. Electronic apparatus for biological research . Figure the voltage gain, <5Kout/(5f^in, and the power gain (6Fout ^/out)/(^in ^^in). In the earthed base configuration a transistor gives a voltage gain, but a current gain less than unity. In the earthed collector configuration, a current gain, but a voltage gain less than unity. In the earthed emitter arrangement the device shows both voltage gain and current gain, so that the power gain is maximal: power gains of over 40 dB's are possible. Further, an earthed base transistor has a low input resistance and a high output resistance, hig
. Electronic apparatus for biological research . Figure the voltage gain, <5Kout/(5f^in, and the power gain (6Fout ^/out)/(^in ^^in). In the earthed base configuration a transistor gives a voltage gain, but a current gain less than unity. In the earthed collector configuration, a current gain, but a voltage gain less than unity. In the earthed emitter arrangement the device shows both voltage gain and current gain, so that the power gain is maximal: power gains of over 40 dB's are possible. Further, an earthed base transistor has a low input resistance and a high output resistance, higher by about 3 orders, whilst in an earthed collector arrange- ment the situation is reversed. With the emitter earthed, the input and output resistance are more comparable (though the latter is still at least 10 times greater). This facilitates the coupling of transistors in cascade, since the matching conditions for optimum power transfer are most nearly met. The earthed emitter configuration is therefore of the greatest interest, and will receive the bulk of our attention. With transistors occupying their present low level of importance in biological work, a full treatment involving all three configurations would produce a chapter of dispropor- tionate length. There is no dearth of textbooks in which common base and common collector configurations are properly discussed; our purpose here is to introduce transistors and to cover sufficient mathematics to enable simple transistor design work to be carried out. Before beginning it must be mentioned that considerable confusion exists at present (September 1957) about the meaning of transistor symbols. A number of groups of transistor engineers have developed their own transistor symbolism and it is as yet too early for standardizing authorities to decide upon, and lay down, the best system. This confusion accounts for much of the difficulty in under- standing transistors at the present time. THE EARTHED EMITTER TRANSISTOR Suppos
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