TEE104 Unit 2 v2.pdffundamentalelectronicsfund

TEE104/03
FUND. OF
ELECTRONICS
BJT TRANSISTOR
CIRCUIT
Course Coordinator: Dr Arjuna Marzuki
Tutor: Dr Roshakimah Mohd Isa

BJT Transistor Circuit
OBJECTIVES
Explain various model parameters essential for
the amplifier circuit.
Differentiate between small-signal and large-
signal models.
Describe the common-emitter, common-base,
and common drain amplifier circuits and their
properties.
Explain the effect of coupling and bypass
capacitors on amplifier gain.

Objectives
Draw the Bode plot.
Explain the Miller effect and the need for proper
operating point.
Draw the frequency response curve.
Able to design different class of power amplifier.

Objectives
1
Explain hybrid-
parameter
models.
2
Draw a T-
equivalent
circuit.
3
Calculate the
amplifier’s
electrical
parameters.
4
Analyze and
measure the
performance of
amplifier
circuits.

Introduction
A model is a combination of specific operating conditions. Once the proper ac equivalent circuit has been
determined, the schematic symbol for the device can be replaced with this equivalent circuit and the
basic methods of circuit analysis is applied to determine the desired electrical parameters.
Hybrid-parameter models and T-equivalent circuit for amplifier circuits will be learnt.
Using these models to analyze and determine the various electrical parameters.
Understand one-port network and two-port network.

One port network
A port is defined as the pair of
terminals at which a signal
(Voltage/Current) can leave or enter.
If a network has only one such pair of
terminals, then it is called a one-port
network as shown in figure below.

The network is shown in the figure is used to describe the
relationship between a pair of terminals.
The left-hand side is the input port, while the right-hand side is
the output port. For the analysis of this network, the following
conditions must be satisfied.
Linearity.
No independent source inside the network.
No stored energy inside the network.
I1 = I1’ and I2 = I2’.
Two-port
network

Two-port network
If V1 and V2 are taken as independent variables and the
linear network contains no independent sources, the
independent and dependent variables are related by the
open-circuit impedance parameters (or simply, the Z-
parameters) Z11, Z12, Z21 and Z22 through the equation set:
V1 = Z11I1 + Z12I2
V2 = Z21I1 + Z22I2

Z-parameter
• A Two-port used for Z-parameter analysis is
shown in the figure.
• Z-parameters are called impedance
parameter, and these parameters are
measured as a complex function of frequency.

Z-parameter
• Each of the Z-parameters can be evaluated by setting the
proper current to zero (or equivalently, by open-circuiting an
appropriate port of the network).
• The Z-parameters are

Z-parameter

H-parameter
• In a similar manner, if V1 and I2 are taken as the
independent variables, a characterization of the two-port
network via the hybrid parameters (or simply, the h-
parameters) results in:
• V1 = h11I1 + h12V2
• I2 = h21I1 + h22V2
• Two of the h-parameters are determined by short-
circuiting port 2 while the remaining two parameters are
found by open-circuiting port 1.

H-parameter
• Two of the h-parameters are
determined by short-circuiting port 2
while the remaining two parameters are
found by open-circuiting port 1.

Hybrid-parameter models of
different amplifier circuits
• Actually, different sets of h-parameters are defined,
depending on which element of the transistor
(Emitter E, Base B or Collector C) shares a common
point with the amplifier input and output terminals.

Common-emitter (CE) transistor configuration
• In the common emitter (CE) transistor configuration, if iC and vBE are taken as dependent
variables, then:
• VBE = f1 (iB, vCE)
• iC = f2(iB ,vCE)

Common-emitter (CE)
transistor configuration
• The four partial derivatives, evaluated
at the Q (operating or quiescent) point,
that occur in the above equations are
called CE hybrid-parameters and are
denoted as follows

Common-emitter
(CE) transistor
configuration
• The equivalent circuit using h-parameters for CE amplifier is
shown in figure below.
• The circuit is valid for use with signals whose excursion about
the Q point is sufficiently small so that the h parameters may be
treated as constants.

