Resistor Capacitor coupled amplifier for electronics

DEPARTMENT OF
ELECTRONICS AND COMMUNICATION
ENGINEERING
Instructor
Mr. D V S RAMANJANEYULU
Assistant Professor
Accredited by NBA & NAAC with “A” Grade
CS301ES : ANALOG & DIGITAL ELECTRONICS

P.KIRAN KUMAR,ECE DEPARTMENT 2
BJTs: Transistor characteristics: The junction transistor,
transistor as an amplifier, CB, CE, CC configurations,
comparison of transistor configurations, the operating
point, self-bias or Emitter bias, bias compensation, thermal
runaway and stability, transistor at low frequencies, CE
amplifier response, gain bandwidth product, Emitter
follower, RC coupled amplifier, two cascaded CE and
multi stage CE amplifiers.
UNIT - II

Bipolar Junction transistor
Holes and electrons
determine device characteristics
Three terminal device
Control of two terminal currents

• 3 adjacent regions of doped Si
(each connected to a lead):
– Base. (thin layer,less doped).
– Collector.
– Emitter.
• 2 types of BJT:
– npn.
– pnp.
• Most common: npn
Bipolar Junction Transistor (BJT)
npn bipolar junction transistor
pnp bipolar junction transistor

Working Principal of Transistor
Transistor with no bias

Modes of operation of Transistor

current components transistor

Operation of NPN transistor

Operation of PNP transistor

Transistor Configuration
1. Common base configuration(CB)
2. Common emitter configuration(CE)
3. Common collector configuration(CC)

Common base configuration(CB)

Common-Base Configuration
• The common-base configuration with pnp and npn
transistors are shown in the figures in the previous slide..
• The term common-base is derived from the fact that the
base is common to both the input and output sides of the
configuration.
• The arrow in the symbol defines the direction of emitter
current through the device.
• The applied biasing are such as to establish current in
the direction indicated for each branch.
• That is, direction of IE is the same as the polarity of VEE
and IC to VCC .
• Also, the equation IE = IC + IB still holds.

Input characteristics
• The driving point or input
parameters are shown in the
figure.
• An input current (IE) is a function
of an input voltage (VBE) for
various of output voltage (VCB ).
• This closely resembles the
characteristics of a diode.
• In the dc mode, the levels of
IC and IE at the operation point
are related by:
αdc = IC /IE
•Normally, α 1.
•For practical devices, α is
typically from 0.9 to 0.998. Figure: Input characteristics for
common-base transistor

Output
characteristics
Figure: Output characteristics for common-base
transistor

Output characteristics
• The output set relates an output current (IC ) to an output voltage (VCB) for
various of level of input current (IE ).
There are three regions of interest:
Active region
• In the active region, the b-e junction is forward-biased, whereas the c-b junction
is reverse-biased.
• The active region is the region normally employed for linear amplifier. Also, in
this region,
I C  IE
Cutoff region
• The cutoff region is defined as that region where the collector current is 0A.
• In the cutoff region, the B-E and C-B junctions of a transistor are both reverse-
biased.
Saturation region:
• It is defined as that region of the characteristics to the left of VCB= 0 V.
• In saturation region, the B-E and C-B junctions of a transistor are both forward
biased.

Base Width Modulation: “Early” Effect
• When bias voltages change, depletion widths change and the
effective base width will be a function of the bias voltages
• Most of the effect comes from the C-B junction since the bias on
the collector is usually larger than that on the E- B junction
Base width gets smaller as applied voltages get
larger

The Early Effect
Range is -100V to -200 V
Converge ~ at single point called "Early Voltage" (after James Early)
Large "Early Voltage" = Absence of "Base Width Modulation"

common-emitter configurations
– Most common configuration
of transistor is as shown
– emitter terminal is common
to input and output circuits
this is a common-emitter
configuration
– we will look at the
characteristics of the device
in this configuration
– The current relations are still
applicable, i.e.,
– IE = IC + IB and IC =α IE
Figure: Common-emitter configuration of pnp
transistor
Figure: Common-emitter
configuration of npn
transistor

• Input
characteristics
– the input takes the form of a
forward- biased pn junction
– the input characteristics are
therefore similar to those of
a semiconductor diode
An input current (IB) is a
function of an input voltage
(VBE) for various of output
voltage (VCE ).

Cutoff Region
Figure: Output characteristics for common-
emitter transistor
Saturatio
n Region
Active
Region

• Output
characteristics
– The magnitude of IB is in μA and not as horizontal as IE
in common-base circuit.
– The output set relates an output current (IC) to an output voltage
(VCE) for various of level of input current (IB ).
• There are three portions as shown:
Active region
 The active region, located at upper-right quadrant, has
the greatest linearity.
 The curve for IB are nearly straight and equally spaced.
 In active region, the B-E junction is forward-biased,
whereas the C-B junction is reverse-biased.
 The active region can be employed for voltage, current or power
amplification.

