ELECTRONICS DEVICES AND CIRCUITS

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MODULE 2

TRANSISTOR AMPLIFIERS

Amplifier is a circuit that is used for amplifying a signal. The input

signal to an amplifier will be a current or voltage and the output will be an

amplified version of the input signal. An amplifier circuit which is purely based

on a transistor or transistors is called a transistor amplifier. Transistors

amplifiers are commonly used in applications like RF (radio frequency), audio,

OFC (optic fiber communication) etc. Anyway the most common application

we see in our day to day life is the usage of transistor as an audio amplifier. As

you know there are three transistor configurations that are used commonly i.e.

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

COMMON EMITTER TRANSISTOR AMPLIFIER (SINGLE STAGE TRANSISTOR AMPLIFIER)

CIRCUIT DIAGRAM

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INPUT CHARACTERISTICS

• Plot IB as f(VBE, VCE)

• As VCE increases, more VBE required to turn the BE on so that IB>0.

• Looks like a pn junction volt-ampere characteristic.

OUTPUT CHARACTERISTICS



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• Plot IC as f(VCE, IB)

• Cutoff region (off)

– both BE and BC reverse biased

• Active region

– BE Forward biased

– BC Reverse biased

• Saturation region (on)

– both BE and BC forward biased

COMMON COLLECTOR TRANSISTOR AMPLIFIER



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COMMON BASE TRANSISTOR AMPLIFIER

TRANSISTOR AS A SWITCH

• Transistor is a semiconductor device used for switching and amplification of weak signals.

CIRCUIT DIAGRAM



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OPERATION

• We applied a dc voltage to the collector of the transistor through resistor R.

• When an input is applied to the base of the transistor , transistor turns ON and current flows through the transistor and LED became OFF.

• If we remove the signal from base , transistor became in OFF position.

• Then current flows through LED and it turns ON.

BJT CIRCUITS AT DC

The BJT operation mode depends on the voltages at EBJ and BCJ.

The I-V characteristics are strongly nonlinear.

Simplified models and classifications are needed to speed up the

hand-calculation analysis.



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EQUIVALENT CIRCUIT OF NPN TRANSISTOR IN DIFFERENT MODES

TRANSISTOR BIASING

• Biasing simply means applying the relevant voltages and/or currents to make the transistor work the way we want it to.

• Transistors must be fed the correct or appropriate levels of voltages to their various regions in order to function properly and amplify signals to the correct level.

• This controlled amount of voltage fed to the different junctions of a transistor is called transistor biasing.

DIFFERENT TYPES OF BIASING



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1.FIXED BIASING

This is the most rarely used biasing method with transistor

amplifiers, but it is widely used when the transistor operates as a switch. The

base current IB is controlled by the base resistor RB. From the second law of

Kirchhoff, we have:

VCC = VB + VBE

VB = IB x RB

2. COLLECTOR TO BASE BIAS



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The next method that the researchers used to stabilize the Q point is the collector feedback bias. According to this method, the base resistor is not connected at the power supply, instead it is connected at the collector of the transistor

3. VOLTAGE DIVIDER BIAS



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The most effective method to bias the base of a transistor amplifier is using a voltage divider. The idea is that the voltage divider maintains a very stable voltage at the base of the transistor, and since the base current is many times smaller than the current through the divider, the base voltage remains practically unchanged. The resistor Re provides the negative feedback. The collector and emitter currents change just a few, and the Q point remains practically stable.

4.EMITTER BIAS

Emitter bias is similar to voltage divider bias. An additional capacitor is added in the emitter rejoin across the emitter resistor.



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SMALL SIGNAL OPERATION AND MODELS

Consider the given bjt amplifier,

Base-Emitter Junction is forward biased using VBE (Battery)

Collector-Base junction is reverse biased using power supply VCC through RC

The input signal is vbe

Consider only the dc bias condition by letting vbe=0 The voltage and current relations are given by



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THE HYBIRD π MODEL

This model represents the BJT as a voltage controlled current source and explicitly includes the input resistance looking into the base , rπ

THE T MODEL

The resistance between base and emitter is shown.



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LOAD LINE ANALYSIS

DC LOAD LINE

• Consider the common emitter amplifier circuit.

• DC load line is the graphical representation of VCE versus Ic .

• Where VCE =collector emitter voltage

Ic= collector current

Consider the basic equation;

Vcc =VCE + IcRc

The dc load line is a graph that represents all the possible combinations of IC and VCE for a given amplifier.



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The intersection of the dc bias value of IB with the dc load line determines the Q-point

• When a circuit is designed to have a centered Q-point, the amplifier is said to be midpoint biased.

• Midpoint biasing allows optimum ac operation of the amplifier.


Q-Point (Static Operation Point)

When a transistor does not have an ac input, it will have specific dc values of IC and VCE.

These values correspond to a specific point on the dc load line. This point is called the Q-point. The letter Q corresponds to the word (Latent) quiescent, meaning at rest. A quiescent amplifier is one that has no ac signal applied and therefore has constant dc values of IC and VCE.


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AC LOAD LINE

• The ac load line is used to tell you the maximum possible output voltage swing for a given common-emitter amplifier.

• In other words, the ac load line will tell you the maximum possible peak-to-peak output voltage (Vpp ) from a given amplifier.

• This maximum Vpp is referred to as the compliance of the amplifier.

CORRECT AMPLIFICATION METHOD


IC

VCE

Q - point

ac load line

dc load line


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BIAS STABILITY

Through proper biasing, a desired quiescent operating point of the transistor amplifier in the active region (linear region) of the characteristics is obtained. It is desired that once selected the operating point should remain stable. The maintenance of operating point stable is called Stabilisation.

The selection of a proper quiescent point generally depends on the following factors:

(a) The amplitude of the signal to be handled by the amplifier and distortion level in signal

(b)The load to which the amplifier is to work for a corresponding supply voltage

The operating point of a transistor amplifier shifts mainly with changes in temperature,

The Thermal Stability of Operating Point

Stability Factor S:- The stability factor S, as the change of collector current with respect to the reverse saturation current, keeping β and VBE constant. This can be written as:

The Thermal Stability Factor : SIco



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Engineering

ELECTRONICS DEVICES AND CIRCUITS