Chapter 3: Bipolar Junction Transistors

Transcription

1 Chapter 3: Bipolar Junction Transistors

2 Transistor Construction There are two types of transistors: pnp npn pnp The terminals are labeled: E - Emitter B - Base C - Collector npn 2

3 Transistor Operation With the external sources, V EE and V CC, connected as shown: The emitter-base junction is forward biased The base-collector junction is reverse biased 3

4 Currents in a Transistor Emitter current is the sum of the collector and base currents: E C + B The collector current is comprised of two currents: C C + majority CO minority 4

5 Common-Base Configuration The base is common to both input (emitter base) and output (collector base) of the transistor. 5

6 Common-Base Amplifier nput Characteristics This curve shows the relationship between of input current ( E ) to input voltage (V BE ) for three output voltage (V CB ) levels. 6

7 Common-Base Amplifier Output Characteristics This graph demonstrates the output current ( C ) to an output voltage (V CB ) for various levels of input current ( E ). 7

8 Operating Regions Active Operating range of the amplifier. Cutoff The amplifier is basically off. There is voltage, but little current. Saturation The amplifier is full on. There is current, but little voltage. 8

9 Approximations Emitter and collector currents: C E Base-emitter voltage: V BE 0.7 V (for Silicon) 9

10 Alpha (α) Alpha (α) is the ratio of C to E : α dc C E deally: α 1 n reality: α is between 0.9 and Alpha (α) in the AC mode: αac C E 10

11 Transistor Amplification Currents and Voltages: E C L V L i E i R L V i R i 10 ma 200mV 20Ω ( 10 ma)( 5 kω) 10mA 50 V Voltage Gain: A v V L V i 50V 200mV

12 Common Emitter Configuration The emitter is common to both input (base-emitter) and output (collectoremitter). The input is on the base and the output is on the collector. 12

13 Common-Emitter Characteristics Collector Characteristics Base Characteristics 13

14 Common-Emitter Amplifier Currents deal Currents E C + B C α E Actual Currents C α E + CBO where CBO minority collector current CBO is usually so small that it can be ignored, except in high power transistors and in high temperature environments. When B 0 µa the transistor is in cutoff, but there is some minority current flowing called CEO. CEO CBO 1 α B 0µA 14

15 Beta (β) β represents the amplification factor of a transistor. (β is sometimes referred to as h fe, a term used in transistor modeling calculations) n DC mode: β dc C B n AC mode: β C CE ac V constant B 15

16 Beta (β) Determining β from a Graph β AC (3.2mA (30µA 1mA 10µA 100 V CE 2.2mA) 20µA) ma 25µ A β DC V CE 16

17 Beta (β) Relationship between amplification factors β and α α β β+ 1 β α α 1 Relationship Between Currents E (β+ 1) C β B B 17

18 Common Collector Collector Configuration The input is on the base and the output is on the emitter. 18

19 Common Collector Collector Configuration The characteristics are similar to those of the common-emitter configuration, except the vertical axis is E. 19

20 Operating Limits for Each Configuration V CE is at maximum and C is at minimum ( Cmax CEO ) in the cutoff region. C is at maximum and V CE is at minimum (V CE max V CEsat V CEO ) in the saturation region. The transistor operates in the active region between saturation and cutoff. 20

21 Power Dissipation Common-base: P Cmax V CB C Common-emitter: P Cmax V CE C Common-collector: P Cmax V CE E 21

22 Transistor Specification Sheet 22

23 Transistor Specification Sheet 23

24 Transistor Testing Curve Tracer Provides a graph of the characteristic curves. DMM Some DMMs measure β DC or h FE. Ohmmeter 24

25 Transistor Terminal dentification 25