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1. Chapter 6
BJT Amplifiers

2. Objectives
 Understand the concept of amplifiers
 Identify and apply internal transistor parameters
 Understand and analyze common-emitter

3. 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.

4. 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.

5. 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.

6. 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.

7. 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. (Appendix
B)
re = 25 mV/IE

8. Transistor Equivalent Circuits
The two graphs best illustrate the difference
between DC and ac. The two only differ slightly.

9. 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 – hre/hoe(1+ hfe)

10. 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.

11. 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 voltage-
divider circuit.

12. 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.

13. 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.
S
inS
in
in V
RR
R

V

14. 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
Example 6-3
The output
resistance is
for all practical
purposes the
value of
Rc=RC||RL.

15. 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

16. Common Emitter Amplifier
AC Equivalent Circuit
v
totalins
totalin
v A
RR
R
)(
)('
A


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
inS
S
S
S
C
i
RR
V
I
I
I

 ,A ivp AAA

17. Summary
 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.
 Most transistors amplifiers are designed to operate
in the linear region.

18. 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