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BJT, AC behavior

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AGENDA

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BJT AC behaviourDC & AC signalsCharacteristicsDC input characteristicsAC input characteristicsHow to draw and ac circuitConfigurationsCommon EmitterExamplesCalculating

BJTImpedance reflection p 171Input/output resistanceSource resistianceCE exampleCommon CollectorExampleCalculationsCommon BaseExampleCalculations

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BJT, AC behavior

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Signal operation can be seen as a small variation on the DC bias of a circuit. If the signals are very small to the DC bias, transistor parameters can be considered as constant. While the BJT is a non-lineair behavior component this appoximation is only valid for very small variations on the DC-bias point.

The first thing to do is to explain the AC model of a BJT, afterwards we make applications an calculations on a complete AC circuit.

P.J.F. PaapP.J.F. Paap

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BJT, DC & AC signals

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BJT, DC & AC signals

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BJT, characteristics

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DCDC model model acac model model

DCDC model; Vbe = 0V7 Ube, Uce, Ic, Ib, Ie model; Vbe = 0V7 Ube, Uce, Ic, Ib, Ie CapitalsCapitals

acac model; re = 26mV/Ie ube, uce, ic, ib, ie model; re = 26mV/Ie ube, uce, ic, ib, ie Low Low casescases

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BJT, DC input characteristics

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Vbe = 0V7

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BJT, AC input characteristics

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re = 26mV/Ic

The dynamic resistor can be calculated by the DC current

Ic

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BJT, characteristics

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BJT, how to draw an ac circuit 1/4

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1 Kill all DC sources (ΔU = 0 V) replacing each dc voltage source with a short

circuit and each dc current source with an open circuit

2 Replace all Capacitors ( xc = 1/2.pi.f )

3 Use the ac model of the active devices T model for BJT

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BJT, how to draw an ac circuit 2/4

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1 Kill all DC sources (ΔU = 0 V)

replacing each dc voltage source with a short circuit

and

each dc current source with an open circuit

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BJT, how to draw an ac circuit 3/4

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2 Replace all Capacitors ( xc = 1/2.pi.f )

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BJT, how to draw an ac circuit 4/4

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3 Use the ac model of the active devices, T model for BJT

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BJT, configurations

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1 Common Emitter Input = base Output = collector

2 Common Collector Input = base Output = emitter

or Emitter follower3 Common Base Input = emitter Output =

collector

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BJT, CE examples

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BJT, CE calculating

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Au

x

x

i something

i something different

Law of Bollenhere ix = ib

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BJT, CE calculating

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Au x

x

i something

i something different

Ib does NOT flow through Rb

When Rb changes in value, base current will NOT change

Through re, base current & collector current

If input voltage increases, output voltage decreases

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BJT, CE calculating

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Au x

x

i something

i something different

A

U

U

i R R

i ruout

in

b c load

b e

1

AR R

ruc load

e

There is NO Rb !!

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BJT, CE example

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BJT, CE example

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A

U

U

i R

i r

i R

i ruout

in

b c

b e

c c

e e

1

e

cu r

RA

NO Rb1 en Rb2 in the formula !!

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BJT, CE example

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e

cu r

RA

ee

cu Rr

RA

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BJT, impedence reflection page 171

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If you stand in base you see all resistors in the emitter (ß+1) magnified

If you stand in emitter you see all resistors in the base 1/(ß+1) magnified

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BJT, input / output resistance

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BJT, input / output resistance

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ee

cu Rr

RA

r R R r Rin b b e e 1 2 r Rout c

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BJT, source resistance

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Rs causes an attenuation in combination with the input resistance of the circuit.

Then the signal is amplificated !

out b out

s s b

u u u in cu u u

in s e

r R

r R r

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BJT, CE example

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BJT, CC example

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BJT, CC

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BJT, CC

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BJT

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BJT, CB

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BJT, CB

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