Common-base (CB)
transistor
configuration
• If vEB and iC are taken as the dependent
variables for the CB transistor, then as in the
CE case, equations can be found specifically
for small excursions about the Q point.
• The results are:
veb = hibie + hrbvcb
ic = hfbie + hobvcb

Common-base (CB)
transistor
configuration
• The partial-derivative definitions of the CB h-
parameters are:

Common-base (CB)
transistor
configuration
• The small-signal h-parameter equivalent
circuit of CB amplifier circuit is shown in figure
below

Common-collector (CC)
transistor configuration
• The common-collector (CC) or emitter-
follower (EF) amplifier, with the
universal bias circuitry can be modeled
for small-signal ac analysis by replacing
the CE-connected transistor with its h-
parameter model.
• Assuming for simplicity that hre = hoe =
0, the equivalent circuit is shown in
figure below.

This model is mainly used to provide a more accurate model for high-frequency effects.
The hybrid  model of the small signal amplifier is shown below.
Hybrid  model

The T-equivalent circuit or re-parameter model is a circuit realization based on the Z-
parameters.
T-model of a common base small signal amplifier is shown below.
T-equivalent circuit

•A simplified CE amplifier circuit with bias arrangement is shown
in figure below and the associated small-signal equivalent circuit
is shown another figure below.
(a) Common emitter circuit and (b) it small signal model
Calculation of various electrical
parameters

Current gain
• Current gain of this amplifier circuit is
defined as (iL/ib). Consider only the
output part of the small-signal model.

Current gain
• The half-adder circuit takes two binary
digits on its inputs and produces two
binary digits on its outputs, a sum bit
and a carry bit.

Voltage gain
• Taking the input part of the small-signal model:
• Now apply KVL, we obtain

Voltage gain
• The base current and voltage gain are

Input impedance
• Input impedance is defined as

Output
impedance
• Output impedance can be determined by shorting the
voltage source shown in figure below and replace the load
resistance RL by test voltage source of value Vdp so that Vdp =
vce.

Output impedance

Analysis of Small-Signal
Amplifiers

Objectives
Describe various configuration types of small signal amplifiers, which are common-
emitter (CE), common-base (CB), and common-collector (CC) configurations.
Establish the relationship between the various currents in CE amplifiers.
Explain the bias conditions.
Analysis and draw the dc and ac load lines.
Explain the role of various capacitances in ac and dc analysis.

Introduction
A bipolar junction transistor (BJT) has three terminals connected to input and
output circuit loops.
A transistor amplifier is a two-port network; one of the three transistor terminals
must be shared by both input and output ports as the common terminal.
This results in three possibilities, namely, common-emitter (CE) amplifier,
common-base (CB) amplifier and common-collector (CC) amplifier circuits.
In electronics, a common-emitter amplifier circuit is generally used.

Introduction
It is one of the three basic single-stage BJT amplifier topologies, typically used as
a voltage amplifier.
The CE amplifier exhibits high voltage gain and high current gain.
The voltage gain of the common-collector (emitter follower) amplifier is
approximately unity and high output current.
The common-base (CB) amplifier provides high voltage gain with a maximum
current gain of unity.

Common Emitter
Amplifier
• A common emitter amplifier is
shown in figure below.
• Boolean expression can be
expressed by the product-of-sum
terms.

dc Bias and
dc load lines
The BJT is an excellent amplifier when biased in the
forward-active region. dc bias is provided to stabilize
the operating point in the desired operation region.
The dc Q-point also determines:
1. The small-signal parameters of the transistor.
2. The voltage gain, input resistance and output
resistance.
3. The maximum input and output signal amplitudes.
4. The overall power consumption of the amplifier.

dc Bias and dc load lines
The operating point is
determined by drawing a load
line on the output
characteristics of the CE
amplifier circuit.
To draw the load line, we have
to find the saturation current
and cut-off voltage.
The load line is drawn by joining
these two points.
The dc parameter of the
amplifier can be determined
the dc circuit shown below.

dc Bias and dc load
lines
The dc circuit of CE amplifier is

dc Analysis
The dc
parameters
are

ac Analysis
The ac circuit of CE amplifier is shown below.

ac analysis
• The ac parameters of CE amplifier are

ac and dc load lines
The combined ac and dc load lines are as follow.

