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Elektronica II Ch.5 BJT AC Analysis. 5.1 Introduction Ch.3 Transistor: basic construction, appearance, characteristics Ch.4 Transistor: biasing Ch.5 AC.

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Presentation on theme: "Elektronica II Ch.5 BJT AC Analysis. 5.1 Introduction Ch.3 Transistor: basic construction, appearance, characteristics Ch.4 Transistor: biasing Ch.5 AC."— Presentation transcript:

1 Elektronica II Ch.5 BJT AC Analysis

2 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 r e, hybrid π and hybrid equivalent

3 5.2 Amplification in the AC domain

4 5.3 BJT transistor modeling hybrid equivalent model: specification sheets include parameters, defined for a specific set of operating conditions (Ic, Vce, f) r e model: important parameter determined by actual operating conditions hybrid π model: high-frequency analysis

5 5.3 BJT transistor modeling (contd) AC equivalent model is obtained by: 1. Setting all dc sources to zero and replacing them by a short-circuit equivalent 2. Replacing all capacitors by short-circuit equivalent 3. Removing all elements bypassed by the short- circuit equivalents introduced by steps 1 and 2 4. Redrawing the network in a more convenient and logical form Parameters: V i, I i, Z i, V o, I o, Z o, A v, A i

6 5.3 BJT transistor modeling (contd)

7 5.4 The r e transistor model Common-Base Configuration Common-Emitter Configuration Common-Collector Configuration (use model defined by CE configuration)

8 5.4 The r e transistor model (contd) Common-Base Configuration

9 Typical values of Z i range from a few ohms to a maximum of about 50 Ω Typical values of Z o are in the mega-ohm range In general, the input impedance is relatively small and the output impedance quite high

10 5.4 The r e transistor model (contd) CB-Amplifier

11 5.4 The r e transistor model (contd) CB-Amplifier (example 5.11)

12 5.4 The r e transistor model (contd) Common-Emitter Configuration

13 Typical values of Zi defined by βr e 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

14 5.4 The r e transistor model (contd) CE-Amplifier: Fixed-bias

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17 5.4 The r e transistor model (contd) CE-Amplifier: Fixed-bias (example 5.4)

18 5.4 The r e transistor model (contd) CE-Amplifier: Voltage-divider

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20 5.4 The r e transistor model (contd) CE-Amplifier: Voltage-divider (Example 5.5)

21 5.4 The r e transistor model (contd) CE-Amplifier: Voltage-divider (Example 5.8)

22 5.4 The r e transistor model (contd) CE-Amplifier: Voltage-divider (Example 5.9)

23 5.4 The r e transistor model (contd) 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

24 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

25 5.15 Determining the current gain (contd)

26 5.16 Effect of R L and R S Two approaches can be used: By inserting the (r e 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

27 5.16 Effect of R L and R S (contd) 1 st approach

28 5.16 Effect of R L and R S (contd)

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30 5.16 Effect of R L and R S (contd) Fixed-bias

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32 5.16 Effect of R L and R S (contd) Fixed-bias: example Voltage divider - Emitter follower

33 nd (Two-port systems) approach (effect of R L and R S )

34 5.17 Two-port systems approach (effect of R L and R S ) (contd)

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37 5.17 Two-port systems approach (effect of R L and R S ) (contd) Example 5.15

38 5.18 Summary tables

39 5.18 Summary tables (contd)

40 5.19 Cascaded systems


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