Detailed Notes: Multistage & Power Amplifiers | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


 
MULTISTAGE AND POWER AMPLIFIERS 
Classification of Amplifiers, Distortion in amplifiers, Different coupling schemes 
used in amplifiers, Frequency response and Analysis of multistage amplifiers, 
Cascade amplifier, Darlington pair. 
Transistor at High Frequency: Hybrid - model of Common Emitter transistor 
model, f
a
, ß and unity gain bandwidth, Gain band width product. Differential 
Amplifiers, Power amplifiers - Class A, Class B, Class C, Class AB. 
 
In order to realize the function of amplification, the transformer may appear to be a potential 
device. However, in a transformer, though there is magnification of input voltage or current, the 
power required for the load has to be drawn from the source driving the input of the transformer. 
The output power is always less than the input power due to the losses in the core and windings. 
The situation in amplification is that the input source is not capable of supplying appreciable 
power. Hence the functional block meant for amplification should not draw any power from the 
input source but should deliver finite out power to the load. 
Thus the functional block required should have input power 
Pi = Vi Ii = 0 
And give the output 
P0 = V0 I0 = finite 
 
Such a functional block is called an ideal amplifier, which is shown in Fig.1 below. 
 
Power gain is G = P0/Pi 
 
The power gain of an ideal amplifier being infinite may sound like witchcraft in that something 
can be produced from nothing. The real fact is that the ideal amplifier requires dc input power. It 
converts dc power to ac power without any demand on the signal source to supply the power for 
the load. 
 
 
 
 
CLASSIFICATION OF AMPLIFIERS: 
Page 2


 
MULTISTAGE AND POWER AMPLIFIERS 
Classification of Amplifiers, Distortion in amplifiers, Different coupling schemes 
used in amplifiers, Frequency response and Analysis of multistage amplifiers, 
Cascade amplifier, Darlington pair. 
Transistor at High Frequency: Hybrid - model of Common Emitter transistor 
model, f
a
, ß and unity gain bandwidth, Gain band width product. Differential 
Amplifiers, Power amplifiers - Class A, Class B, Class C, Class AB. 
 
In order to realize the function of amplification, the transformer may appear to be a potential 
device. However, in a transformer, though there is magnification of input voltage or current, the 
power required for the load has to be drawn from the source driving the input of the transformer. 
The output power is always less than the input power due to the losses in the core and windings. 
The situation in amplification is that the input source is not capable of supplying appreciable 
power. Hence the functional block meant for amplification should not draw any power from the 
input source but should deliver finite out power to the load. 
Thus the functional block required should have input power 
Pi = Vi Ii = 0 
And give the output 
P0 = V0 I0 = finite 
 
Such a functional block is called an ideal amplifier, which is shown in Fig.1 below. 
 
Power gain is G = P0/Pi 
 
The power gain of an ideal amplifier being infinite may sound like witchcraft in that something 
can be produced from nothing. The real fact is that the ideal amplifier requires dc input power. It 
converts dc power to ac power without any demand on the signal source to supply the power for 
the load. 
 
 
 
 
CLASSIFICATION OF AMPLIFIERS: 
 
Amplifiers are classified in many ways based on different criteria as given below. 
I In terms of frequency range: 
1. DC amplifiers. (0 Hz to 20 Hz) 
2. Audio amplifiers (20 Hz to 20 KHz) 
3. Radio frequency amplifiers (Few KHz to hundreds of KHz) 
4. Microwave amplifiers (In the range of GHz) 
5. Video amplifiers (Hundreds of GHz) 
II In terms of signal strength: 
1. Small signal amplifiers. 
2. Large signal mplifiers 
III.  In terms of  coupling: 
 
