Module 8 - Introduction to Amplifiers
Pages i - ix,
1-1 to 1-10,
1-11 to 1-20,
1-21 to 1-30,
1-31 to 1-40,
2-1 to 2-10,
2-11 to 2-20,
2-21 to 2-30,
2-31 to 2-35,
3-1 to 3-10,
3-11 to 3-20,
3-21 to 3-30,
3-31 to 3-40,
3-41 to 3-50,
3-51 to 3-60,
3-61 to 3-70,
AI-1 to AI-3,
Figure 1-9. - Direct-coupled transistor amplifiers.
Notice that the output (collector) of Q1 is connected directly to the input (base) of Q2. The network of R4,
R5, and R6 is a voltage divider used to provide the bias and operating voltages for Q1 and Q2. The entire circuit
provides two stages of amplification.
Direct coupling provides a good frequency response since no
frequency-sensitive components (inductors and capacitors) are used. The frequency response of a circuit using
direct coupling is affected only by the amplifying device itself.
Direct coupling has several
disadvantages, however. The major problem is the power supply requirements for direct-coupled amplifiers. Each
succeeding stage requires a higher voltage. The load and voltage divider resistors use a large amount of power and
the biasing can become very complicated. In addition, it is difficult to match the impedance from stage to stage
with direct coupling. (Impedance matching is covered a little later in this chapter.)
amplifier is not very efficient and the losses increase as the number of stages increase. Because of the
disadvantages, direct coupling is not used very often.
The most commonly used coupling in amplifiers is RC coupling. An RC-coupling
network is shown in figure 1-10.
Figure 1-10. - RC-coupled transistor amplifier.
The network of R1, R2, and C1 enclosed in the dashed lines of the figure is the coupling network. You
may notice that the circuitry for Q1 and Q2 is incomplete. That is intentional so that you can concentrate on the
R1 acts as a load resistor for Q1 (the first stage) and develops the output signal of
that stage. Do you remember how a capacitor reacts to ac and dc? The capacitor, C1, "blocks" the dc of Q1's
collector, but "passes" the ac output signal. R2 develops this passed, or coupled, signal as the input signal to
Q2 (the second stage). This arrangement allows the coupling of the signal while it isolates the biasing of each
stage. This solves many of the problems associated with direct coupling.
RC coupling does have a few
disadvantages. The resistors use dc power and so the amplifier has low efficiency. The capacitor tends to limit
the low-frequency response of the amplifier and the amplifying device itself limits the high-frequency response.
For audio amplifiers this is usually not a problem; techniques for overcoming these frequency limitations will be
covered later in this module.
Before you move on to the next type of coupling, consider the capacitor in
the RC coupling. You probably remember that capacitive reactance (X C) is determined by the following formula:
This explains why the low frequencies are limited by the capacitor. As frequency decreases, XC increases. This
causes more of the signal to be "lost" in the capacitor.
The formula for XC also shows that the value of
capacitance (C) should be relatively high so that capacitive reactance (XC) can be kept as low as possible. So,
when a capacitor is used as a coupling element, the capacitance should be relatively high so that it will couple
the entire signal well and not reduce or distort the signal.
Impedance coupling is very similar to RC coupling. The difference is the use of an impedance device
(a coil) to
replace the load resistor of the first stage.
Figure 1-11 shows an impedance-coupling network between two stages of amplification. L1 is the load
for Q1 and develops the output signal of the first stage. Since the d.c. resistance of a coil is low, the
efficiency of the amplifier stage is increased. The amount of signal developed in the output of the stage depends
on the inductive reactance of L1. Remember the formula for inductive reactance:
Figure 1-11. - Impedance-coupled transistor amplifier.
The formula shows that for inductive reactance to be large, either inductance or frequency or both must
be high. Therefore, load inductors should have relatively large amounts of inductance and are most effective at
high frequencies. This explains why impedance coupling is usually not used for audio amplifiers.
of the coupling network (C1 and R1) functions just as their counterparts (C1 and R2) in the RC-coupling network.
C1 couples the signal between stages while blocking the d.c. and R1 develops the input signal to the second stage
Figure 1-12 shows a transformer-coupling network between two
stages of amplification. The transformer action of T1 couples the signal from the first stage to the second stage.
In figure 1-12, the primary of T1 acts as the load for the first stage (Q1) and the secondary of T1 acts as the
developing impedance for the second stage (Q2). No capacitor is needed because transformer action couples the
signal between the primary and secondary of T1.
Figure 1-12. - Transformer-coupled transistor amplifier.
