Chapter3:Bipolar Junction Transistors (BJTs) 1

Figure 3.1 A simplified structure of the npn transistor. sedr42021_0501.jpg Figure 3.2 A simplified structure of the pnp transistor.

Figure 3.4 Cross-section of an npn BJT. Figure 3.3 Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.) sedr42021_0503.jpg Figure 3.4 Cross-section of an npn BJT.

sedr42021_0504.jpg Figure 3.5 Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode: vBE > 0 and vCB ³ 0.

Figure 3.5 Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode. sedr42021_0505a.jpg

sedr42021_0509.jpg Figure 3.6 The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters the saturation mode of operation for vCB < –0.4 V, and the collector current diminishes.

Figure 3.8 Circuit symbols for BJTs. Figure 3.7 Large-signal model for the pnp transistor operating in the active mode. sedr42021_0512.jpg Figure 3.8 Circuit symbols for BJTs.

Figure 3.10 Circuit for Example 5.1. Figure 3.9 Voltage polarities and current flow in transistors biased in the active mode. Example: β=100, VBE=0.7v at Ic=1mA For Ic=2mA and Vc= 5v, what are the different values of Rc=?, VBE=? IE=?, RE=? sedr42021_0514a.jpg Figure 3.10 Circuit for Example 5.1.

Application 1: In the circuit shown below (Fig E3.15), the voltage at the emitter was measured and found to be -0.7v. If β=50, find IE?, IC?, IB?, VC? sedr42021_e0510.jpg Figure 3.11

Application 1: P 423 sedr42021_0534a.jpg Figure 12 Analysis of the circuit for Example : (a) circuit; (b) circuit redrawn to remind the reader of the convention used in this book to show connections to the power supply

Application 2: P 422 sedr42021_0535a.jpg Figure 12 Analysis of the circuit for Example 12. Note that the circled numbers indicate the order of the analysis steps.

Figure 3.13 Example 5.7: (a) circuit Application 3: Determine the voltage at all nodes and the current through all branches sedr42021_0537a.jpg Figure 3.13 Example 5.7: (a) circuit

Application 4: sedr42021_0538a.jpg Figure 14 Example : (a) circuit; (b) analysis with the steps indicated by the circled numbers.

Figure 5.15 Example: (a) circuit; (b) analysis with steps numbered. Application 5: For the emitter bias network determine a. lB b. IC c. VCE d. VC e. VE f. VB g. VBC sedr42021_0539a.jpg Figure 5.15 Example: (a) circuit; (b) analysis with steps numbered.

Figure 5.16 Circuits for Example Application 5: sedr42021_0540a.jpg Figure 5.16 Circuits for Example

Transistor Switch A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0 V in saturation. Fig 4-22 VCE(cutoff) = VCC IC(sat) = (VCC – VCE(sat))/βDC IB(min) = Ic (sat)/βDC Figure 3.17

Biasing in BJT Amplifier circuits: Transistor circuit under examination in this introductory discussion. sedr42021_0543a.jpg Figure 3.18

The Classical biasing for BJTs using a single power supply: (a) circuit; (b) circuit with the voltage divider supplying the base replaced with its Thévenin equivalent. Figure 3.19: (a) circuit; (b) circuit with the voltage divider supplying the base replaced with its Thévenin equivalent

Common-base re equivalent circuit. Figure 3. 20

Defining Zo. Figure 3. 21

Defining Av = Vo/Vi for the common-emitter configuration. Figure 3. 22

re model for the common-emitter transistor configuration. Figure 3. 23

Common Emitter Configurations Common-emitter fixed-bias configuration. Figure 3. 24

Network of the previous figure following the removal of the effects of VCC, C1 and C2.

Substituting the re model into the network Figure 3. 26

Determining Zo for the network Figure 3. 27

Demonstrating the 180° phase shift between input and output waveforms. Figure 3. 28

Example Figure 3. 29

Voltage-divider bias configuration. Figure 3. 29

Substituting the re equivalent circuit into the ac equivalent network Figure 3. 30

Example Figure 3. 31

CE emitter-bias configuration. Figure 3. 32

Substituting the re equivalent circuit into the ac equivalent network Figure 3. 33

Defining the input impedance of a transistor with an un-bypassed emitter resistor. Figure 3. 34

Example Figure 3. 35

Common Collector Configurations Emitter-follower configuration. Figure 3. 36

Substituting the re equivalent circuit into the ac equivalent network Figure 3. 37

Defining the output impedance for the emitter-follower configuration. Figure 3. 38

Example Figure 3. 39

Emitter-follower configuration with a voltage-divider biasing arrangement. Figure 3. 40

Emitter-follower configuration with a collector resistor RC Figure 3. 41

Collector feedback configuration. Figure 3. 42

Substituting the re equivalent circuit into the ac equivalent network Figure 3. 42

Defining Zo for the collector feedback configuration. Figure 3. 43

Example Figure 3. 44

Collector feedback configuration with an emitter resistor RE. Figure 3. 45

Homework: Present the AC equivalent circuit Figure 3. 46

Resolve the all the numerical parameters for the circuit shown below Figure 3. 47

Common-base configuration. Figure 3. 48

Substituting the re equivalent circuit into the ac equivalent network Figure 3. 49

Example Figure 3. 50

General overview of the small signal Analysis (CE, CC, CB Common Emitter Input resistance moderate/small Output resistance large Open Circuit Voltage gain large Short Circuit Current gain Figure 3.51: A common-emitter amplifier using the structure (b) Equivalent circuit obtained by replacing the transistor with its hybrid-p model. large Large voltage and current gain but Rin and Ro not good for voltage amplifier.

Common Emitter with RE Re increases Rin but reduces open circuit voltage gain. Current gain and output resistance are unchanged. Input resistance Open Circuit Voltage gain reduced increased Rib greatly increased by resistance reflection rule (Miller) Voltage gain reduced by ~ (1+gmRe); Rib increased by this factor. Figure (a) A common-emitter amplifier with an emitter resistance Re. (b) Equivalent circuit obtained by replacing the transistor with its T model.

Common Base Input resistance Open Circuit Voltage gain Short Circuit Current gain small large unity Output resistance Non-inverting version of common emitter. large Good for unity gain current buffer. Figure (a) A common-base amplifier using the structure (b) Equivalent circuit obtained by replacing the transistor with its T model.

Common Collector Figure (a) An emitter-follower circuit based on the structure (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model augmented with ro. (c) The circuit in (b) redrawn to emphasize that ro is in parallel with RL. This simplifies the analysis considerably.

Open Circuit Voltage gain Current gain Input resistance large Output resistance small Open Circuit Voltage gain Current gain ~unity large Voltage gain ~1 so emitter follows base input voltage (emitter follower) Good for amplifier output stage: large Rin, small Rout.