1. Direct coupling. 
2. Resistance – capacitance (RC) coupling. 
3. Transformer coupling. 
 
IV. In terms of parameter: 
 
1. Voltage amplifiers. 
2. Current amplifiers. 
3. Power amplifiers. 
 
V. In terms of biasing condition: 
 
1. Class A amplifier 
2. Class B amplifier 
3. Class AB amplifier 
4. Class C amplifier. 
 
VI. In terms of tuning: 
 
1. Single tuned amplifier 
2. Double tuned amplifier 
3. Stagger tuned amplifier. 
 
DECIBEL NOTATION: 
 
The power gain of an amplifier is expressed as the ratio of the output power to the input power. 
When we have more than one stage of amplification i.e. when the output of one stage becomes 
the input to the next stage, the overall gain has to be obtained by multiplying the gains of the 
Page 3


 
MULTISTAGE AND POWER AMPLIFIERS 
Classification of Amplifiers, Distortion in amplifiers, Different coupling schemes 
used in amplifiers, Frequency response and Analysis of multistage amplifiers, 
Cascade amplifier, Darlington pair. 
Transistor at High Frequency: Hybrid - model of Common Emitter transistor 
model, f
a
, ß and unity gain bandwidth, Gain band width product. Differential 
Amplifiers, Power amplifiers - Class A, Class B, Class C, Class AB. 
 
In order to realize the function of amplification, the transformer may appear to be a potential 
device. However, in a transformer, though there is magnification of input voltage or current, the 
power required for the load has to be drawn from the source driving the input of the transformer. 
The output power is always less than the input power due to the losses in the core and windings. 
The situation in amplification is that the input source is not capable of supplying appreciable 
power. Hence the functional block meant for amplification should not draw any power from the 
input source but should deliver finite out power to the load. 
Thus the functional block required should have input power 
Pi = Vi Ii = 0 
And give the output 
P0 = V0 I0 = finite 
 
Such a functional block is called an ideal amplifier, which is shown in Fig.1 below. 
 
Power gain is G = P0/Pi 
 
The power gain of an ideal amplifier being infinite may sound like witchcraft in that something 
can be produced from nothing. The real fact is that the ideal amplifier requires dc input power. It 
converts dc power to ac power without any demand on the signal source to supply the power for 
the load. 
 
 
 
 
CLASSIFICATION OF AMPLIFIERS: 
 
Amplifiers are classified in many ways based on different criteria as given below. 
I In terms of frequency range: 
1. DC amplifiers. (0 Hz to 20 Hz) 
2. Audio amplifiers (20 Hz to 20 KHz) 
3. Radio frequency amplifiers (Few KHz to hundreds of KHz) 
4. Microwave amplifiers (In the range of GHz) 
5. Video amplifiers (Hundreds of GHz) 
II In terms of signal strength: 
1. Small signal amplifiers. 
2. Large signal mplifiers 
III.  In terms of  coupling: 
 
1. Direct coupling. 
2. Resistance – capacitance (RC) coupling. 
3. Transformer coupling. 
 
IV. In terms of parameter: 
 
1. Voltage amplifiers. 
2. Current amplifiers. 
3. Power amplifiers. 
 
V. In terms of biasing condition: 
 
1. Class A amplifier 
2. Class B amplifier 
3. Class AB amplifier 
4. Class C amplifier. 
 
VI. In terms of tuning: 
 
1. Single tuned amplifier 
2. Double tuned amplifier 
3. Stagger tuned amplifier. 
 
DECIBEL NOTATION: 
 
The power gain of an amplifier is expressed as the ratio of the output power to the input power. 
When we have more than one stage of amplification i.e. when the output of one stage becomes 
the input to the next stage, the overall gain has to be obtained by multiplying the gains of the 
individual stages. When large numbers are involved, this calculation becomes cumbersome. 
Also, when we have passive coupling networks between amplifier stages, there will be 
attenuation of the signal that is gain less than unity. To find the overall gain of a typical 
multistage amplifier such as the one given below. 
 
