partiiilectures multistage amplifiers · in multistage amplifier, where it is used as a final...

University of Missan Multistage Amplifiers

Electrical Engineering Department

Second Year, Electronic II,2015-2016 Maab Alaa Hussain

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University of missan


Electronic II, Second year

2015-2016

PartIIILectures

Multistage Amplifiers

Assistant Lecture: Maab Alaa Hussain




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Multistage and Compound Amplifiers

Basic Definitions:

Fig.(1-1) A general Multistage Amplifier

1- Gain of Multistage Amplifier:




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Example: A given amplifier arrangement has the following voltage gains.

Av1=10, Av2=20 and Av3=40. What is the overall gain? Also express each

gain in dB and determine the total dB voltage gain.

Solution:

Example: Three amplifier stages are working in cascade with 0.05v peak

to peak input providing 150v peak-to-peak output. If the voltage gain of

the first stage is 20and input to the third stage is 15v peak-to-peak.

Determine:

i. The overall voltage gain.

ii. Voltage gain of 2nd

and 3rd

stages.

iii. Input voltage of the 2nd

stage.

Solution:




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2- Different Coupling schemes Used in Amplifiers:

1- Resistance –Capacitance (RC) coupling:

It is the most important method of coupling the signal from one stage to

the next stage and is shown in Fig.(1-2)a. in this method the signal

developed across the collector resistor of each stage is coupled through

capacitor into the base of the next stage. The cascaded stages amplify the

signal and overall gain is equal to the product of individual stage gains.

The amplifier using this coupling scheme, are called RC-coupled

amplifiers.

Fig.(2-1)




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2. Impedance coupling:

This coupling is shown in Fig.(1-2)b. it may be noted that the Rc is

replaced by an inductor (L). as frequency increases the XL(equal to W.L)

increases. In other word inductor pass direct current but block alternating

current. The amplifier using this scheme, are called impedance-coupling

amplifiers.

3. Transformer Coupling:

This coupling is shown in fig.(1-2)c. in this method, primary winding

of the transistor acts as a collector load and secondary winding conveys

the a.c. output signal directly to the base of the next stage. The amplifiers

using this scheme of coupling are called transformer coupled amplifiers.

4. Direct Coupling:

This is shown in Fig.(1-2)d. in this method the a.c. output signal is fed

directly to the next stage. This type is used low frequency signales to be

amplified.




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1- RC coupled amplifier:

Fig.(1-3)




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1-1 calculation of Voltage Gain for RC Coupled Amplifies:

Fig.(1-4)

The parameters re1' and re2' represent the a.c. emitter diode transmittance

of Q1 and Q2 respectively.

Re1'=26/IE1 and re2

'=26/IE2

Now the Ri of the first stage as in fig.(1-4)a

Ri2=RB // (1.re1')=1.re1' (if RB>> 1re1')

Ro=Rc//Ri2

Where: Ri2 the input of the 2nd

stage.

Ri1=RB//(2.re2') =2.re2' (if RB>>2.re2')

Ro2=Rc//RL




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AV1=1.1

1

i

o

R

R=1.

'

1

1

'

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1

. e

o

o

o

r

R

r

R

Av2= '

2

2

e

o

r

R

Av=Av1.Av2= '

2

2

'

1

1 .e

o

e

o

r

R

r

R

If Q1 and Q2 are identical , then re1'=re2'=re'

And :

Av=Av1.Av2= 2'

21.

e

oo

r

RR

Example: Determine Ri, Ro, Ai, Av, for the first stage and second stage

for the circuit shown in Fig.(1-5). Also determine the overall voltage gain

and the gain in dB.

Solution:

Given Vcc=10v, Rc=5kΩ, RB=1MΩ, RE=1kΩ, RL=10kΩ, 1=2=100.

Fig.(1-5)

Ri for first and second stages:




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The input resistance for the first stage:

The input resistance for the second stage:

The output resistance for the first and second stages:

H.W: A radio receiver uses a two stage RC coupled amplifier

shown in the figure below, determine the values of voltage gain

of each stage and overall voltage gain.




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1-2 Advantages and Disadvantages of RC Coupled Amplifier

a. Advantages:

It is the most convenient and least expensive multistage amplifier.

It has wide frequency response.

It provides less frequency distortion.

b. Disadvantages:

The overall gain of amplifier is comparatively small because of the

loading effect of successive stages.

