Download Bipolar Transistor Operation and DC Biasing and more Essays (high school) Physics in PDF only on Docsity! Bipolar Junction Transistors Bipolar Transistor Operation • We will consider npn transistors – pnp devices are similar but with different polarities of voltage and currents – when using npn transistors • collector is normally more positive than the emitter • VCE might be a few volts • device resembles two back-to-back diodes • with the base open-circuit, negligible current flows from the collector to the emitter Bipolar Transistor Operation • Transistor action Bipolar Transistor Operation • Figure shows a bias arrangement for both npn and pnp BJTs for operation as an amplifier. • In both cases the base-emitter (BE) junction is forward- biased and the base-collector (BC) junction is reverse- biased. • This condition is called forward-reverse bias. Bipolar Transistor Operation
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DC Beta βDC & DC Alpha αDC • The dc current gain of a transistor is the ratio of the dc collector current to the dc base current (IB) and is designated dc beta (βDC ). • Typical values of βDC range from less than 20 to 200 or higher. In data sheets it is written as DC Beta βDC & DC Alpha αDC • The ratio of the dc collector current to the dc emitter current is the dc alpha (αDC). The alpha is a less-used parameter than beta in transistor circuits. • Typically, values of αDC range from 0.95 to 0.99 or greater, but αDC is always less than 1. • The reason is that IC is always slightly less than IE by the amount of IB. For example, if IE = 100 mA and IB = 1 mA, then IC = 99 mA and αDC = 0.99. Example • Determine the dc current gain and βDC the emitter current IE for a transistor where IB 50 mA and IC 3.65 mA. BJT CIRCUIT ANALYSIS • In the basic transistor bias circuit configuration the three transistor dc currents and three dc voltages can be identified. • IB: dc base current • IE: dc emitter current • IC: dc collector current • VBE: dc voltage at base with respect to emitter • VCB: dc voltage at collector with respect to base • VCE: dc voltage at collector with respect to emitter BJT CIRCUIT ANALYSIS BJT CIRCUIT ANALYSIS • The base-bias voltage source, VBB, forward-biases the base-emitter junction, and the collector-bias voltage source, VCC, reverse-biases the base-collector junction. • When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltage drop of VBE = 0.7 V Example • Determine IB, IC, IE, VBE, VCE, and VCB in the circuit. The transistor has a βDC = 150. Collector Characteristic Curve • Output characteristics – region near to the origin is the saturation region – this is normally avoided in linear circuits – slope of lines represents the output resistance Modes of Operation • Most important mode of operation • Central to amplifier operation • The region where current curves are practically flat Active: Saturation: • Barrier potential of the junctions cancel each other out causing a virtual short Cutoff: • Current reduced to zero • Ideal transistor behaves like an open switch