We have to multiply the various gains and attenuations. Moreover, when we wish to plot the 
gain of an amplifier versus frequency, using large numbers for plotting is not convenient. Hence 
it has been the practice to use a new unit called the decibel (usually abbreviated as dB) for 
measuring the power gain of a four terminal network. The power gain in decibels is given by 
 
          G = 10 log10 P0 / Pi  dB 
 
This new notation is also significant in the field of acoustics as the response of the human 
ear to sound intensity is found to be following this logarithmic pattern. The overall gain in 
decibel notation can be obtained for the amplifier gain of the figure1 by simply adding the 
decibel gains of the individual networks. If any network attenuates the signal, the gain will be 
less than the unity and the decibel gain will be negative. Thus the overall gain for the 
amplifier chain shown above is given by 
Overall gain = 10 – 6 + 30 – 10 + 20 = 44 dB 
The absolute power level of the output of an amplifier is sometimes specified in dBm, i.e. 
decibels with reference to a standard power power level, which is usually, 1 Mw dissipated in a 
600  load. Therefore, if an amplifier has 100 Mw, its power level in dBm is equal to 10 log 
100/1 = 20 dBm. 
MULTISTAGE AMPLIFIERS: 
 
In real time applications, a single amplifier can’t provide enough output. Hence, two or more 
amplifier stages are cascaded (connected one after another) to provide greater output Such an 
arrangement is known as multistage amplifier Though the basic purpose of this arrangement is 
increase the overall gain, many new problems as a consequence of this, are to be taken care. For 
e.g. problems such as the interaction between stages due to impedance mismatch, cumulative 
hum & noise etc. 
 
Page 4


 
MULTISTAGE AND POWER AMPLIFIERS 
Classification of Amplifiers, Distortion in amplifiers, Different coupling schemes 
used in amplifiers, Frequency response and Analysis of multistage amplifiers, 
Cascade amplifier, Darlington pair. 
Transistor at High Frequency: Hybrid - model of Common Emitter transistor 
model, f
a
, ß and unity gain bandwidth, Gain band width product. Differential 
Amplifiers, Power amplifiers - Class A, Class B, Class C, Class AB. 
 
In order to realize the function of amplification, the transformer may appear to be a potential 
device. However, in a transformer, though there is magnification of input voltage or current, the 
power required for the load has to be drawn from the source driving the input of the transformer. 
The output power is always less than the input power due to the losses in the core and windings. 
The situation in amplification is that the input source is not capable of supplying appreciable 
power. Hence the functional block meant for amplification should not draw any power from the 
input source but should deliver finite out power to the load. 
Thus the functional block required should have input power 
Pi = Vi Ii = 0 
And give the output 
P0 = V0 I0 = finite 
 
Such a functional block is called an ideal amplifier, which is shown in Fig.1 below. 
 
Power gain is G = P0/Pi 
 
The power gain of an ideal amplifier being infinite may sound like witchcraft in that something 
can be produced from nothing. The real fact is that the ideal amplifier requires dc input power. It 
converts dc power to ac power without any demand on the signal source to supply the power for 
the load. 
 
 
 
 
CLASSIFICATION OF AMPLIFIERS: 
 
Amplifiers are classified in many ways based on different criteria as given below. 
I In terms of frequency range: 
1. DC amplifiers. (0 Hz to 20 Hz) 
2. Audio amplifiers (20 Hz to 20 KHz) 
3. Radio frequency amplifiers (Few KHz to hundreds of KHz) 
4. Microwave amplifiers (In the range of GHz) 
5. Video amplifiers (Hundreds of GHz) 
II In terms of signal strength: 
1. Small signal amplifiers. 
2. Large signal mplifiers 
III.  In terms of  coupling: 
 
1. Direct coupling. 
2. Resistance – capacitance (RC) coupling. 
3. Transformer coupling. 
 
IV. In terms of parameter: 
 
1. Voltage amplifiers. 
2. Current amplifiers. 
3. Power amplifiers. 
 
V. In terms of biasing condition: 
 
1. Class A amplifier 
2. Class B amplifier 
3. Class AB amplifier 
4. Class C amplifier. 
 
VI. In terms of tuning: 
 
1. Single tuned amplifier 
2. Double tuned amplifier 
3. Stagger tuned amplifier. 
 
DECIBEL NOTATION: 
 