It has tendency to become noisy with age, especially in moist

climates.

It provides poor resistance matching between stages.

1-3 Application of RC Coupled Amplifiers

The RC coupled amplifier has excellent frequency response from 50

HZ to 20KHZ. This property makes it very useful in the initial stages of

all the public address systems. However, it may be noted that a coupled

amplifier cannot be used as a final stage of that amplifier because of its

poor matching characteristics.




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2- Impedance Coupled:

Fig. (1-6)shows the two stage impedance coupled amplifier. The

circuit consists of two single stage C.E amplifiers. The inductor (L) the

capacitor (Cc) and resistor (RB) forms the coupling network. The function

of C1,C2, and CE are the same as explained in RC coupled amplifier.

Fig.(1-6)

The expression of overall voltage gain and individual gains may be

obtained in the same way as obtained for RC coupled amplifier. The only

change to be made is that Rc should be obtained by inductive reactance

(XL=WL). Thus voltage gain of the first stage ,

Av2=Ro2/re2'

Av=Av1.Av2




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3- Transformer coupled Amplifier

Fig.(1-7) shows a two stage transformer coupled amplifier. The circuit

consists of two single stage C.E amplifiers. The function of transformer

(T1) is to couple the a.c. output signal from the output of the first stage to

the input of the second stage, while transformer (T2) couples the output

signal to the load. The input coupling capacitor is C1, while the emitter

bypass capacitor is CE.

Fig.(1-7)

3-1 calculation of Voltages Gain for Transformer coupled Amplifier:




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Example: Fig.(1-8) shows the circuit diagram of a two stage transformer

coupled transistor amplifier.

Calculate the values of individual stage gains and the overall voltage gain

in dB. Neglect VBE, take re'=25/IE




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Fig.(1-8)

Solution:




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3-2 Advantages and Disadvantages of Transformer Coupled Amplifier:

a. Advantages:

No signal power is lost in the collector or base resistors, because of

the low winding resistance of the transformer.

It provides a higher voltage gain than RC coupled amplifier.

It provides an excellent resistance matching between the stages.

b. Disadvantages:

The coupling transformer is expensive and bulky, particularly

when operated at audio frequencies.

At radio frequencies, the winding inductance and distributed

capacitance produce reverse frequency distortion.

It tends to produce 'hum' in the circuit.




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3-3 Application of Transformer Coupled Amplifier:

The transformer coupled amplifier provide excellent impedance

matching between the individual stages. This ability makes it very useful

in multistage amplifier, where it is used as a final stage. It is used to

transfer power to the low impedance load (such as speaker). The

impedance of the speaker varies from 4Ω to 16Ω, where as the output

resistance of a transistor amplifier is several hundred ohms. In order to

match the load impedance, with that of amplifier output, a step-down

transformer of proper turns ratio is used. The resistance of the secondary

winding of the transformer is made equal to the speaker impedance, while

that of the primary winding is made equal to the output resistance of the

amplifier.

4- Direct Coupled Amplifier:

It is also called DC amplifier and used to amplify very low frequency

(below 10 HZ) signals including direct current or zero frequency. It may

be noted that the capacitors, inductors and transformers cannot be used as

a coupling network at very low frequencies because the electrical size of

these devices, at low frequencies, become very large. Fig. (1-8) shows the

two stage direct coupled amplifier the operation of this circuits is

discussed below:

The signal to be amplified is applied directly to the input of the first

stage. Due to the transistor action, it appears in the amplified form across

the collector resistor of Q1. This voltage then drives the base of the




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second transistor Q2 and the amplified output is obtained across the

collector resistor of Q2.

Fig.(1-8)

4-1 calculation of Voltage Gain of Direct Coupled Amplifier:

Consider Fig.(1-9).

Fig.(1-9)




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Example: for the following figure determine the individual stages a.c.

voltage gains and the overall voltage gain. Neglect VBE, take re'=26/IE in

mA, 1=2=100.

