View
240
Download
1
Category
Preview:
Citation preview
7/29/2019 Acoples de Transistores
1/34
Topic 5
MULTISTAGEAMPLIFIERS
AV18-AFC ANALOG FUNDAMENTALS C
1
7/29/2019 Acoples de Transistores
2/34
Overview
This topic covers multistage amplifier circuits and the methods oftransferring a signal from one stage to the next and the advantages and
disadvantages of these coupling methods.
ANALOG FUNDAMENTALS C AV18-AFC
2 17 Feb 10
7/29/2019 Acoples de Transistores
3/34
Topic Learning Outcome
LO 5. Explain the operation of multistage amplifiers.
Assessment Criteria
LO 5.1. Describe the advantages/disadvantages of the following coupling
methods:
LO 5.1.1. RC,
LO 5.1.2. transformer and
LO 5.1.3. direct.
.2 Describe the operation of the following multistage amplifier
.2.1 RC coupled,
.2.2 transformer coupled and
.2.3 direct coupled.
.3 Determine the following parameters for multistage amplifiers:
.3.1 voltage gain,
.3.2 current gain and
.3.3 power gain.
.4 Explain the operation of multistage valve amplifiers.
AV18-AFC ANALOG FUNDAMENTALS C
3
7/29/2019 Acoples de Transistores
4/34
Multistage Amplifiers
Single stage amplifiers are generally limited to voltage gains in the region of100. Typical radio antenna receiver signals are often as low as 10 V, while
the required voltage to drive the output speaker is around 10 V.
So, to amplify the 10 V from the antenna to the 10 V required to drive an
output speaker, a gain of approximately 1,000,000 is required.
This kind of gain is achieved by cascading (or joining) several amplifiers
together in series. That is to say, the output of one single stage amplifier is
fed to the input of the following single stage amplifier.
To join, or cascade these amplifiers together the most common methods
used are:
RC coupling ( resistive capacitive coupling ),
transformer coupling, and
direct coupling.
ANALOG FUNDAMENTALS C AV18-AFC
4
7/29/2019 Acoples de Transistores
5/34
Figure 51 illustrates the three coupling techniques.
Figure 51Multistage Amplifier Coupling Techniques
In a cascaded system, the first amplifier is called the firststage and the
second amplifier is called the secondstage.
Amplifiers are cascaded together to achieve an overall higher gain than that
possible with one amplifier. Figure 52 illustrates the cascaded amplifier
system.
Figure 52
Cascaded Amplifiers
AV18-AFC ANALOG FUNDAMENTALS C
5
7/29/2019 Acoples de Transistores
6/34
Assume that the input voltage is 0.1 V, the amplitude at the output of the
first stage will be:
NOTE
The output voltage of the first stage becomes the input
voltage to the second stage. This voltage is now
amplified by the second amplifier to become the final
output signal.
Therefore:
This indicates that the overall circuit gain is equal to:
For all cascaded amplifier systems, the overall amplifier gain can be
determined by multiplying the individual stage gains together.
If the single stage amplifier gains are expressed in decibels, the overall
amplifier gain is determined by adding the single stage dB gains.
ANALOG FUNDAMENTALS C AV18-AFC
6
7/29/2019 Acoples de Transistores
7/34
For example, referring to Figure 52, each stage has a gain of 20. In
decibels, this is equal to:
Stage 1 Stage 2
For the same example, the overall amplifier gain was determined to equal
400. This equates to:
From this example we can see that the addition of the individual stage dB
gains will equal the overall amplifier dB gain.
If a multistage amplifier consists of a stage which has a negative dB gain, it
is treated as a negative addition.
For example, an amplifier consists of three stages. Each stage has a voltage
gain of 10 dB, 22 dB and -11 dB respectively. The overall amplifier voltagegain is therefore:
To determine the current or power gain of a multistage amplifier, the same
procedure is used.