The power gain of an amplifier is expressed as the ratio of the output power to the input power. 
When we have more than one stage of amplification i.e. when the output of one stage becomes 
the input to the next stage, the overall gain has to be obtained by multiplying the gains of the 
individual stages. When large numbers are involved, this calculation becomes cumbersome. 
Also, when we have passive coupling networks between amplifier stages, there will be 
attenuation of the signal that is gain less than unity. To find the overall gain of a typical 
multistage amplifier such as the one given below. 
 
We have to multiply the various gains and attenuations. Moreover, when we wish to plot the 
gain of an amplifier versus frequency, using large numbers for plotting is not convenient. Hence 
it has been the practice to use a new unit called the decibel (usually abbreviated as dB) for 
measuring the power gain of a four terminal network. The power gain in decibels is given by 
 
          G = 10 log10 P0 / Pi  dB 
 
This new notation is also significant in the field of acoustics as the response of the human 
ear to sound intensity is found to be following this logarithmic pattern. The overall gain in 
decibel notation can be obtained for the amplifier gain of the figure1 by simply adding the 
decibel gains of the individual networks. If any network attenuates the signal, the gain will be 
less than the unity and the decibel gain will be negative. Thus the overall gain for the 
amplifier chain shown above is given by 
Overall gain = 10 – 6 + 30 – 10 + 20 = 44 dB 
The absolute power level of the output of an amplifier is sometimes specified in dBm, i.e. 
decibels with reference to a standard power power level, which is usually, 1 Mw dissipated in a 
600  load. Therefore, if an amplifier has 100 Mw, its power level in dBm is equal to 10 log 
100/1 = 20 dBm. 
MULTISTAGE AMPLIFIERS: 
 
In real time applications, a single amplifier can’t provide enough output. Hence, two or more 
amplifier stages are cascaded (connected one after another) to provide greater output Such an 
arrangement is known as multistage amplifier Though the basic purpose of this arrangement is 
increase the overall gain, many new problems as a consequence of this, are to be taken care. For 
e.g. problems such as the interaction between stages due to impedance mismatch, cumulative 
hum & noise etc. 
 
DISTORTION IN AMPLIFIERS: 
 
In any amplifier, ideally the output should be a faithful reproduction of the input. This is called 
fidelity. Of course there could be changes in the amplitude levels. However in practice this never 
happens. The output waveform tends to be different from the input. This is called as the 
distortion. The distortion may arise either from the inherent non – linearity in the transistor 
characteristics or from the influence of the associated circuit. 
 
The distortions are classified as: 
 
1. Non – linear or amplitude distortion 
2. Frequency distortion 
3. Phase distortion 
4. Inter modulation distortion 
NON – LINEAR DISTORTION: 
 
This is produced when the operation is over the non-linear part of the transfer characteristics of 
the transistor. (A plot between output v/s input is called as the transfer characteristics). Since the 
amplifier amplifies different parts of the input differently. For example, there can be 
compression of the positive half cycle and expansion of the negative half cycle. Sometimes, the 
waveform can become clipped also. (Flattening at the tips). Such a deviation from linear 
amplification produces frequencies in the output, which are not originally present in the output. 
Harmonics (multiples) of the input signal frequency are present in the output. The percentage 
harmonic distortion for the n
th
 
Harmonic is given by 
 
Dn = An (amplitude of the n the harmonic)  100% 
 
A1(amplitude of the fundamental) 
And the total harmonic distortion by 
D
T =
 
 
Where D
2, 
D
3 
   are harmonic components. 
D 
2 
? D 
2 
? ? ? ? ? ? ? D 
2
 
2 3 n 
Page 5


 
MULTISTAGE AND POWER AMPLIFIERS 
Classification of Amplifiers, Distortion in amplifiers, Different coupling schemes 
used in amplifiers, Frequency response and Analysis of multistage amplifiers, 
Cascade amplifier, Darlington pair. 
Transistor at High Frequency: Hybrid - model of Common Emitter transistor 
model, f
a
, ß and unity gain bandwidth, Gain band width product. Differential 
Amplifiers, Power amplifiers - Class A, Class B, Class C, Class AB. 
 