The voltage gain of the CE amplifier circuit is analyzed from the ac
equivalent small-signal model as shown in figure below.
Voltage gain analysis of
CE amplifier

Voltage
gain
The voltage gain is
determined from the
following equations

Common
collector
amplifier
• Common-collector
amplifier is also known as
emitter follower because it
has practically closed to
unity gain and its input
impedance is high which can
be used as buffer to reduce
loading effect. Consider the
common-collector amplifier
shown in figures below.

Input/Output
impedance of
common
collector
amplifier

Voltage gain of
common collector
amplifier
• The ac voltage gain is

Common Base Amplifier
Common-base amplifier is shown in figures below.

•The input impedance of the emitter is
Rin(emitter) = re||RE =
•The output impedance is Rout = RC.
( )
r RE
 
/ ( ) ||
+1
Input/Output impedance of
Common Base Amplifier

Voltage gain of
common base
amplifier
• The input voltage Vin is
Vin = Ie(re||RE)
• The output voltage Vout is
Vout = -IeRC
• ac voltage gain (AV) of the
amplifier is
AV = -RC/(re||RE)

Frequency Response

Objectives
• Explain the Miller effect.
• Use Miller’s concept in the
analysis of amplifier circuit.
• Draw Bode plots.
• Draw the lower and higher
frequency response of the
amplifier circuit.
• Explain the complete frequency
response of the amplifier circuit.
• Determine the lower and higher
cut-off frequencies.

Investigation of the frequency
effects introduced by the larger
capacitive elements of the
network at low frequencies and
smaller capacitive elements of
the active device at high
frequencies will be studied.
A brief review of s-domain
analysis and study of amplifier
transfer functions, the low-
capacitively coupled CE amplifier
will be discussed.
We will also see the high
amplifier circuit.
To draw the amplifier response,
Bode plots will be studied.
The Miller effect and its
importance in the amplifier’s
analysis will be discussed in
detail.
Introduction

Miller effect
Miller’s Theorem is used to simplify the analysis of the inverting
amplifiers at high frequencies where the parasitic capacitances
are important.
In analog electronics, the Miller effect accounts for an
increase in the equivalent input capacitance of an
inverting voltage amplifier due to the amplification of
capacitance between the input and output terminals. In
a BJT, Cbc is the capacitance between the input (base)
node and output (collector) node.

Miller effect
In an FET, this corresponding capacitor is Cgd (in between
gate and drain).
Miller effect normally refers to capacitance, any
impedance connected between the input and
another node exhibiting high gain can modify
the amplifier input impedance via the Miller
effect.

• Miller’s Theorem states that capacitance C, connected in between input and
output nodes, effectively appears as a capacitance from input to ground.
• The input capacitance is given by Cin(Miller) = CinM = C(1 + Av).
• The output capacitance is given by
Miller effect

• The circuit used to illustrate Miller theorem is shown below.
Miller effect

Bode plot
Bode plots are the most widely used means of displaying and
communicating frequency response information.
A Bode plot is usually a combination of a Bode magnitude plot and a Bode
phase plot.
A Bode magnitude plot is a graph of log magnitude versus frequency in
which the radial frequency is plotted along the X-axis and the gain of the
circuit (in decibel) at that frequency is plotted on the Y-axis.

Bode plot
The maximum difference is 3 dB and occurs at
corner frequencies. To correct a straight-line
amplitude plot:
1. At every zero, put a point 3 dB above the
line.
2. At every pole, put a point 3 dB below the
line.
3. Draw a smooth line through those points
using the straight lines as asymptotes (lines
which the curve approaches).

