. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. TRANSISTORS Of the possible circuits to compensate for these temperature effects, the most widespread is shown in Figure A stabilizing resistor R^ is connected in the emitter circuit, and the base bias is derived from a potential divider R1-R2 across the battery. If R^ and Ro are chosen so that the current up them is much larger than the base current, then the mean potential of the base below earth is merely {/?2/(^i + -^2)}^- Since the base-emitter voltage with transistors is small
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. TRANSISTORS Of the possible circuits to compensate for these temperature effects, the most widespread is shown in Figure A stabilizing resistor R^ is connected in the emitter circuit, and the base bias is derived from a potential divider R1-R2 across the battery. If R^ and Ro are chosen so that the current up them is much larger than the base current, then the mean potential of the base below earth is merely {/?2/(^i + -^2)}^- Since the base-emitter voltage with transistors is small, about 100 mV, the expression. Figure {R^liRi + ^2)}^ ^Iso gives with sufficient accuracy the voltage across Rg, defining the emitter current at i?2 R1 + R2 V Of this, a fraction given very roughly by 1//5 flows out of the base, and the remainder out of the collector. The total collector current is composed of the true transistor collector current /94, plus the leakage current (/? + l)4(o)- Any tendency for the leakage current to increase produces an increase in the potential difference across R^ and a reduction in the emitter-base voltage. Because the emitter-base junction is a diode biased in the forward direction, the voltage-current relation is exponential and a small proportional reduc- tion in voltage produces a much larger proportional reduction in base current. Thus an increase in (/? + 1)7^(0) is partially offset by a reduction in ^I,. At first sight it appears that the compensation breaks down when ^I^ is reduced to zero, and that therefore the collector current can never be less than (^ + l)/c(o). In fact this is not so, because of the peculiar shape of the base-emitter characteristic for transistors in the earthed-emitter mode. It is clear from Figure that the direction of the base current reverses before the emitter-base voltage has fallen to zero. Thus as {^ + l)^c(o) increases, a point is reached at which base current begins to flow into, instead of
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