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Topic 5

MULTISTAGEAMPLIFIERS

AV18-AFC ANALOG FUNDAMENTALS C

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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

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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.


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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.


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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


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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.


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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.


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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

.


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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.


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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.


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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



Equipment

LabVolt Classroom Equipment.


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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.


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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


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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.


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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.


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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



1 Transistor Amplifier Circuits, RC Coupling Exercise 1



Equipment

LabVolt Classroom Equipment


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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


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.


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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


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.


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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.


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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.


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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



1 Transistor Amplifier Circuits, Transformer Coupling Exercise 1



Equipment

LabVolt Classroom Equipment


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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.


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Driver/Phase Splitter Stage

The driver/phase splitter portion of the circuit is shown in

Figure 511.

Figure 511

Driver/Phase Splitter Section


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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

.


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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


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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.


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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.


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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.


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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.


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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.


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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.

__________________________________________________

__________________________________________________

__________________________________________________


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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


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