In order to realize the function of amplification, the transformer may appear to be a potential 
device. However, in a transformer, though there is magnification of input voltage or current, the 
power required for the load has to be drawn from the source driving the input of the transformer. 
The output power is always less than the input power due to the losses in the core and windings. 
The situation in amplification is that the input source is not capable of supplying appreciable 
power. Hence the functional block meant for amplification should not draw any power from the 
input source but should deliver finite out power to the load. 
Thus the functional block required should have input power 
Pi = Vi Ii = 0 
And give the output 
P0 = V0 I0 = finite 
 
Such a functional block is called an ideal amplifier, which is shown in Fig.1 below. 
 
Power gain is G = P0/Pi 
 
The power gain of an ideal amplifier being infinite may sound like witchcraft in that something 
can be produced from nothing. The real fact is that the ideal amplifier requires dc input power. It 
converts dc power to ac power without any demand on the signal source to supply the power for 
the load. 
 
 
 
 
CLASSIFICATION OF AMPLIFIERS: 
 
Amplifiers are classified in many ways based on different criteria as given below. 
I In terms of frequency range: 
1. DC amplifiers. (0 Hz to 20 Hz) 
2. Audio amplifiers (20 Hz to 20 KHz) 
3. Radio frequency amplifiers (Few KHz to hundreds of KHz) 
4. Microwave amplifiers (In the range of GHz) 
5. Video amplifiers (Hundreds of GHz) 
II In terms of signal strength: 
1. Small signal amplifiers. 
2. Large signal mplifiers 
III.  In terms of  coupling: 
 
1. Direct coupling. 
2. Resistance – capacitance (RC) coupling. 
3. Transformer coupling. 
 
IV. In terms of parameter: 
 
1. Voltage amplifiers. 
2. Current amplifiers. 
3. Power amplifiers. 
 
V. In terms of biasing condition: 
 
1. Class A amplifier 
2. Class B amplifier 
3. Class AB amplifier 
4. Class C amplifier. 
 
VI. In terms of tuning: 
 
1. Single tuned amplifier 
2. Double tuned amplifier 
3. Stagger tuned amplifier. 
 
DECIBEL NOTATION: 
 
The power gain of an amplifier is expressed as the ratio of the output power to the input power. 
When we have more than one stage of amplification i.e. when the output of one stage becomes 
the input to the next stage, the overall gain has to be obtained by multiplying the gains of the 
individual stages. When large numbers are involved, this calculation becomes cumbersome. 
Also, when we have passive coupling networks between amplifier stages, there will be 
attenuation of the signal that is gain less than unity. To find the overall gain of a typical 
multistage amplifier such as the one given below. 
 
We have to multiply the various gains and attenuations. Moreover, when we wish to plot the 
gain of an amplifier versus frequency, using large numbers for plotting is not convenient. Hence 
it has been the practice to use a new unit called the decibel (usually abbreviated as dB) for 
measuring the power gain of a four terminal network. The power gain in decibels is given by 
 
          G = 10 log10 P0 / Pi  dB 
 
This new notation is also significant in the field of acoustics as the response of the human 
ear to sound intensity is found to be following this logarithmic pattern. The overall gain in 
decibel notation can be obtained for the amplifier gain of the figure1 by simply adding the 
decibel gains of the individual networks. If any network attenuates the signal, the gain will be 
less than the unity and the decibel gain will be negative. Thus the overall gain for the 
amplifier chain shown above is given by 
Overall gain = 10 – 6 + 30 – 10 + 20 = 44 dB 
The absolute power level of the output of an amplifier is sometimes specified in dBm, i.e. 
decibels with reference to a standard power power level, which is usually, 1 Mw dissipated in a 
600  load. Therefore, if an amplifier has 100 Mw, its power level in dBm is equal to 10 log 
100/1 = 20 dBm. 
MULTISTAGE AMPLIFIERS: 
 
