Transcript common-base amplifier

Chapter 6
BJT Amplifiers
Objectives
 Understand the concept of amplifiers
 Identify and apply internal transistor parameters
 Understand and analyze common-emitter,
common-base, and common-collector amplifiers
 Discuss multistage amplifiers
 Troubleshoot amplifier circuits
Introduction
One of the primary uses of a transistor is to
amplify ac signals. This could be an audio
signal or perhaps some high frequency radio
signal. It has to be able to do this without
distorting the original input.
Amplifier Operation
Recall from the previous chapter that the purpose of
dc biasing was to establish the Q-point for operation.
The collector curves and load lines help us to relate
the Q-point and its proximity to cutoff and saturation.
The Q-point is best established where the signal
variations do not cause the transistor to go into
saturation or cutoff.
What we are most interested in is the ac signal itself.
Since the dc part of the overall signal is filtered out in
most cases, we can view a transistor circuit in terms
of just its ac component.
Amplifier Operation
For the analysis of transistor circuits from both dc and ac
perspectives, the ac subscripts are lower case and italicized.
Instantaneous values use both italicized lower case letters
and subscripts.
Amplifier Operation
The boundary between cutoff and saturation is called the
linear region. A transistor which operates in the linear
region is called a linear amplifier. Note that only the ac
component reaches the load because of the capacitive
coupling and that the output is 180º out of phase with
input.
Transistor Equivalent Circuits
We can view transistor circuits by use of resistance
or r parameters for better understanding. Since
the base resistance, rb is small it normally is not
considered and since the collector resistance, rc is
fairly high we consider it as an open. The emitter
resistance, rc is the main parameter that is viewed.
You can determine rc
from this simplified
equation.
rc = 25 mV/IE
Transistor Equivalent Circuits
The two graphs best illustrate the difference
between DC and ac. The two only differ slightly.
Transistor Equivalent Circuits
Since r parameters are used throughout the rest of the
textbook we will not go into deep discussion about h
parameters. However, since some data sheets include or
exclusively provide h parameters these formulas can be
used to convert them to r parameters.
r’e = hre/hoe
r’c = hre + 1/hoe
r’b = hie - (1+ hfe)
The Common-Emitter Amplifier
The common-emitter amplifier exhibits high voltage and
current gain. The output signal is 180º out of phase with the
input.
Now let’s use our dc and ac analysis methods to view this type
of transistor circuit.
The Common Emitter Amplifier
DC Analysis
The dc component of the
circuit “sees” only the part
of the circuit that is within
the boundaries of C1, C2,
and C3 as the dc will not
pass through these
components. The equivalent
circuit for dc analysis is
shown.
The methods for dc analysis
are just are the same as
dealing with a voltagedivider circuit.
Common Emitter Amplifier
AC Equivalent Circuit
The ac equivalent
circuit basically
replaces the
capacitors with
shorts, being that ac
passes through
easily through them.
The power supplies
are also effectively
shorts to ground for
ac analysis.
Common Emitter Amplifier
AC Equivalent Circuit
We can look at the input voltage in terms of the equivalent
base circuit (ignore the other components from the previous
diagram). Note the use of simple series-parallel analysis skills
for determining Vin.
Common Emitter Amplifier
AC Equivalent Circuit
The input resistance as seen by the input voltage
can be illustrated by the r parameter equivalent circuit.
The simplified formula below is used.
Rin(base) = acr’e
The output
resistance is
for all practical
purposes the
value of RC.
Common Emitter Amplifier
AC Equivalent Circuit
Voltage gain can be
easily determined by
dividing the ac output
voltage by the ac input
voltage.
Av = Vout/Vin = Vc/Vb
Voltage gain can also be
determined by the
simplified formula below.
Av = RC/r’e
Common Emitter Amplifier
AC Equivalent Circuit
Taking the attenuation
from the ac supply internal
resistance and input
resistance into consideration
is included in the overall
gain.
A’v = (Vb/Vs)Av
or
A’v = Rin(total)/Rs + Rin(total)
The Common-Emitter Amplifier
The emitter bypass capacitor helps increase the gain
by allowing the ac signal to pass more easily.