Cutoff region
• The region below IB = 0μA is defined as cutoff
region.
• For linear amplification, cutoff region should be
avoided.
Saturation region:
• The small portion near the ordinate, is the saturation
region, which should be avoided for linear application.
• In the dc mode, the levels of IC and IB at the operation
point are related by: Normally,  ranges from 50 to
400.
dc = IC / IB
For ac situations,  is
defined as
IB
IC
V constan t
ac 
CE 

Common-Collector Configuration
• The common-collector configuration with npn
and pnp transistors are shown in the figures.
Figure: Common-collector
configuration of npn
transistor
Figure: Common- collector
configuration of pnp
transistor

• It is used primarily for impedance-matching
purpose since it has a high input impedance and
low output impedance.
• The load resistor can be connected from emitter to
ground.
• The collector is tied to ground and the circuit
resembles
common-emitter circuit.
• The output set relates an output current (IE) to an
output voltage (VCE) for various of level of input
current (IB ).
Common-Collector

• It is a curve whichshows the relationship between
the base current,IB and the collector base voltage VCB at
constant VCE This method of determining the characteristic is as
follows.
• First, a suitable voltage is applied between the emitter and the
collector.
• Next the input voltage
VCB
is increased in a number
of steps and corresponding values of IE are noted.
•The base current is taken on the y-axis, and the input voltage
is taken on the x-axis. Fig. shows the family of the input
characteristic at different collector- emitter voltages.

• This is almost the same as the output characteristics of
common-emitter circuit, which are the relations between
IC and VCE for various of level of input current IB.
Since that: IE  IC .
Figure: Output
characteristics
for common-
collector
transistor

Relation between

Comparison between CB CE CC

•The purpose of the common emitter amplifier is to provide good
current, voltage, and power gain.
• 180° phase shift
TRANSISTOR AMPLIFIERS
Common Emitter Amplifier

Common Emitter Amplifier
Components
R1 determines forward bias
R2 aids in developing bias
R3 is the collector load resistor
used to develop the output
signal
R4 is the emitter resistor used for
thermal stability
C1,c2 are blocking capacitors

Load line

Biasing of BJT
➢Biasing refers to the application of D.C. voltages to setup the operating
point in such a way that output signal is undistorted throughout the
whole operation.
➢Also once selected properly, the Q point should not shift because of
change of IC due to
(i)variation due to replacement of the transistor of same type
(ii)Temperature variation

Stabilization
The process of making operating point independent of
temperature changes or variation in transistor
parameters is known as stabilization.
➢Stabilization of operating point is necessary due to
❖Temperature dependence of IC
❖Individual variations
❖Thermal runaway
.
Stabilization

Stabilization
Temperature dependence of IC & Thermal runaway
IC  IB  ( 1)ICBO
➢ICBO is strong function of temperature. A rise of 10oC doubles the ICBO and IC
will increase ( +1) times of ICBO
➢The flow of IC produce heat within the transistor and raises the transistor
temperature further and therefore, further increase in ICBO
➢This effect is cumulative and in few seconds, the IC may become large
enough to burn out the transistor.
➢The self destruction of an unstablized transistor is known as thermal
runaway.

Stability Factor
➢The rate of change collector current IC with respect to the collector
leakage current ICBO is called stability factor, denoted by S.
Stability Factor
Lower the value of S, better is
the stability of the transistor.
S  ( C
)
dICBO
dI

Stability Factor
➢The rate of change collector current IC with respect to the collector
leakage current ICBO at constant and IB is called stability factor, denoted by S.
IC  IB  ( 1)ICBO (1)
Differentiating equation (1) w.r.t
IC
1)
dICBO
dI
1  (
B
) 
(
dIB
( 1)
1 
(
) 
d I
(
B
S 
(   1 )
Different biasing schemes
(i)Fixed bias (base resistor biasing)
(ii)Collector base bias
(iii)Emitter bias
(iv)Voltage divider bias
4
dIC
dIC
S
dIC
)
d I
C
1 

Fixed Bias
This form of biasing is also called base bias. The single power source is used for both collector and
base of transistor, although separate batteries can also be used.
5
Using KVL in the base-emitter loop VCC
– IBRB –VBE = 0 ; IB = (VCC-VBE)/RB
IC = IB= (VCC-VBE)/RB
Using KVL in the collector-emitter
loop VCC – ICRC –VCE = 0; VCE = VCC -
ICRC
Q(VCE,IC) is set