The inductors that make up the primary and secondary of the transformer have very little dc resistance, so the
efficiency of the amplifiers is very high. Transformer coupling is very often used for the final output (between
the final amplifier stage and the output device) because of the impedance-matching qualities of the transformer.
The frequency response of transformer-coupled amplifiers is limited by the inductive reactance of the transformer
just as it was limited in impedance coupling.
Q-12. What is the purpose of an amplifier-coupling
network? Q-13. What are four methods of coupling amplifier stages?
Q-14. What is the most common form of
Q-15. What type coupling is usually used to couple the output from a power amplifier?
type coupling would be most useful for an audio amplifier between the first and second stages?
What type of coupling is most effective at high frequencies?
IMPEDANCE CONSIDERATIONS FOR AMPLIFIERS
It has been mentioned that efficiency and
impedance are important in amplifiers. The reasons for this may not be too clear. You have been shown that any
amplifier is a current-control device. Now there are two other principles you should try to keep in mind. First,
there is no such thing as "something for nothing" in electronics. That means every time you do something to a
signal it costs something. It might mean a loss in fidelity to get high power. Some other compromise might also be
made when a circuit is designed. Regardless of the compromise, every stage will require and use power. This brings
up the second principle-do things as efficiently as possible. The improvement and design of electronic circuits is
an attempt to do things as cheaply as possible, in terms of power, when all the other requirements (fidelity,
power output, frequency range, etc.) have been met.
This brings us to efficiency. The most efficient
device is the one that does the job with the least loss of power. One of the largest losses of power is caused by
impedance differences between the output of one circuit and the input of the next circuit. Perhaps the best way to
think of an impedance difference (mismatch) between circuits is to think of different-sized water pipes. If you
try to connect a one-inch water pipe to a two-inch water pipe without an adapter you will lose water. You must use
an adapter. An
impedance-matching device is like that adapter. It allows the connection of two devices with different
impedances without the loss of power.
Figure 1-13 shows two circuits connected together. Circuit number 1
can be considered as an a.c. source (ES) whose output impedance is represented by a resistor (R1). It can be
considered as an a.c. source because the output signal is an a.c. voltage and comes from circuit number 1 through
the output impedance. The input impedance of circuit number 2 is represented by a resistor in series with the
source. The resistance is shown as variable to show what will happen as the input impedance of circuit number 2 is
Figure 1-13. - Effect of impedance matching in the coupling of two circuits.
The chart below the circuit shows the effect of a change in the input impedance of circuit number 2 (R2)
on current (I), signal voltage developed at the input of circuit number 2 (ER2), the power at the output of
circuit number 1 (PR1), and the power at the input to circuit number 2 (PR2).
Two important facts are
brought out in this chart. First, the power at the input to circuit number 2 is greatest when the impedances are
equal (matched). The power is also equal at the output of circuit number 1 and the input of circuit number 2 when
the impedance is matched. The second fact is that the largest voltage signal is developed at the input to circuit
number 2 when its input impedance is much larger than the output impedance of circuit number 1. However, the power
at the input of circuit number 2
is very low under these conditions. So you must decide what conditions you want in coupling two
circuits together and select the components appropriately.
Two important points to remember about
impedance matching are as follows. (1) Maximum power transfer requires matched impedance. (2) To get maximum
voltage at the input of a circuit requires an intentional impedance mismatch with the circuit that is providing
the input signal.
Impedance Characteristics of Amplifier Configurations
Now that you have seen the
importance of impedance matching the stages in an electronic device, you may wonder what impedance characteristics
an amplifier has. The input and output impedances of a transistor amplifier depend upon the configuration of the
transistor. In Module 7, Introduction to
Solid-State Devices and Power Supplies, you were introduced to the
three transistor configurations; the common emitter, the common base, and the common collector. Examples of these
configurations and their impedance characteristics are shown in figure 1-14.
Figure 1-14. - Transistor amplifier configurations and their impedance characteristics.
NOTE: Only approximate impedance values are shown. This is because the exact impedance values will vary
from circuit to circuit. The impedance of any particular circuit depends upon the device (transistor) and the
other circuit components. The value of impedance can be computed by dividing the signal voltage by the signal
The common-emitter configuration provides a medium input impedance and a medium output impedance. The
common-base configuration provides a low input impedance and a high output impedance. The common-collector
configuration provides a high input impedance and a low output impedance. The common-collector configuration is
often used to provide impedance matching between a high output impedance and a low input impedance.
amplifier stage is transformer coupled, the turns ratio of the transformer can be selected to provide impedance
matching. In NEETS Module 2, Introduction to Alternating Current and Transformers, you were shown the relationship
between the turns ratio and the impedance ratio in a transformer. The relationship is expressed in the following
As you can see, impedance matching between stages can be accomplished by a combination of the amplifier
configuration and the components used in the amplifier circuit.
