1	Chapter 28 Basic Transistor Theory
2	Transistor Construction Bipolar Junction Transistor (BJT) 3 layers of doped semiconductor 2 p-n junctions Layers are: Emitter, Base, and Collector Can be NPN or PNP Emitter and Collector both P or both N type
3	Transistor Construction Structure and Electronic Symbol
4	Transistor Operation Amplifier B-E junction forward biased V_BE ≈ 0.7 V for Si C-B junction reverse biased KCL: I_E = I_C + I_B
5	Transistor Operation Transistor Bias Circuits
6	Transistor Operation dc Beta (β_dc) I_E = I_C + I_B I_B << I_E I_C ≈ I_E 40 < β_dc < 400
7	Transistor Operation 2N3904 NPN transistor spec 100 < β_dc < 300 β_dc dependent on Operating point Temperature
8	Transistor Operation dc Alpha (α_dc) α < 1 α-β Relationship
9	Transistor Specifications Maximum voltage between C & E with Base open, V_CEO Maximum reverse voltage between C & B with Emitter open, V_CBO Maximum reverse voltage between E & B with Collector open, V_EBO
10	Transistor Specifications Maximum collector current, I_C Maximum power dissipated, P_D P_D = I_C * V_CE
11	Transistor Specifications Minimum C-E voltage for breakdown, V_(BR)CEO Carefully examine absolute max ratings dc current gain variable β = h_FE in specs
12	Collector Characteristic Curves Saturation region I_C increases rapidly for small values of V_CE BJT behaves like closed switch
13	Collector Characteristic Curves Active region BJT acts as a signal amplifier B-E junction is forward biased & C-B junction is reverse biased
14	Collector Characteristic Curves β_dc not constant β_dc dependent on dc operating point Quiescent point = operating point Active region limited by Maximum forward current, I_C(MAX) Maximum power dissipation, P_D
15	dc Load Line Drawn on characteristic curves Component values in a bias circuit Determine quiescent point, Q Q is between saturation and cutoff Best Q for a linear amplifier Midway between saturation and cutoff
16	DC Load Line Characteristic curve with Load Line Q-point, and current gain
17	Transistor Biasing Fixed-Bias Circuit Single power supply Coupling capacitors
18	Transistor Biasing Equations for Fixed-Bias circuit
19	Transistor Biasing Fixed Bias Circuit highly dependent on β_dc Emitter-Stabilized Bias Circuit Add emitter resistor Greatly reduces effect of change of β Equations
20	Transistor Biasing
21	Transistor Biasing Universal-Bias circuit Sometimes referred to as voltage divider bias Most stable Equations:
22	Transistor Biasing Universal-Bias circuit Need I_B << I_C Make Simple Voltage divider between V_CC, Base, and ground
23	Transistor Biasing
24	Transistor Biasing Common Collector Circuit Less common than CE circuit Collector connected to ground Similar analysis Voltage gain < 1
25	Transistor Biasing Common Base Circuit Least common High frequency applications Current gain < 1
26	The Transistor Switch BJT less used as amplifiers IC amplifiers available Switching is a principal application of BJT’s Current amplifier turn on LED’s Power amplifier to turn on small motors
27	The Transistor Switch A buffer has high input impedance and low output impedance
28	The Transistor Switch BJT as a buffer between digital input and LED
29	Testing a Transistor with a Multimeter Ohmmeter dc voltage generates small current Test CB and BE junctions Forward bias = small resistance Reverse bias = large resistance
30	Testing a Transistor with a Multimeter Fail test BJT will not operate correctly Pass test Not a guarantee that BJT is good
31	Testing a Transistor with a Multimeter Six measurements required An O.C. between two terminals (both directions) means other terminal is B Only two low Ω readings if BJT is good
32	Testing a Transistor with a Multimeter Lower of the two low Ω readings is C Other one of low Ω readings is E
33	Junction Field Effect Transistor Construction and Operation Construction and symbols
34	Junction Field Effect Transistor Construction and Operation BJT Current amplification BE junction forward biased Input impedance (Common Emitter) low
35	Junction Field Effect Transistor Construction and Operation JFET Voltage amplification GS junction reverse biased Input impedance very high
36	Junction Field Effect Transistor Construction and Operation Basic operation of an n-channel JFET
37	Junction Field Effect Transistor Construction and Operation I_S = I_D Decrease V_GS from 0 to –4 Decrease current flowing Pinchoff voltage reached
38	Junction Field Effect Transistor Construction and Operation I_D vs V_GS (Transconductance curve) described by Shockley’s equation
39	Junction Field Effect Transistor Construction and Operation Self-bias circuit
40	Junction Field Effect Transistor Construction and Operation Load line
41	Junction Field Effect Transistor Construction and Operation Another biasing circuit: similar to universal bias circuit for BJT’s Voltage divider Resistor from from V_DDto the Gate Resistor from Gate to ground
42	Junction Field Effect Transistor Construction and Operation Basic JFET circuit analysis: use KVL and KCL I_G = 0 I_D= I_S
43	MOSFETs Metal Oxide Semiconductor Field Effect Transistors Small Low power Higher current capability, I_DS Do not have to reverse bias the gate Depletion or Enhancement types
44	MOSFETs Construction and symbols
45	MOSFETs Depletion MOSFETs Have a channel Shockley’s equation still valid Depletion mode Enhancement mode
46	MOSFETs Enhancement MOSFETs No channel Positive V_GS required prior to current Enhancement mode only No depletion mode Shockley’s equation no longer valid
47	MOSFETs Biasing Voltage Divider Drain-feedback circuit shown here
48	MOSFETs
49	MOSFETs Handling precautions Subject to damage by electrostatic charges Packaged in static resistant bags Handle at static safe workstation Use grounded wrist strap
50	Troubleshooting a Transistor Circuit Ensure correct biasing Measure V_BE Determine V_CEQ and I_CQ Determine I_BQ Calculate β