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EE141© 2010, Доц.д-р. T.Василева

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Bipolar Junction Transistor

Amplifiers

Semiconductor

Elements


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What is an Amplifier?

An amplifier is a circuit that can increase the peak-to-peak voltage, current, or

power of a signal. It allows a small signal to control a much larger, high-powered

one. Definitions of voltage, current and power gain coefficients are also given in

figure. Lowercase italic letters indicate ac voltage and alternating currents.

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Amplifier Configurations

There are three configurations of a BJT amplifier circuit: common-

emitter (CE), common-collector (CC) and common-base (CB). The

configuration is named for the electrode that is common for input and

output networks. The CE is the most widely used for amplifiers because

it has the best combination of current gain and voltage gain. In CE the

input and output voltage are 180° out of phase, called an inversion.


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Transistor Biasing

For the transistor to operate properly as an amplifier, the base-emitter junction

should be forward-biased and the base-collector junction – reverse-biased. This is

called forward-reverse bias. The three dc voltages for the biased transistor are

the emitter voltage UE, the collector voltage UC and the base voltage UB. These

voltages are measured with respect to ground.

Circuit with fixed

base current

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Voltage-Divider Biasing (VDB)

The voltage-divider bias (VDB) configuration uses only a single dc source

to provide forward-reverse bias to the transistor. Resistors R1 and R2 form a

voltage divider that provides the base bias voltage UB. Resistor RE allows

the emitter to rise above the ground potential.

Circuit with fixed base

voltage.

Voltage

divider

Iдел >> IB


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Examples – Biasing

IC , UCE = ? IC, UCE = ?UB, IB, IC, UCE = ?

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DC Load Line

A straight line drawn on the collector curves between the cutoff and saturation

point of a transistor is called the load line. Notice that the load line is only

determined by the resistors RC, RE and UCC and not by the transistor itself.

CEECCC URRIE )( Load line equation


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The slope and position of the load line depends only on collector circuit resistance

RC and supply voltage UCC and not on the transistor itself.

When UCC varies with constant RC value the load line moves parallel to itself.

Changing the collector resistor with the same collector supply voltage produces

load lines of different slopes but the same cutoff value.

Load Line – Slope and Position

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Operating Point

The base current IB is established by the base bias. The intersection point

between the collector current curve (at this IB) and the dc load line is called the

quiescent or Q-point or operating point. Coordinates of the Q-point are the

values IBQ, ICQ, UCEQ.

The load line contains every possible operation point for the circuit.


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AC Operationdc base voltage UB = const ac signal voltage ub Instant value uB = UB + ub

Coupling

capacitor

A coupling capacitor allows an ac signal to be coupled into an amplifier without

disturbing its Q point. The capacitor acts as an open to dc and as a short to ac.

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Signal Operation on the Load Line

The input signal varies the base current above and below its dc value. This

causes much larger variation in the collector current because of the

transistor current gain. The variation in collector current produces a

corresponding variation in the voltage across RC. As a result, the collector-

emitter voltage UCE also changes.


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Waveforms & DC Levels

Input and output signal waveforms

An input signal with an amplitude of 50 mV is applied to the CE amplifier.

The output signal is with amplitude of 500 mV. The amplifier voltage gain

Au = 10.

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Two Load Lines – DC & AC

Every amplifier has a dc equivalent circuit and an ac equivalent circuit. Because

of this, it has two load lines: a dc load line and an ac load line. The ac collector

resistance rC is less than the dc collector resistance RC. For this reason the ac

load line has a higher slope than the dc load line. The peak-to-peak sinusoidal

current and voltage are determined by the ac load line.


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Clipping Large Signals

An amplifier is a class A amplifier if it is biased such that it always operates in

the linear region where the shape of the output signal is an amplified copy of

the input signal.

If the Q point is not centered, the output signal is limited. If the Q-point is

moved higher or lower, a large signal will drive the transistor into saturation or

cutoff. Thus saturation or cutoff clipping occurs. Both types of clipping are

undesirable because they distort the signal.

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CE, CB and CC Amplifier Circuits

RTRT RT

Amplifier CЕ Amplifier CB Amplifier CC

AI – high

AU - high

AI < 1

AU - highAI - high

AU < 1

what is an amplifier? - lark.tu-sofia.bg · pdf filewhat is an amplifier? ... (cc) and...

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