Q-18. What impedance relationship
between the output of one circuit and the input of another circuit will provide the maximum power transfer?
Q-19. If maximum current is desired at the input to a circuit, should the input impedance of that circuit be lower
than, equal to, or higher than the output impedance of the previous stage?
Q-20. What are the input- and
output-impedance characteristics of the three transistor configurations?
& Q-21. What transistor circuit
configuration should be used to match a high output impedance to a low input impedance?
Q-22. What type
of coupling is most useful for impedance matching?
Perhaps you have been around a public address system when
a squeal or high-pitched noise has come from the speaker. Someone will turn down the volume and the noise will
stop. That noise is an indication that the amplifier (at least one stage of amplification) has begun oscillating.
Oscillation is covered in detail in NEETS Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits. For
now, you need only realize that the oscillation is caused by a small part of the signal from the amplifier output
being sent back to the input of the amplifier. This signal is amplified and again sent back to the input where it
is amplified again. This process continues and the result is a loud noise out of the speaker. The process of
sending part of the output signal of an amplifier back to the input of the amplifier is called FEEDBACK.
There are two types of feedback in amplifiers. They are POSITIVE FEEDBACK, also called REGENERATIVE FEEDBACK, and
NEGATIVE FEEDBACK, also called DEGENERATIVE FEEDBACK. The difference between these two types is whether the
feedback signal is in phase or out of phase with the input signal.
Positive feedback occurs when the
feedback signal is in phase with the input signal. Figure 1-15 shows a block diagram of an amplifier with positive
feedback. Notice that the feedback signal is in phase with the input signal. This means that the feedback signal
will add to or "regenerate" the input signal. The result is a larger amplitude output signal than would occur
without the feedback. This type of feedback is what causes the public address system to squeal as described above.
Figure 1-15. - Positive feedback in an amplifier.
Figure 1-16 is a block diagram of an amplifier with negative feedback. In this case, the feedback signal
is out of phase with the input signal. This means that the feedback signal will subtract from or "degenerate" the
input signal. This results in a lower amplitude output signal than would occur without the feedback.
Figure 1-16. - Negative feedback in an amplifier.
Sometimes feedback that is not desired occurs in an amplifier. This happens at high frequencies and limits the
high-frequency response of an amplifier. Unwanted feedback also occurs as the result of some circuit components
used in the biasing or coupling network. The usual solution to unwanted feedback is a feedback network of the
opposite type. For example, a positive feedback network would counteract unwanted, negative feedback.
Feedback is also used to get the ideal input signal. Normally, the maximum output signal is desired from an
amplifier. The amount of the output signal from an amplifier is dependent on the amount of the input signal.
However, if the input signal is too large, the amplifying device will be saturated and/or cut off during part of
the input signal. This causes the output signal to be distorted and reduces the fidelity of the amplifier.
Amplifiers must provide the proper balance of gain and fidelity.
Figure 1-17 shows the way in which
feedback can be used to provide the maximum output signal without a loss in fidelity. In view A, an amplifier has
good fidelity, but less gain than it could have. By adding some positive feedback, as in view B, the gain of the
stage is increased. In view C, an amplifier has so much gain and such a large input signal that the output signal
is distorted. This distortion is caused by the amplifying device becoming saturated and cutoff. By adding a
negative feedback system, as in view D, the gain of the stage is decreased and the fidelity of the output signal
Figure 1-17A.-Feedback uses in amplifiers.
Figure 1-17B. - Feedback uses in amplifiers.
Figure 1-17C. - Feedback uses in amplifiers.
Figure 1-17D. - Feedback uses in amplifiers.
Positive and negative feedback are accomplished in many ways, depending on the reasons requiring the
feedback. A few of the effects and methods of accomplishing feedback are presented next.
As you have seen, positive feedback is accomplished by adding part of the output signal
in phase with the input signal. In a common-base transistor amplifier, it is fairly simple to provide positive
feedback. Since the input and output signals are in phase, you need only couple part of the output signal back to
the input. This is shown in figure 1-18.
NEETS Table of Contents
- Introduction to Matter, Energy,
and Direct Current
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection,
Control, and Measurement
- Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
- Introduction to Generators and Motors
- Introduction to Electronic Emission, Tubes,
and Power Supplies
- Introduction to Solid-State Devices and
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- Introduction to Wave Propagation, Transmission
Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics