Electronic circuit chapt2_anaume.pdf

7/27/2019 Electronic circuit chapt2_anaume.pdf

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Dayang Nur Salmi Dharmiza


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Transistor is a VCCS (voltagecontrolled current source)

To cause changes in current, VBE is

changed.

Base or emitter terminals can be usedas input terminal

Collector is not used as an inputterminal because collector voltage hasnegligible effect on terminal currents

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Large changes in Ic or Iecreate large voltage dropsacross collector and emitterresistors

Collector or emitter can beused as ouput terminal

Base is not used as outputterminal due to small Ib

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Constraints for signal injection and extractionyield three families of amplifiers

Type of amplifier Input Output

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Steps for using small-signal models: Determine DC operating point of BJT

Decide the collector current

Calculate small-signal model parameters for this

DC operating point Eliminate DC sources

Replace DC voltage sources with short circuits

Replace DC current sources with open circuits

Replace BJT with an equivalent small-signal model Choose most convenient one depending on

surrounding circuitry

Analyze

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Redraw network by replacing capacitors with shortcircuit and dc voltage to ground.

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Substitute with re

model

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Zi: the figure clearly reveals that

= ohms

usually RB is greater than re. So,

RB>10 ohms

Zo: output impedance isdetermined when Vi=0.

= ohms

usually ro is greater than RC. So,

ro>10 ohms

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Av: resistors ro and RC are in parallel, and since

= = =-

=

, substitute into =

Thus, =

=

If 10, =

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Ai: apply current divider rule to the input and output circuits

Therefore,

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Since = , and =

,

so =

+and = -

(+ )

Replace in Ai above, then

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Negative sign in the resulting equation for Avreveals that a 180o phase shift occurs betweeninput and output signals

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Find: (a) re, with ro=, determine (b)Zi, (c)Zo, (d)Av, (e)Ai,

Repeat parts (c)-(e) including ro=50 in allcalculations and compare results

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Which is only slightly different due to the accuracycarried through the calculations

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Biased using four-resisternetwork (voltage divider)

AC input is injected to

base through a couplingcapacitor

AC output is taken fromcollector

Emitter is grounded

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All capacitors open circuits, disconnecting Vi, R1 and R3from circuit

Q-point can be found from dc equivalent circuit by using DCmodel for BJT

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Replace all capacitors by shortcircuits, inductors by opencircuits

Set all independent DC sources

to 0.

Replace transistor by small-signal model

Use small-signal AC equivalentto analyze AC characteristics ofamplifier


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Ifro>10 , ro>10 ohms

Av: Since RC and ro are in parallel,

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Ifro>10 ,

Since the network is so similar with fixed bias CEexcept for the fact that R=R1||R2=RB, theequation will have the same format

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Since = , and =

,

so =

+and = -

(+ )

Replace in Ai above, then

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Find: (a) re, (b)Zi with ro=, determine (c)Zo, (d)Av, (e)Ai,

Repeat parts (b)-(e) if ro=1/hoe=50k and compare results

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-10-

-10 mins break-

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CE bias configuration Emitter-follower configuration

CB configuration

Collector DC feedback configuration

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Apply Kirchhoffs law:

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Magnitude of BR is often too closeto Zb to permit the approximationIb=Ic. Applying current divider ruleto the input will result in

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The output is taken fromemitter terminal

Output voltage is always

slightly less than the inputsignal due to the dropfrom base to emitter

Voltage is in phase withinput signal follower

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Zi (same with CE emitter-bias):

Zo: Vi is set to 0V,

Ai: apply voltage divider rule

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Zi:

Zo:

Av:

Ai: assuming

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=

=

=

= 1


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Employs feedback path from collector to base toincrease stability

Quite difficult to analyze network

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Determine re, Zi, Zo, Av, Ai Determine again with ro=20k and compare

results.

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DC feedback resistor for increased stability Capacitor C3 shifts portions of the feedback

resistance to the input and output

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Determine:

Electronic circuit chapt2_anaume.pdf

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