AV18-AFC ANALOG FUNDAMENTALS C
7
7/29/2019 Acoples de Transistores
8/34
Direct Coupling
With direct coupling, the output of one transistor is connected directly to theinput of the next transistor, as shown in Figure 53.
Figure 53
Direct Coupling
NOTE
The collector terminal (output) of transistor Q1
connects directly to the base terminal of transistor Q2.
The first stage of the circuit (Q1) is a common emitter amplifier which hasthe voltage divider resistors to set and stabilise the DC bias. The second
stage (Q2) does not have the voltage divider resistors. The bias of the
second stage is set by the collector voltage of Q1.
Referring to Figure 53, for Q2 to function, the collector voltage must
therefore be higher than the base voltage. As the base voltage is determined
by the collector voltage of Q1, the collector voltage of Q2 will be closer to
the applied VCC
.
ANALOG FUNDAMENTALS C AV18-AFC
8
7/29/2019 Acoples de Transistores
9/34
Note that for each added stage, the collector voltage progressively
approaches the supply voltage (VCC
). This will be a limiting factor in the
number of stages for a direct coupled amplifier.
The absence of the voltage divider network makes the DC bias of the circuit
more sensitive to temperature changes.
As each stage is linked, if a change occurs at the collector of Q1 this change
will be amplified by Q2, thereby making an even larger change at the
collector of Q2.
With direct coupling, the setting of the Q point to enable maximum signal
swing at the output is difficult to achieve over three or more stages.
Although successive common emitter stages are shown directly coupled in
Figure 53, this configuration is rarely used. A more commonly usedconfiguration is shown in Figure 54.
Figure 54
Direct Coupled Amplifiers
Note in Figure 54 that the second stage of the circuit now has a PNP
transistor for Q2. Consequently the emitter voltage of Q2 is approximately
0.6 V higher than the base voltage. As the collector base junction is reverse
biased for normal operation, the collector voltage will be less than the base
voltage.
AV18-AFC ANALOG FUNDAMENTALS C
9
7/29/2019 Acoples de Transistores
10/34
Advantages
The main advantage of direct coupling is the ability to amplify a DC voltage
and low frequency signals. The operational amplifier (discussed later) is an
example of an amplifier circuit specifically designed to amplify DC bydirect coupling.
A typical frequency response for a direct coupled amplifier ranges from DC
to 100 kHz.
Disadvantages
The main disadvantage of direct coupled amplifiers is their poor
temperature stability, as leakage current and increases for increases in
temperature.
With an increase in temperature, the collector voltage of the first stage will
change. This change will be amplified by the following amplifier stages and
may drive the last stage out of its linear operating region.
Another disadvantage is the difficulty in matching the collector voltage of
one stage to the required base voltage of the next stage to set the Q point
through, out the amplifier.
ANALOG FUNDAMENTALS C AV18-AFC
10
7/29/2019 Acoples de Transistores
11/34
Practical Exercise
Direct Coupled Amplifiers
Overview
The following practical exercises will reinforce the theory on direct coupled
amplifiers and will form part of your performance assessment for this
module.
Procedure
Your Instructor will nominate which of the following Lab-Volt practical
exercises you are to carry out:
1 Transistor Amplifier Circuits, Direct Coupling Exercise 1
2 Transistor Amplifier Circuits, Direct Coupling Exercise 2
3 Transistor Amplifier Circuits, Direct Coupling Exercise 3
Equipment
LabVolt Classroom Equipment.
AV18-AFC ANALOG FUNDAMENTALS C
11
7/29/2019 Acoples de Transistores
12/34
RC Coupling
When a capacitor and one or more resistors connect the output of the firststage to the input of the second stage, the amplifier is RC (resistance-
capacitance) coupled. Figure 55 illustrates RC coupling components.
Figure 55
RC Coupling
Figure 55 consists of two cascaded common emitter NPN amplifiers
(Q1 and Q2). The coupling capacitor connects the output of the first stage
to the input of the second stage.