Fixed Bias
Advantages:
➢Operating point can be shifted easily anywhere in the active region by merely
changing the base resistor (RB).
➢A very small number of components are required.
Disadvantages:
➢Poor stabilization
➢High stability factor(S= +1 because IB is constant so dIB/dIC =0 ), hence prone
to thermal runaway
Usage:
➢Due to the above inherent drawbacks, fixed bias is rarely used in linear
circuits (i.e., those circuits which use the transistor as a current source).
Instead, it is often used in circuits where transistor is used as a switch.
How the Q point is affected by changes in VBE and ICBO in fixed bias?
IB = (VCC-VBE)/RB IC = IB

Collector base bias
➢This configuration employs negative feedback to prevent thermal runaway
and stabilize the operating point.
➢ In this form of biasing, the base resistor R is connected to the collector
instead
B
of connecting it to the DC source V
.
cc
So any thermal runaway will induce a voltage drop across the R C resistor that
will throttle the transistor's base current.
Applying KVL
VCC = (IC+IB)RC + VCE (1)
VCE
= IBRB + VBE (2)
Since, IC = IB so from equation (1) & (2)
C
4
3
B
 V BE
V CC
B
R  (1   )
R
I

Q(V ,I ) is
set
CE C

Collector base bias
• Advantages:
➢ Better stabilization compared to fixed bias
• Disadvantages:
➢ This circuit provides negative feedback which reduces the gain of the amplifier.
• Usage:
➢ The feedback also decreases the input impedance of the amplifier as seen
from the base, which can be advantageous. Due to the gain reduction from
feedback, this biasing form is used only when the trade-off for stability is
warranted.

Voltage Divider Bias
➢This is the most widely used method to provide biasing and stabilization to a
transistor.
➢In this form of biasing, R1 and R2 divide the supply voltage VCC and voltage
across R2 provide fixed bias voltage VB at the transistor base.
Also a resistance R E is included in series with the emitter that provides the
stabilization.
(R1  R
2 )
R
2
V 
V
CC
B
VB = Voltage across R2
(ignoring base current)
Voltage across R2
R
4
5
E
R
C
R1
R2

Small Signal Analysis
Small Signal Analysis of Amplifiers
• Small signal response is analyzed using the h-parameter model
• Response of an amplifier depends on frequency considerations.
• Frequency response curves of RC Coupled amplifier ,
•There are 3 regions of frequency : low , mid and high
•The difference between high and low frequency is the bandwidth

Hybrid h-Parameter model for an amplifier
• The equivalent circuit of a transistor can be dram using simple
approximation by retaining its essential features.
• These equivalent circuits will aid in analyzing transistor
circuits easily and rapidly.
• A transistor can be treated as a two part network. The
terminal behavior of any two part network can be specified by
the terminal voltages V1 & V2 at parts 1 & 2 respectively and
current i1 and i2, entering parts 1 & 2, respectively, as shown in
figure.

Two Port Network

Hybrid Parameters or h-parameters
If the input current i1 and output Voltage V2 are takes as
independent variables, the input voltage V1 and output current i2 can
be written as
V1 = h11 i1 + h12 V2
i2 = h21 i1 + h22 V2
The four hybrid parameters h11, h12, h21 and h22 are defined as
follows.
h11 = [V1 / i1] with V2 = 0
= Input Impedance with output part short circuited.

h22 = [i2 / V2] with i1 = 0
= Output admittance with input part open circuited.
h12 = [V1 / V2] with i1 = 0
= reverse voltage transfer ratio with input part open
circuited.
h21 = [i2 / i1] with V2 = 0
= Forward current gain with output part short circuited.

The dimensions of h – parameters are as follows:
h11 - Ω
h22 – mhos
h12, h21 – dimension less.
as the dimensions are not alike, (i.e) they are hybrid in
nature, and these parameters are called as hybrid parameters.

The Hybrid Model for Two-port Network:-
V1 = h11 i1 + h12 V2
I2 = h1 i1 + h22 V2
↓
V1 = hi i1 + hr V2
I2 = hf i1 + h0 V2

The Hybrid Model for Two-port Network:-

Transistor Hybrid Model
Use of h – parameters to describe a transistor have the following
advantages:
•h – parameters are real numbers up to radio frequencies .
•They are easy to measure
•They can be determined from the transistor static characteristics
curves.
•They are convenient to use in circuit analysis and design.
•Easily convert able from one configuration to other.
•Readily supplied by manufactories.

