Common Base Amplifier Static Characteristics Common Base Static
Common Base Amplifier
Static Characteristics �Common Base Static characteristics ◦ Input characteristics. IE varies with VBE when voltage VCB is held constant �VCB is adjusted with the help of R 1 �VBE is increased and corresponding values of IE are noted �The plot gives input characteristics �Similar to the forward characteristics of P-N diode �This characteristics is used to find the input resistance of the transistor. Its value is given by the reciprocal of its slope Rin= VBE / IE
BJT Input Characteristics C VEE R 2 + IE E VBE IC VCB R 1 B IE 8 m. A 6 m. A 4 m. A 2 m. A 0. 7 VBE VCC
Static Characteristics �Common Base Static characteristics ◦ Output characteristics. IC varies with VCB when IE is held constant �VBE is adjusted with the help of R 2 and IE is held constant �VCB is increased and corresponding values of IC are noted �The plot gives output characteristics �Then IE is increased to a value little higher and whole process is repeated �The output resistance of the transistor is given by Rout= VCB / IC C VEE R 2 IC + IE E VCB VBE B R 1 VCC
BJT Output Characteristics Ø IC flows even when VCB=0 for different values of IE(due to internal junction voltage at CB junction) Ø IC flows even when IE=0 (Collector leakage current or reverse saturation current ICBO) ØThe output resistance is very high (500 k ) IC Saturation Region Active Region IE Cutoff IE = 0 VCB
Static Characteristics – It can be seen that IC flows even when VCB is zero – It is due to the fact that electrons are being injected into base due to forward biased E-B junction and are collected by collector due to action of internal junction voltage at C-B junction – Another important feature is that a small amount of collector current flows even when the emitter current IE is zero called collector leakage current (ICBO) – When VCB is permitted to increase beyond a certain value, IC increases rapidly due to avalanche breakdown • This characteristics may be used to find ac = IC/ IE
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