The purpose of the coupling capacitor is to block the collector DC current of
Q1 from the base of Q2. This prevents any DC interaction of the two
stages, thereby preventing any shifting of the Q points of each amplifierstage.
The voltage divider biasing networks (R1, R2, R5 and R6) determine the
bias and, as such, the Q point of each individual stage.
With the application of an AC signal at the input of the amplifier, the
resulting waveform at the collector of Q1 is 180 out of phase with the
input. This phase inversion is passed through the coupling capacitor to the
second stage of the amplifier.
ANALOG FUNDAMENTALS C AV18-AFC
12
7/29/2019 Acoples de Transistores
13/34
With the signal at the base of Q2, the resulting waveform at the collector of
Q2 is 180 out of phase with the base signal. Figure 56 illustrates the
circuit waveforms.
Figure 56
RC Coupled Amplifiers-Waveforms
AV18-AFC ANALOG FUNDAMENTALS C
13
7/29/2019 Acoples de Transistores
14/34
Note that with two CE amplifiers cascaded, the output signal has undergone
a 360 phase shift; the output remains in phase with the input signal.
Frequency Response
The gain of an amplifier is not the same for all input signal frequencies.The way in which the gain varies for frequency is called the frequency
response.
The band width of an amplifier is the range of signal frequencies over which
the gain of the amplifier remains relatively constant.
The size of the coupling capacitor can affect the frequency response and
bandwidth of an amplifier. Too small a capacitor increases the capacitive
reactance at lower frequencies, resulting in the coupling capacitor forming a
voltage divider with the input impedance of the second stage of theamplifier. This causes a reduction in the voltage gain and a narrower
bandwidth.
It is best to select a capacitor which will have a low reactance at the lowest
input signal frequency. This ensures good performance over the frequency
range of the amplifier.
The coupling capacitors used in transistor circuits are often electrolytic.
This is especially true in low frequency amplifiers, because high values of
capacitance are needed to pass the AC signals.
ANALOG FUNDAMENTALS C AV18-AFC
14
7/29/2019 Acoples de Transistores
15/34
Advantages
The advantages of RC coupled amplifiers are:
their ability to amplify uniformly over the entire audio range,
RC coupling is small, light and inexpensive,
there is no magnetic field produced to interfere with the signal and
they provide an output with minimal frequency distortion.
Disadvantages
The disadvantages of RC coupled amplifiers are:
the supply voltage is dropped by the load resistor thus the collector
operates at a reduced voltage and
frequencies below 20 Hz to DC cannot be amplified as they cannot
pass through the capacitor.
AV18-AFC ANALOG FUNDAMENTALS C
15
7/29/2019 Acoples de Transistores
16/34
Practical Exercise
RC Coupled Amplifiers
Overview
The following practical exercises will reinforce the theory on RC coupled
amplifiers and will form part of your performance assessment for this
module.
Procedure
Your Instructor will nominate which of the following Lab-Volt practical
exercises you are to carry out:
1 Transistor Amplifier Circuits, RC Coupling Exercise 1
2 Transistor Amplifier Circuits, RC Coupling Exercise 2
3 Transistor Amplifier Circuits, RC Coupling Exercise 3
Equipment
LabVolt Classroom Equipment
ANALOG FUNDAMENTALS C AV18-AFC
16
7/29/2019 Acoples de Transistores
17/34
Transformer Coupling
When a transformer connects the output of the first stage of an amplifier tothe input of the second stage of the amplifier, the amplifiers are transformer
coupled.
Figure 57 illustrates transformer coupling.
Figure 57
Transformer Coupling
The primary coil of transformer T1 is connected between the first stage
amplifier Q1 collector terminal and VCC
. The transformer secondary coil
connects to the base terminal of the second stage amplifier Q2. The
transformer's AC ground is found through the DC blocking capacitor C1.
The purpose of C1 is to ensure that the second stage DC bias is not shorted
to earth by the transformer secondary winding during AC operation.