•The Bode phase plot is a graph where the radial frequency is
plotted along the X-axis and phase shift of the circuit at that
frequency is plotted on the Y-axis.
•The phase shift is almost always represented in terms of
radians. Likewise, the frequency axis may be in units of hertz
or radians per second. So, the axes must be labeled correctly.
Bode plot

•Let’s consider the common- emitter amplifier circuit as shown
in figure below.
Low/high frequency response

In the circuit, there are three external capacitors, C1, CC and CE. These
capacitors are known as input capacitor, coupling capacitor and bypass
capacitor respectively.
At low frequencies, coupling capacitors (Cs, CC) and bypass capacitors (CE) will
have capacitive reactance (XC) that affects the circuit impedances.
These three capacitors will determine the low frequency response of the
amplifier circuit. The analysis of the low-frequency response of the common-
emitter amplifier is complicated by the fact that the BJT has a finite .

Low/high frequency
response
• The parasitic capacitors associated with bipolar transistors are Cbe,
Cbc and Cce, and wiring capacitors. The capacitance that is inherent
within the transistor, amplifier current and voltage gain decreases in
magnitude as the frequency of the input signal increases beyond
the mid-frequency range.
• At high frequency, capacitors C1, Cc and CE are assumed to be in the
short-circuit state. The high-frequency cut-off point H is the
frequency at which the ratio is equal to square root 2 multiplied by
the mid-frequency gain, or at which gain in dB decreased by 3dB
from its maximum gain.

•The range of frequencies above H is called the high-frequency
region. H is called higher cut-off frequency, corner frequency or
break point.

•All amplifiers typically exhibit a band-pass frequency response
as in figure.
Complete frequency response of
amplifier circuit

•The cut-off frequency on the low end is usually determined by
the coupling and bypass capacitors (if there are no such
capacitors, the low end extends all of the way to dc).
•The high frequency limit is typically determined by internal
capacitances in the transistor itself.
•The bandwidth is the difference between the high and low
corner frequencies, (f2 – f1).
Complete frequency response
of amplifier circuit

•As the signal frequency drops below mid-band, the impedances
of the coupling capacitors C1 and CC and emitter bypass capacitor
CE increase.
•The coupling capacitors drop more signal voltage and the
emitter bypass capacitor begins to open up, causing increased
series-series feedback. This results in a reduction of the gain.
•The internal capacitances of a transistor, having values in the
picofarad (pF) range, will begin to decrease the gain of the
amplifier for frequencies above mid-band.
Complete frequency response
of amplifier circuit

•The impedance of coupling capacitors increases as frequency
decreases, therefore, the voltage gain of a BJT amplifier
decreases as frequency decreases.
•At very low frequencies, the capacitive reactance of the
coupling capacitors may become large enough to drop some
amount of the input or output voltage.
•Also, the emitter bypass capacitor may become large enough so
that it no longer shorts the emitter resistor to ground,
decreasing the amplifier voltage gain.
Lower cut-off frequencies

•At low frequencies, coupling capacitors (C1, CL) and bypass
capacitors (CE) will have capacitive reactance (XC) that affects the
circuit’s gain.
•Capacitor C1 is normally connected in between the applied
source and the active device.
•The general form of the RC configuration is established by the
network as shown in figure.

•The lower cutoff frequency due to coupling capacitor C1, where
the voltage gain drops by 3dB from the mid-band value or 0.707
multiplied by the mid-band Av.

•The coupling capacitor is connected between the output of the
active device and load. The RC network for considering the effect
of CC is shown in figure below.
•The cut-off frequency due to CC is given by

•The lower cutoff frequency due to coupling capacitor CE is given
by

Higher cut-off frequencies
•Figure below shows the high-frequency equivalent model for
the network

•High cut-off frequency at the input side is
•At very high frequencies, the effect of Ci is to reduce the total
impedance of the parallel combination of R1, R2, Ri and Ci. This
results in a reduced voltage drop across Ci and a reduction in
gain.

•High cut-off frequency at the output is

TEE104 Unit 2 v2.pdffundamentalelectronicsfund

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