ELECTRONICS DEVICES AND CIRCUITS

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MODULE 2
TRANSISTORAMPLIFIERS
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 EMITTERTRANSISTORAMPLIFIER (SINGLE STAGE
TRANSISTORAMPLIFIER)
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 COLLECTORTRANSISTORAMPLIFIER

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COMMON BASE TRANSISTOR AMPLIFIER
TRANSISTORAS 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
TRANSISTORBIASING
• 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 corrector 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.EMITTERBIAS
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
Considerthe given bjt amplifier,
-Emitter Junction is forward biased using VBE (Battery)
-Base junction is reverse biased using power supply VCC through
RC
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.

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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.
CORRECTAMPLIFICATION 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. Thiscan be written as:
The Thermal Stability Factor: SIco

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