Transformer T1 electrically couples the first stage to the second stage for
AC signals only. DC current flow between the stages is prevented by the
isolation of the transformer primary and secondary windings.
The function of the transformer is to match the low impedance of the second
stage base circuit with the high impedance of the first stage collector circuit.
AV18-AFC ANALOG FUNDAMENTALS C
17
7/29/2019 Acoples de Transistores
18/34
The impedance of the primary transformer coil (ZP) in the collector circuit is
equal to the impedance of the secondary coil (ZS) times the square of the
transformer turns ratio (NP/N
S).
What this means is that the impedance seen by the collector of Q1 will equal
the transformer turns ratio squared times the load. The load is a
combination of the transformer secondary coil impedance and the input
impedance of the second stage of the amplifier.
Figure 58 illustrates the AC load seen by the collector circuit.
Figure 58
Transformer Coupling
As shown in Figure 58, the impedance of the secondary winding is
affected by the parallel resistance of:
The final impedance seen by the collector equals the transformer turns ratio
(squared) times the secondary coil impedance.
With an AC signal applied at the input, the resulting signal at the collector
of Q1 will be approximately 180 out of phase. The collector signal of the
first stage is not exactly 180 out of phase due to the inductive reactance of
the transformer primary.
ANALOG FUNDAMENTALS C AV18-AFC
18
7/29/2019 Acoples de Transistores
19/34
The transformer secondary coil signal is either in phase or 180 out of phase
with the primary signal, depending on the connection point to the secondary
winding. In Figure 58 the dot on the bottom of the primary coil and the
dot on the top of the secondary coil indicate that the signals at these two
points are in phase.
Due to the turns ratio of T1, the peak output voltage of the secondary is
stepped down from the peak voltage applied at the primary winding.
The resulting output signal of the second stage of the amplifier is not quite
in phase with the input signal applied at the base of transistor Q1. This is
due to the inductive reactance of the primary coil of the transformer.
Figure 59 illustrates the waveforms throughout the circuit.
Figure 59
Transformer Coupled Amplifiers-Waveforms
A transformer coupled amplifier uses less power than a RC coupled
amplifier. This is because the DC voltage drop across the primary windingis considerably less than that of a collector resistor. Transformer coupling
also allows the use of a smaller supply voltage.
AV18-AFC ANALOG FUNDAMENTALS C
19
7/29/2019 Acoples de Transistores
20/34
Advantages
The advantages of a transformer coupled amplifier are:
Use of a transformer allows the impedance matching of the output andinput impedances of the respective amplifier stages.
Less power consumption as no collector resistor is used.
The use of a capacitor across the primary winding can make a
frequency selective amplifier.
Disadvantages
The disadvantages of transformer coupling are:
Larger, heavier and cost more than RC coupling networks.
To prevent the magnetic field of the transformer affecting the signal,
they must be wound on an iron core inside a shielded can.
Transformers are frequency sensitive (impedance changes with
frequency). Therefore, the frequency range of the transformer-
coupled amplifiers is limited.
Summary
Capacitor
Coupling
Direct
Coupling
Transformer
Coupling
DC
Amplification
No Yes No
Impedancematching
No No Yes
Advantages Easy to use.
DC biasing of each
stage unaffected.
Outputs at different
DC levels can be
coupled.
Uniform gain over
audio frequencies.
Simplicity when a few
are used.
Provides DC
amplification.
High efficiency.
Can be tuned to make
a selective amplifier.
ANALOG FUNDAMENTALS C AV18-AFC
20
7/29/2019 Acoples de Transistores
21/34
Disadvantages Require high values of
capacitance for low
frequencies.
Cannot amplify DC
and low frequencies.
Difficult to design for
many stages.
Poor temperature
sensitivity.
Cost, size, and weight
can be a problem.
Frequency response.
Cannot amplify DC
and low frequencies.
Practical Exercise
Transformer Coupled Amplifiers
Overview
The following practical exercises will reinforce the theory on transformer
coupled amplifiers and will form part of your performance assessment for
this module.
