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Elektronica II Ch.5 BJT AC Analysis

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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

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5.2 Amplification in the AC domain

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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

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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

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5.3 BJT transistor modeling (contd)

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

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

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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

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

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

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

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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

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

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

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

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

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

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

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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

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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

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5.15 Determining the current gain (contd)

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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

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

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5.16 Effect of R L and R S (contd)

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

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

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

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

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

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5.18 Summary tables

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5.18 Summary tables (contd)

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5.19 Cascaded systems

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