. Electronic apparatus for biological research . (a) (b) Figure multi-vibrator (see below), the coupled amplifier employs 5 transistors, and two further transistors in earthed collector follow the rectifier and filter system to secure an output impedance of only a few tens of ohms. The apparatus is used to amplify the output of a thermocouple and, since the latter is a low resistance device, the performance is expressed by Burton in terms of voltages. The basic voltage gain of about 1,000 is reduced to 55 by negative feedback. Feedback as heavy as this enables Burton to claim a drif
. Electronic apparatus for biological research . (a) (b) Figure multi-vibrator (see below), the coupled amplifier employs 5 transistors, and two further transistors in earthed collector follow the rectifier and filter system to secure an output impedance of only a few tens of ohms. The apparatus is used to amplify the output of a thermocouple and, since the latter is a low resistance device, the performance is expressed by Burton in terms of voltages. The basic voltage gain of about 1,000 is reduced to 55 by negative feedback. Feedback as heavy as this enables Burton to claim a drift equivalent to only 1 mV at the input over the ambient temperature range —12°C to -\-50°C. Even allowing for the fact that 1 mV across the low resistance of a thermocouple (10 Q) represents a current change of 10~* amps, the figure given augurs adequate independence of room temperature. converters These are devices for producing a direct output from a direct input at some other voltage. As normally used, they are arranged to produce a voltage step-up. For example, a small converter described by Johnston^* supplied an output of 30 V, 100 //A from an input at 3 V. The object was to supersede the relatively expensive HT battery in thermionic hearing aids by power derived by the converter from the relatively cheap LT cells. The efficiency of conversion was 60 per cent and the unit measured only 1 f x 1t6 X \ in. At the other end of the scale, a converter employing a power transistor^^ delivered 10 kV; 100 [jiK for an input of 12 V, 150 mA; efficiency, 55 per cent. Much higher conversion efficiencies are possible. Perhaps the classical British paper on converters is by Light and Hooker^^, but see also Light^'. The simplest type is shown in Figure ; in principle it is a transistor oscillator, with feedback achieved from collector circuit to base circuit by mutual induction, arranged so that the collector winding, L^, forms the primary of a step-up transformer. A h
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