Procedure
Your Instructor will nominate which of the following Lab-Volt practical
exercises you are to carry out:
1 Transistor Amplifier Circuits, Transformer Coupling Exercise 1
2 Transistor Amplifier Circuits, Transformer Coupling Exercise 2
3 Transistor Amplifier Circuits, Transformer Coupling Exercise 3
Equipment
LabVolt Classroom Equipment
AV18-AFC ANALOG FUNDAMENTALS C
21
7/29/2019 Acoples de Transistores
22/34
Multi-Stage Valve Audio Amplifier
Multi-stage valve audio amplifiers operate in a similar manner to FETamplifiers. The main difference is that the higher supply voltages used by
valves provide higher levels of gain.
This means that valve amplifiers require fewer stages to achieve the same
level of amplification.
A typical example of a multi-stage valve audio amplifier is shown in
Figure 510.
Figure 510
Multi-Stage Valve Audio Amplifier
This amplifier consists of a:
driver/phase splitter stage and
push-pull power amplifier stage.
ANALOG FUNDAMENTALS C AV18-AFC
22
7/29/2019 Acoples de Transistores
23/34
Driver/Phase Splitter Stage
The driver/phase splitter portion of the circuit is shown in
Figure 511.
Figure 511
Driver/Phase Splitter Section
AV18-AFC ANALOG FUNDAMENTALS C
23
7/29/2019 Acoples de Transistores
24/34
How it Works
V1
is a double triode. This basically means that there are two valves
in the one glass envelope.
These valves work independently of each other but share the heater,cathode resistor and cathode bypass capacitor.
The audio input is coupled from the input to the grid of V1a
by the
transformer T1.
V1a
and V1b
are cathode biased in class "A".
R8
is the cathode resistor for V1.
C13 is an AC bypass for R8 to improve the gain of the driver stage.
R9
is the plate/anode resistor for V1a
.
The heater/filament voltage for V1, V
2and V
3is -6.3 VDC, and is
provided from the power supply via fuse F1, and SW
1.
R10
is the plate/anode resistor for V1b
.
The anode signal from V1a
is 180o phase shifted from the input audio,
and is fed via C14, to the control grid of the pentode V2.
This signal is voltage divided by R11
and R12
to provide a lower
voltage, anti-phase signal to the grid of V1b
.
This will cause the anode signals at V1a
and V1b
to be phase shifted by
180o from each other.
The anode signal of V1b
is fed to the control grid of V3
via C15
.
ANALOG FUNDAMENTALS C AV18-AFC
24
7/29/2019 Acoples de Transistores
25/34
Push-Pull Power Amplifier Stage
The push-pull power amplifier portion of the circuit is shown in
Figure 512.
Figure 512
Push-Pull Power Amplifier
AV18-AFC ANALOG FUNDAMENTALS C
25
7/29/2019 Acoples de Transistores
26/34
How it Works
V2
and V3
are configured and biased as a class "AB" push-pull
amplifier.
R11 and R12 form the grid resistance for V2, while R13 is the gridresistance for V
3.
R14
is the cathode resistor for both V2
and V3, and is bypassed by C
16
to provide extra gain.
The screen grids of V2
and V3
are connected (DC wise) to the +300V
rail via R15
and AC wise to earth via C17
.
The suppressor grids of V2
and V3
are connected internally to their
own cathodes.
When the AC signal to V2's control grid drives in a positive direction,
the input to V3's control grid will drive negative because of the phase
splitting action of V1a
and V1b
.
This will cause V2
to draw more anode current from the +300V rail
via the primary of the transformer T2, and V
3to stop drawing anode
current because it will be driven below its cut-off voltage.
ANALOG FUNDAMENTALS C AV18-AFC
26
7/29/2019 Acoples de Transistores
27/34
The current paths and waveforms when V2
is ON and V3
is OFF are shown
in Figure 513.
