Chapter 5 Bipolar Junction Transistors Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Microelectronic Circuit Design McGraw-Hill

Circuit Representations for the Transport Models In npn transistor (expressions analogous for pnp transistors), total current traversing base is modeled by a current source given by: Diode currents correspond directly to the two components of base current. Microelectronic Circuit Design McGraw-Hill

Operation Regions of Bipolar Transistors Base-Emitter Junction Base-Collector Junction Reverse Bias Forward Bias Forward-Active Region (Good Amplifier) Saturation Region (Closed Switch) Cutoff Region (Open Switch) Reverse-Active Region (Poor Amplifier) Binary Logic States Microelectronic Circuit Design McGraw-Hill

Microelectronic Circuit Design i-v Characteristics of Bipolar Transistor: Common-Emitter Output Characteristics For iB = 0, transistor is cutoff. If iB > 0, iC also increases. For vCE > vBE, npn transistor is in forward-active region, iC = bF iB is independent of vCE. For vCE < vBE, transistor is in saturation. For vCE < 0, roles of collector and emitter reverse. Microelectronic Circuit Design McGraw-Hill

Microelectronic Circuit Design i-v Characteristics of Bipolar Transistor: Common-Emitter Transfer Characteristic Defines relation between collector current and base-emitter voltage of transistor. Almost identical to transfer characteristic of pn junction diode Setting vBC = 0 in the collector-current expression yields Collector current expression has the same form as that of the diode equation Microelectronic Circuit Design McGraw-Hill

Simplified Forward-Active Region Model In forward-active region, emitter-base junction is forward-biased and collector-base junction is reverse-biased. vBE > 0, vBC < 0 If we assume that then the transport model terminal current equations simplify to BJT is often considered a current-controlled device, though fundamental forward-active behavior suggests a voltage- controlled current source. Microelectronic Circuit Design McGraw-Hill

Simplified Forward-Active Region Model (Example 1) Problem: Estimate terminal currents and base-emitter voltage Given data: IS =10-16 A, aF = 0.95, VBC = VB - VC = -5 V, IE = 100 mA Assumptions: Simplified transport model assumptions, room temperature operation, VT = 25.0 mV Analysis: Current source forward-biases base-emitter diode, VBE > 0, VBC < 0, we know that transistor is in forward-active operation region. Microelectronic Circuit Design McGraw-Hill

Simplified Forward-Active Region Model (Example 2) Problem: Estimate terminal currents, base-emitter and base-collector voltages. Given data: IS = 10-16 A, aF = 0.95, VC = +5 V, IB = 100 mA Assumptions: Simplified transport model assumptions, room temperature operation, VT = 25.0 mV Analysis: Current source causes base current to forward-bias base-emitter diode, VBE > 0, VBC <0, we know that transistor is in forward-active operation region. Microelectronic Circuit Design McGraw-Hill

Simplified Circuit Model for Forward-Active Region Jaeger/Blalock 4/26/07 Microelectronic Circuit Design McGraw-Hill

Microelectronic Circuit Design Jaeger/Blalock 4/26/07 Microelectronic Circuit Design McGraw-Hill

Microelectronic Circuit Design Jaeger/Blalock 4/26/07 Microelectronic Circuit Design McGraw-Hill

Microelectronic Circuit Design Jaeger/Blalock 4/26/07 Microelectronic Circuit Design McGraw-Hill

Microelectronic Circuit Design Jaeger/Blalock 4/26/07 Microelectronic Circuit Design McGraw-Hill