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Lab Report # 04

To construct a Common Emitter amplifier and calculate

its voltage gain with & without feedback

Theory:

  A common emitter is a type of electronic amplifier stage based on

a bipolar transistor in series with a load element such as a resistor. The

term "common emitter" refers to the fact that the emitter node of the

transistor indicated by an arrow symbol! is connected to a "common" power

rail typically the # volt reference or ground node. The collector node is

connected to the output load and the base node acts as input.

Explanation of circuit:

  The electronic circuit diagram given below shows

a common emitter configuration with voltage divider bias CE$%!. 'n the

figure the common emitter circuit comprises the load resistor (C and )*)

transistor with the output connected as shown+ the other circuit elements

are used for biasing the transistor and signal coupling,decoupling.

The resistor (E between the emitter node and the shared ground appears at

first glance to contradict the strict definition of "common emitter" but the

term is still appropriate here because for all fre-uencies of interest the

capacitor CE acts as low impedance by decoupling the emitter to ground. The

emitter resistor provides a form of negative feedback called emitter

degeneration  which increases the stability and linearity of the amplifier

especially in response to temperature changes.

or the common emitter circuit on the right this is necessary to ensure the

transistor is in the active mode and thus prevent it from acting as a

rectifier which would cause clipping on the negative portion of the input

signal resulting in a distorted output.

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The resistors (/ and (0 are chosen to ensure the base1emitter voltage is

appro2imately #.3 volts which is the "on" voltage for a 4T transistor.

These resistors along with (E also determine the -uiescent current flowing

through the transistor and therefore its gain.

eedback concept:

  'f some percentage of an amplifier5s output signal is

connected to the input so that the amplifier amplifies part of its own output

signal we have what is known as feedback . eedback comes in two varieties:

positive  also called regenerative ! and negative  also called degenerative !.

*ositive feedback reinforces the direction of an amplifier5s output voltagechange.

)egative feedback on the other hand has a "dampening" effect on an

amplifier: if the output signal happens to increase in magnitude the

feedback signal introduces a decreasing influence into the input of the

amplifier thus opposing the change in output signal. 6hile positive feedback

drives an amplifier circuit toward a point of instability oscillations!

negative feedback drives it the opposite direction: toward a point of

stability.

An amplifier circuit e-uipped with some amount of negative feedback

is not only more stable but it tends to distort the input waveform to a

lesser degree and is generally capable of amplifying a wider range of

fre-uencies. The tradeoff for these advantages there 7ust has  to be a

disadvantage to negative feedback right8! is decreased gain. 'f a portion of

an amplifier5s output signal is "fed back" to the input in such a way as to

oppose any changes in the output it will re-uire a greater input signal

amplitude to drive the amplifier5s output to the same amplitude as before.

This constitutes a decreased gain. 9owever the advantages of stabilitylower distortion and greater bandwidth are worth the tradeoff in reduced

gain for many applications.

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Circuits

6ithout feedback

6ith feedback

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*rocedure:  The circuit was set up as shown in the circuit diagram. The

/; resistor was connected to Collector. The emitter terminal was

connected to /##; resistor and on its way to the ground which provides a

feedback path to input. Two additional resistances of /#; were inserted

between the base terminals to produce the voltage divider bias. The

coupling capacitors were inserted as well to ensure the ma2imum gain. To

measure the output & input voltages $in & $out! an oscilloscope was connected

across the transistor the readings were reconfirmed using voltmeter

.inally after taking all precautions into account ' got up the readings and

calculated the voltage gain Av!.

The readings are plotted in the observation chart below:

 

<bservations & Calculations

$CC $! $') $! $<=T $! with

f,b

$<=T $!without f,b

A$ with

f,b

A$ without

f,b

/> /0m$ #./0 #.?/ 1/# 1@3.>

/> /#m$ #.#@@ #.? 1@.@ 1?/> #?m$ #.#@ #.@ 1? 1>3.>

/>   14mV 0.13 0.91 -9.2 -65

Applications:

 

Common emitter circuits are used to amplify weak voltagesignals such as the faint radio signals detected by an antenna. They are also

used in a special analog circuit configuration known as a current mirror

where a single shared input is used to drive a set of identical transistors

each of whose current drive output will be nearly identical to each other

even if they are driving dissimilar output loads.