Transistor Hybrid Model CE Configuration
In common emitter transistor configuration, the input signal is
applied between the base and emitter terminals of the transistor and
output appears between the collector and emitter terminals. The
input voltage (Vbe) and the output current (ic) are given by the
following equations:
Vbe = hie.ib + hre.Vc
ie = hfe.ib + hoe.Vc

Hybrid Model and Equations for the transistor in three different configurations are
are given below.

CE amplifier response
•Frequency Response of an electric or electronics circuit
allows us to see exactly how the output gain and the phase
changes at a particular single frequency, or over a whole
range of different frequencies depending upon the design
characteristics of the circuit.
• Frequency response analysis of a circuit or system is
shown by plotting its gain against a frequency scale.
• The circuits gain, (or loss) at each frequency point helps
us to understand how well (or badly) the circuit can
distinguish between signals of different frequencies.

• There are many different ways for the
calculations of the frequency depending on the
combination of components.
• The -3dB frequency for resistance and
capacitance (the most common in amplifier
design) is determined by
fo = 1 / (2 Π R C)
• where fo is the -3dB frequency

Effect of Coupling Capacitors
• Coupling capacitors are in series with the signal and
are part of a high-pass filter network. They affect the
low-frequency response of the amplifier.

Effect of Coupling Capacitors
RC
+VCC
R2
Vin
R1
RE
RL
C
C
1
3
C2
Rin
Vin
C1
The equivalent circuit for C1 is a
high-pass filter:

Effect of Bypass Capacitors-contd…
• A bypass capacitor causes reduced gain at low-
frequencies and has a high-pass filter response. The
resistors “seen” by the bypass capacitor include RE,
re’, and the bias resistors.
RC
+VCC
R2
Vin
R1
RE
RL
C1
C3
2
C
RE
Vin
C2
|| r +
e
' R R R
( || || )
1 2 S
b

Effect of Internal capacitances
• The high-frequency response of an amplifier is
determined by internal junction capacitances.
These capacitances form low-pass filters with the
external resistors.
Cbc
Cbe
Cgd
Cgs

Decibel
• The decibel is a logarithmic ratio of two power
levels and is used in electronics work in gain or
attenuation measurements.
• Decibels can be expressed as a voltage ratio when
the voltages are measured in the same impedance.
• To express voltage gain in decibels, the formula is
• Av(dB) = 20 log Av

Typical Frequency response

Typical Frequency response
Gain is more commonly stated using a logarithmic scale, and the
result is expressed in decibels (dB). For voltage gain, this takes
the form
The upper and lower frequencies defining the bandwidth, called the
corner
or cutoff frequencies.

Bandwidth
The range of frequencies with
close to constant gain is known
as the bandwidth.

Bandwidth-contd…
• The bandwidth represents the amount or "width" of
frequencies, or the "band of frequencies," that the
amplifier is most effective in amplifying.
• The bandwidth is not the same as the band of
frequencies that is amplified. The bandwidth (BW) of
an amplifier is the difference between the frequency
limits of the amplifier.
BW= f2 - f1

Low frequency Response Of CE Amplifier- Input
coupling capacitor
RC
+VCC
R2
Vin
R1
RL
Vout
C1
C3
RE C2
Rin = R1 ||R2 ||Rin(base)
Vin
C1
Transistorbase
Vbase

Low frequency Response Of CE Amplifier- Output
coupling capacitor
• The output RC circuit is composed of the series
combination of the collector and load resistors with
the output capacitor. The cutoff frequency due to the
output circuit is
 
C L 3
1
2
c
f
R R C




single stage CE amplifier

Operation of Single stage CE amplifier
• The circuit diagram of a voltage amplifier using single transistor in
CE configuration is shown in figure. It is also known as a small-signal
single-stage CE amplifier or RC coupled CE amplifier. It is also
known as a voltage amplifier.
• The potential divider biasing is provided by resistors R1, R2 and RE.
• It provides good stabilization of the operating point. The capacitors
CC1 and CC2 are called the coupling capacitors used to block the AC
voltage signals at the input and the output sides.
• The capacitor CE works as a bypass capacitor. It bypasses all the AC
currents from the emitter to the ground and avoids the negative
current feedback. It increases the output AC voltage.
• The resistance RL represents the resistance of whatever is
connected at the output. It may be load resistance or input
resistance of the next stage

Two stage RC coupled amplifier

Advantages of RC coupled Amplifier
1. Wide frequency response
2. It is most convenient coupling
3. It is inexpensive way of coupling distortion in
output is low
4. It is high fidelity amplifier
5. No core distortion

Application
1. In Public Address amplifier system
2. Tape Recorder
3. TV, VCR and CD player
4. Stereo amplifiers
5. RC coupled amplifiers are basically voltage
amplifiers

Resistor Capacitor coupled amplifier for electronics

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