In other words, the transistor is a robust voltage buffer. Moreover, the output impedance is low, so it is not destabilized by the following circuit if that happens to be a heavy load. being virtually invisible, to the previous circuit. What’s good about it is its high input impedance, which has the effect of not loading, i.e. In the old world of vacuum tubes, they were common cathode, common grid and common plate.) This particular classification derives from the external circuit configuration.Īn emitter follower circuit, also known as a common-collector amplifier, is the quintessential negative feedback device. (For field-effect transistors, the analogous circuit configurations are common source, common gate and common drain. There are three bipolar junction transistor amplifier topologies: common emitter, common base and common collector. This region lies I E =0 and to the right side of V CB=0. In this region, both the emitter-base junction and collector-base junction are reverse biased. If the forward bias is high, then the hole current from the collector I C becomes positive. Hence, the net hole current fin the transistor decreases. This hole current flows just opposite to the hole current flows from the collector region to the base region. Thus, hole current flows from the collector region to the base region. The holes from the Collector (P region) cross the junction to enter the base region under the condition of forwarding bias. That is why I C increases with a small increase in V CB in this region. As the collector junction is forward biased, so the collector current I C increases exponentially with V CB just like in PN junction diode. Thus, for pnp transistor, the saturation region of the output characteristics lies to the left of V CB = 0 and above I E =0. In this region, both the junction i.e emitter-base junction ( J C) and collected base junction ( J e) is forward biased. The collector current I C depends only on the value of I E and independent of V CB. Where α Is the fraction of total emitter current which represents the holes that have traveled from the emitter to the collector through the base. Then the collector current, I C= I CO– α I E Since only a few holes entering the base region from the emitter Recombine with the electrons in the base region, so most of the emitter current reaches the collector. Now let us suppose, small emitter current I E flows. The magnitude of I CO is constant and independent of V CB. In this case, the collector current I C is equal to the reverse saturation current I CO. If I E = 0, then the transistor behaves as a PN junction diode formed by base and collector part of the transistor. In this region, the emitter-base junction J E is forward biased and collector-base junction J C is reversed biased. It is clear that the output characteristics of a transistor in the common base configuration are divided into three regions : Sr. It can be written as I c = F (V CB, I c ). The variation of collector current I C with the collector-base voltage (V CB ) at constant emitter current ( I E ) is called output characteristics of the transistor in the common base configuration. Output Characteristics of common base configuration
The value of the collector voltage at which punch through takes place is called punch-through voltage. This phenomenon is known as punch through. At a certain reverse bias of collector junction, the depletion layer at the collector junction covers the whole base region so the effective width of the base reduced to zero.Īs a result of this, the potential barrier at the emitter junction decreases and hence large emitter current flows. Since a small change in V EB causes a large change in I E so the Dynamic input resistance of junction Je is very small. The Dynamic input resistance = Δ V CB/ Δ I E. Thus, Emitter current I E increases with the increase in V CB. Due to this effect, the gradient of the holes concentration in the base region increases with the increase in a reverse bias V CB for a given value of V EB. This change of the effective width of the base region by the reverse Bias Voltage of collector junction is known as early effect or base width modulation. if the width of the depletion layer at the collector junction penetrating the base region is W’, then the effective width of the base region is equal to W – W’. As a result of this, the effective width of the base region decreases. However, when V CB increasing, the width of the depletion layer at the collector-base junction increases.
0 kommentar(er)
