Elektronica II

Ch.5 BJT AC Analysis

5.1 Introduction

Ch.3 Transistor: basic construction, appearance, characteristics

Ch.4 Transistor: biasing Ch.5 AC response BJT amplifier by

reviewing the (small-signal) models re, hybrid π and hybrid equivalent

5.2 Amplification in the AC domain

5.3 BJT transistor modeling

hybrid equivalent model: specification sheets include parameters, defined for a specific set of operating conditions (Ic, Vce, f)

re model: important parameter determined by actual operating conditions

hybrid π model: high-frequency analysis

5.3 BJT transistor modeling (cont’d)

AC equivalent model is obtained by:1. Setting all dc sources to zero and replacing them

by a short-circuit equivalent2. Replacing all capacitors by short-circuit equivalent3. Removing all elements bypassed by the short-

circuit equivalents introduced by steps 1 and 24. Redrawing the network in a more convenient and

logical form

Parameters: Vi, Ii, Zi, Vo, Io, Zo, Av, Ai

5.3 BJT transistor modeling (cont’d)

5.4 The re transistor model

Common-Base Configuration Common-Emitter Configuration Common-Collector Configuration (use

model defined by CE configuration)

5.4 The re transistor model (cont’d)Common-Base Configuration

5.4 The re transistor model (cont’d)Common-Base Configuration

Typical values of Zi range from a few ohms to a maximum of about 50 Ω

Typical values of Zo are in the mega-ohm range In general, the input impedance is relatively small and

the output impedance quite high

5.4 The re transistor model (cont’d)CB-Amplifier

5.4 The re transistor model (cont’d)CB-Amplifier (example 5.11)

5.4 The re transistor model (cont’d)Common-Emitter Configuration

5.4 The re transistor model (cont’d)Common-Emitter Configuration

Typical values of Zi defined by βre range from a few hundred ohms to the kilo-ohm range, with a maximum of about 6 kΩ to 7 kΩ

Typical values of Zo are in the range of 40 kΩ to 50 kΩ

5.4 The re transistor model (cont’d)CE-Amplifier: Fixed-bias

5.4 The re transistor model (cont’d)CE-Amplifier: Fixed-bias

5.4 The re transistor model (cont’d)CE-Amplifier: Fixed-bias

5.4 The re transistor model (cont’d) CE-Amplifier:Fixed-bias (example 5.4)

5.4 The re transistor model (cont’d)CE-Amplifier: Voltage-divider

5.4 The re transistor model (cont’d)CE-Amplifier: Voltage-divider

5.4 The re transistor model (cont’d) CE-Amplifier:Voltage-divider (Example 5.5)

5.4 The re transistor model (cont’d) CE-Amplifier:Voltage-divider (Example 5.8)

5.4 The re transistor model (cont’d) CE-Amplifier:Voltage-divider (Example 5.9)

5.4 The re transistor model (cont’d) Other configurations

=> 5.8 common-emitter fixed-bias: Zi, Zo, Av, phase relationship, example 5.4=> 5.9 voltage-divider bias:

=> Re bypassed, with ro : Zi, Zo, Av, phase relationship, example 5.5 en 5.9=> Re unbypassed, with ro : example 5.8

=> 5.10 ce emmitter-bias:=> Re unbypassed, without ro : Zi, Zo, Av, phase relationship=> Re unbypassed, with ro: Zi, Zo, Av=> bypassed => zie ce fixed-bias-> example 5.6 (unbypassed, with ro) en 5.7 (bypassed, with ro)

=> 5.11 emitter-follower:=> without ro : Zi, Zo, Av, phase relationship=> with ro : Zi, Zo, Av-> example 5.10: without en with ro=> variaties: with voltage-divider biasing en with collector resistor Rc

=> 5.12 common-base configuration: Zi, Zo, Av, Ai: example 5.11=> 5.13 collector feedback:

=> without Re:=> without ro: Zi, Zo, Av, phase relationship=> with ro: Zi, Zo, Av-> example 5.12: without en with ro

=> with Re: exercise=> 5.14 collector dc feedback: with ro: Zi, Zo, Av, phase relationship, example 5.13

5.15 Determining the current gain

For each transistor configuration, the current gain can be determined directly from the voltage gain, the defined load and the input impedance

5.15 Determining the current gain (cont’d)

5.16 Effect of RL and RS

Two approaches can be used: By inserting the (re model) equivalent

circuit and use methods of analysis to determine the quantities of interest

By defining a two-port equivalent model and use the parameters determined for the no-load situation

5.16 Effect of RL and RS (cont’d) 1st approach

5.16 Effect of RL and RS (cont’d)

5.16 Effect of RL and RS (cont’d)

5.16 Effect of RL and RS (cont’d) Fixed-bias

5.16 Effect of RL and RS (cont’d) Fixed-bias

5.16 Effect of RL and RS (cont’d) Fixed-bias: example 5.14

- Voltage divider

- Emitter follower

5.17 2nd (Two-port systems) approach (effect of RL and RS)

5.17 Two-port systems approach (effect of RL and RS) (cont’d)

5.17 Two-port systems approach (effect of RL and RS) (cont’d)

5.17 Two-port systems approach (effect of RL and RS) (cont’d)

5.17 Two-port systems approach (effect of RL and RS) (cont’d) Example 5.15

5.18 Summary tables

5.18 Summary tables (cont’d)

5.19 Cascaded systems