The XC(bypass) should be about ten times less than RE.
The Common-Emitter Amplifier
The bypass
capacitor makes the
gain unstable since
transistor amplifier
becomes more
dependent on IE.
This effect can be
swamped or
somewhat alleviated
by adding another
emitter resistor(RE1).
The Common-Collector Amplifier
The common-collector amplifier is usually referred
to as the emitter follower because there is no phase
inversion or voltage gain. The output is taken from the
emitter. The common-collector amplifier’s main
advantages are its high current gain and high input
resistance.
The Common-Collector Amplifier
Because of its high input
resistance the commoncollector amplifier used as a
buffer to reduce the loading
effect of low impedance
loads. The input resistance
can be determined by the
simplified formula below.
Rin(base)  ac(r’e + Re)
The Common-Collector Amplifier
The output resistance is very low. This makes
it useful for driving low impedance loads.
The current gain(Ai) is approximately ac.
The voltage gain is approximately 1.
The power gain is approximately equal to
the current gain(Ai).
The Common-Collector Amplifier
The darlington pair is
used to boost the input
impedance to reduce
loading of high output
impedance circuits. The
collectors are joined
together and the emitter
of the input transistor is
connected to the base of
the output transistor. The
input impedance can be
determined the formula
below.
Rin = ac1ac2Re
The Common-Base Amplifier
The common-base amplifier has high voltage gain with a
current gain no higher than 1. It has a low input resistance
making it ideal for low impedance input sources. The ac
signal is applied to the emitter and the output is taken from
the collector.
The Common-Base Amplifier
The common-base voltage gain(Av) is
approximately equal to Rc/r’e
The current gain is approximately 1.
The power gain is approximately equal to the voltage gain.
The input resistance is approximately equal to r’e.
The output resistance is approximately equal to RC.
Multistage Amplifiers
Two or more amplifiers can be connected to increase the
gain of an ac signal. The overall gain can be calculated by
simply multiplying each gain together.
A’v = Av1Av2Av3 ……
Multistage Amplifiers
Gain can be expressed in decibels(dB).
The formula below can be used to
express gain in decibels.
A v(dB) = 20logAv
Each stage’s gain can now can be
simply added together for the total.
Multistage Amplifiers
The capacitive coupling keeps dc bias voltages separate
but allows the ac to pass through to the next stage.
Multistage Amplifiers
The output of stage 1 is loaded by input of stage 2. This
lowers the gain of stage 1. This ac equivalent circuit helps
give a better understanding how loading can effect gain.
Multistage Amplifiers
Direct coupling between stage improves low frequency gain.
The disadvantage is that small changes in dc bias from
temperature changes or supply variations becomes more
pronounced.
Troubleshooting
Troubleshooting techniques for transistor
amplifiers is similar to techniques covered in
Chapter 2. Usage of knowledge of how an
amplifier works, symptoms, and signal tracing are
all valuable parts of troubleshooting. Needless to
say experience is an excellent teacher but having
a clear understanding of how these circuits work
makes the troubleshooting process more efficient
and understandable.
Troubleshooting
The following slide is a diagram for a two stage commonemitter amplifier with correct voltages at various points.
Utilize your knowledge of transistor amplifiers and
troubleshooting techniques and imagine what the effects
would be with various faulty components—for example, open
resistors, shorted transistor junctions or capacitors. More
importantly, how would the output be affected by these faults?
In troubleshooting it is most important to understand the
operation of a circuit.
What faults could cause low or no output?
What faults could cause a distorted output signal?
Troubleshooting
Summary
 Most transistors amplifiers are designed to operate
in the linear region.
 Transistor circuits can be view in terms of its ac equivalent
for better understanding.
 The common-emitter amplifier has high voltage and
current gain.
 The common-collector has a high current gain and
voltage gain of 1. It has a high input impedance and low
output impedance.
Summary
 The common-base has a high voltage gain and a
current gain of 1. It has a low input impedance and
high output impedance
 Multistage amplifiers are amplifier circuits cascaded to
increased gain. We can express gain in decibels (dB).
 Troubleshooting techniques used for individual
transistor circuits can be applied to multistage amplifiers
as well.