Figure 513
Current Paths And Waveforms When V2 Is On
And V3 Is Off
The increased current draw through V2
causes the flux in the primary
of T2
to expand.
This in turn induces a current in the secondary of T2
which develops
an increasing voltage across the voice coil of the loud speaker.
Now lets consider what will happen for a negative going control grid signal
to V2.
This push-pull stage is a little different to those that you have encountered in
FETS and BJTs.
The fact that FETS can come as N or P types, enables them to be used in
complementary symmetry.
BJTs also have the same capability. Valves on the other hand can only
operate in one direction.
AV18-AFC ANALOG FUNDAMENTALS C
27
7/29/2019 Acoples de Transistores
28/34
The current paths and waveforms when V2
is OFF and V3
is ON are shown
in Figure 514.
Figure 514
Current Paths And Waveforms When V2 Is Off
And V3 Is On
The negative going signal on the control grid of V2 will cut it off.
As the signal at the control grid of V2
goes negative, the phase splitter
supplies a positive going signal to the control grid of V3
.
The phase splitter V1
(see Figure 512) allows anti-phase signals to
be fed to V2
and V3
so that the push-pull stage will amplify both
positive and negative half cycles.
This positive going voltage turns V3
on harder and draws an
increasing current through the primary of T2.
Notice that the direction of current flow through the primary of T2
is
in the opposite direction to that when V2
was ON and V3
was OFF.
This will build L2's primary magnetic flux in the opposite direction.
The flux is coupled to the secondary of T2
and develops a voltage of
opposite polarity across the voice coil of the loudspeaker.
ANALOG FUNDAMENTALS C AV18-AFC
28
7/29/2019 Acoples de Transistores
29/34
Summary
Valve amplifiers are connected together to form multi-stage valve
amplifiers for the following reasons:
higher voltage and power gain,
improved isolation,
impedance matching,
increased stability,
wider frequency response, and
better linearity.
Multi-Stage Audio Amplifiers
Valve amplifiers are always operated in class "A" for a single stage,
and class "AB" for push pull.
A phase shifting driver stage is required if a push pull stage follows
the driver.
AV18-AFC ANALOG FUNDAMENTALS C
29
7/29/2019 Acoples de Transistores
30/34
Trainee Activity
1. What is the method of transistor coupling utilised in the above
circuit?
__________________________________________________
2. Using the circuit shown in Question 1, draw the waveforms at
the base and collector of each transistor for the following input
waveform.
ANALOG FUNDAMENTALS C AV18-AFC
30
7/29/2019 Acoples de Transistores
31/34
AV18-AFC ANALOG FUNDAMENTALS C
31
7/29/2019 Acoples de Transistores
32/34
3. What is the overall amplifier voltage gain (AV) of a transformer
coupled amplifier, consisting of three stages each having the following
stage gains?
Stage 1 = +26dB
Stage 2 = +15dB
Stage 3 = -19dB
__________________________________________________
__________________________________________________
__________________________________________________
4. Draw a three stage direct coupled amplifier.
5. List two advantages of a RC coupled amplifier.
__________________________________________________
__________________________________________________
__________________________________________________
ANALOG FUNDAMENTALS C AV18-AFC
32
7/29/2019 Acoples de Transistores
33/34
6. What amplifier coupling method can amplify DC?
__________________________________________________
__________________________________________________
7. List two disadvantages of a transformer coupled amplifier.
__________________________________________________
__________________________________________________
8. Why is a transformer coupled amplifier more efficient than otheramplifier coupling methods?
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
9. Flywheel effect in multi-stage RF valve amplifiers relates to:
__________________________________________________
10. With a multi stage valve audio amplifier, the driver/phase splitter
stage;
__________________________________________________
__________________________________________________
End of Topic Text
AV18-AFC ANALOG FUNDAMENTALS C
33
7/29/2019 Acoples de Transistores
34/34
THIS PAGE INTENTIONALLY BLANK
ANALOG FUNDAMENTALS C AV18-AFC
Recommended