading the mesh size of the control grid along the axis of the valve it can be shown that the ampHfication obtainable from the valve is a function of the mean grid bias {Figure ). Clearly if the. /so/ Vg-k Vg-k Variable-^ Normal grid Variable-^ grid valve characteristic characteristic Figure Figure input signal is large enough to sweep over an appreciable proportion of this curve, the output of a variable /n stage is distorted. However, if the signal is small enough only a short piece of the curve is used and this may be regarded as being straight. Use of this fact may be made in the Wien bridge oscillator. In Figure , part of the output is tapped off and shunt rectified by the small diode in the second valve to produce a negative bias which controls the gain of the first valve. The capacitance of C^ has to be sufficiently large for its reactance to be negligible compared with the frequency determining R's and 222. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Donaldson, Peter Eden Kirwan.
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