In real time applications, a single amplifier can’t provide enough output. Hence, two or more 
amplifier stages are cascaded (connected one after another) to provide greater output Such an 
arrangement is known as multistage amplifier Though the basic purpose of this arrangement is 
increase the overall gain, many new problems as a consequence of this, are to be taken care. For 
e.g. problems such as the interaction between stages due to impedance mismatch, cumulative 
hum & noise etc. 
 
DISTORTION IN AMPLIFIERS: 
 
In any amplifier, ideally the output should be a faithful reproduction of the input. This is called 
fidelity. Of course there could be changes in the amplitude levels. However in practice this never 
happens. The output waveform tends to be different from the input. This is called as the 
distortion. The distortion may arise either from the inherent non – linearity in the transistor 
characteristics or from the influence of the associated circuit. 
 
The distortions are classified as: 
 
1. Non – linear or amplitude distortion 
2. Frequency distortion 
3. Phase distortion 
4. Inter modulation distortion 
NON – LINEAR DISTORTION: 
 
This is produced when the operation is over the non-linear part of the transfer characteristics of 
the transistor. (A plot between output v/s input is called as the transfer characteristics). Since the 
amplifier amplifies different parts of the input differently. For example, there can be 
compression of the positive half cycle and expansion of the negative half cycle. Sometimes, the 
waveform can become clipped also. (Flattening at the tips). Such a deviation from linear 
amplification produces frequencies in the output, which are not originally present in the output. 
Harmonics (multiples) of the input signal frequency are present in the output. The percentage 
harmonic distortion for the n
th
 
Harmonic is given by 
 
Dn = An (amplitude of the n the harmonic)  100% 
 
A1(amplitude of the fundamental) 
And the total harmonic distortion by 
D
T =
 
 
Where D
2, 
D
3 
   are harmonic components. 
D 
2 
? D 
2 
? ? ? ? ? ? ? D 
2
 
2 3 n 
A distortion factor meter measures the total distortion. The spectrum or wave analyzer can be 
used to measure the amplitude of each harmonic. 
FREQUENCY DISTORTION: 
 
A practical signal is usually complex (containing many frequencies). Frequency distortion occurs 
when the different frequency components in the input signal are amplified differently. This is  
due to the various frequency dependent reactances (capacitive & inductive) present in the circuit 
or the active devices (BJT or FET). 
PHASE DISTRIBUTION: 
 
This occurs due to different frequency components of the input signal suffering different phase 
shifts. The phase shifts are also due to reactive effects and the active devices. This causes 
problems in TV picture reception. To avoid this amplifier phase shift should be proportional to 
the frequency. 
INTERMODULATION DISTORTION: 
 
The harmonics introduced in the amplifier can combine with each other or with the original 
frequencies to produce new frequencies to produce new frequencies that are not harmonics of the 
fundamental. This is called inter modulation distortion. This distortion results in unpleasant 
hearing. 
 
FREQUENCY RESPONSE OF AN AMPLIFIER: 
 
Frequency response of an amplifier is a plot between gain & frequency. If the gain is constant 
(same) for all frequencies of the input signal, then this plot would be a flat line. But this never 
happens in practice. 
 
As explained earlier, there are different reactive effects present in the amplifier circuit and the 
active devices used. Infact there are external capacitors used for blocking, capacitors etc. Also, in 
tuned amplifiers, resonant LC circuits are connected in the collector circuits of the amplifier to 
get narrow band amplification around the resonant frequencies. 
 
Fig below shows a frequency response of a typical amplifier.