Solution:




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Individual stages a.c. voltage gain

The voltage across R2:




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4-2 Direct coupled amplifier using complementary transistor:

Figure (1-10) shows the two stage direct coupled amplifier using

complementary transistors. The term 'complementary' means that one

Fig. (1-10)

stage uses NPN transistor while the second stage uses PNP transistor. The

operation of the circuit may be understood from the condition that when

the positive going input signal is applied to the base of transistor Q1, its

collector current increases. This increases the voltage across R3due to

which the collector voltage of transistor Q1 decreases. Consequently, the

base of transistor Q2 becomes less positive. It means that a negative going

signal is applied to the base of transistor Q2 transistor. It cause a forward

bias across emitter-base junction of transistor Q2 to increase, which in

turn increases the collector current. As result of this, the voltage drop

across R6 increases and we get a positive going amplified output.

Similarly, during the positive going signal, we get a negative going

amplified output across R6.

Example: for the circuit below, determine the d.c. collector currents and

collector to emitter voltage of Q1,Q2. Also determine the approximate




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overall a.c. voltage gain of the cascade amplifier. Assume =100 and C1

and C2 to very large. VBE=0.7v.

Solution:

d.c. collector currents and collector to emitter voltage of Q1,Q2

we know that the voltage drop across the R2:




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IC2=IE2

VC2=IC2. R5= (3.2 x 10 -3

)x 100= 0.32v

Overall voltage gain

AV2=1 (if it is an emitter follower stage)

Then :

AV=AV1.AV2=2.93x1=2.93 Ans.

4-3 Advantages and Disadvantages of Direct coupled Amplifier:

Advantages:

1. The circuit arrangement is very simple because it uses a minimum

number of resistors.

2. The circuit cost is low.

3. It can amplify very low frequency signals down to zero frequency.




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Disadvantages:

1. It cannot amplify high frequency signals.

2. It has poor temperature stability. Because of this, its Q-point shifts.

In a multistage direct coupled amplifier, the Q-point shifts are

amplified in succeeding stages.

4-4 Application of Direct Coupled Amplifier:

The Direct Coupled Amplifier are used in many electronic system that

handle signals, which change very slowly with time. Some of the

important application are given below:

1. Analog computation.

2. Power supply regulators.

3. Bioelectric measurements.

4. Linear integrated circuits.

5- Darlington Amplifier:

The Darlington amplifier consists of two cascade emitter followers as

shown in Fig.(1-11). The darlington amplifier has a high input resistance,

low output resistance, and high current gain. These characteristics make it

very useful as a current amplifier. The voltage gain of this amplifier is

less than unity.

Fig.(1-11)




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5-1 Darlington Amplifier Characteristics:

Consider the darligton amplifier circuit shown in Fig.(1-11).

Let Ib1= Base current of Q1 transistor,




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Example:




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Example:




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Tuned Amplifiers

1. Single-tuned voltage Amplifier:

Figure below shows the circuit of single-tuned voltage amplifier using

a bipolar junction transistor (BJT). The circuit in Fig. (a) is called

capacitively coupled tuned amplifier because the output is taken through

the coupling capacitor (Cc). similarly, the circuit shown in Fig. (b) is

called inductively coupled tuned amplifier because the output is taken

across an inductor. Both these circuits consists of a transistor amplifier

and tuned circuit as the load. The values of capacitance (C) and

inductance(L) of tuned circuit are selected in such a way that the resonant

frequency of the tuned circuit is equal to the frequency to be selected and

amplified. The resistors(R1, R2, and RE) are called biasing resistors. These

resistors provide d.c. operating currents and voltages for for the transistor.




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As in the following figure:

2. Double-tuned Voltage Amplifier:

Figure bellow shows the circuit of a double tuned voltage amplifier. It

consist of a transistor amplifier with two tuned circuits. One of the tuned

(i.e., L1 C1) is shown as the collector load and other (i.e., L2 C2) as the

output. The resistor R1,R2 and RE are used to provided the d.c. currents

and voltages for the transistor operation.

The signal to be amplified is applied at the input terminal through the

coupling capacitor (Cc).

The resonant frequency of the tuned circuit L1C1 is made equal to that

of the signal (by adjusting L1 or C1). Under these conditions, the tuned

circuit offers a very high impedance to the input signal . As a result of




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this, a large output appears across the tuned circuit L1C1. The output from

this tuned circuit is inductively coupled to the L2C2 tuned circuit.

The double tuned voltage amplifiers are extensively used in

intermediate frequency (IF) amplifier.

partiiilectures multistage amplifiers · in multistage amplifier, where it is used as a final...

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