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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Simplified Forward-Active Region Model (Example 2)
Problem: Estimate terminal currents, base-emitter and base-collector
voltages.
Given data: IS = 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

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

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

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

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

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