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Bài giảng Electronics circuits for communications - Chapter 4: Power amplifiers (PA) - Đinh Quốc Hùng

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CHAPTER 4:  
POWER AMPLIFIERS (PA)  
1
Amplifier Terms  
Classes of Operation  
•
Class A operation of an amplifier means that the transistor operates in the active region at all  
times. This implies that collector current flows for 360° of the ac cycle. With a class A amplifier,  
the designer usually tries to locate the Q point somewhere near the middle of the load line. This  
way, the signal can swing over the maximum possible range without saturating or cutting off the  
transistor, which would distort the signal  
•
•
Class B operation is different. It means that collector current flows for only half the cycle (180°).  
To have this kind of operation, a designer locates the Q point at cutoff. Then, only the positive  
half of ac base voltage can produce collector current. This reduces the wasted heat in power  
transistors  
Class C operation means that collector current flows for less than 180°of the ac cycle. With class C  
operation, only part of the positive half cycle of ac base voltage produces collector current. As a  
result, we get brief pulses of collector current  
2
Amplifier Terms  
Classes of Operation  
3
Amplifier Terms  
Classes of Operation  
•
Class A operation of an amplifier means that the transistor operates in the active region at all  
times. This implies that collector current flows for 360° of the ac cycle. With a class A amplifier,  
the designer usually tries to locate the Q point somewhere near the middle of the load line. This  
way, the signal can swing over the maximum possible range without saturating or cutting off the  
transistor, which would distort the signal  
•
•
Class B operation is different. It means that collector current flows for only half the cycle (180°).  
To have this kind of operation, a designer locates the Q point at cutoff. Then, only the positive  
half of ac base voltage can produce collector current. This reduces the wasted heat in power  
transistors  
Class C operation means that collector current flows for less than 180°of the ac cycle. With class C  
operation, only part of the positive half cycle of ac base voltage produces collector current. As a  
result, we get brief pulses of collector current  
4
Amplifier Terms  
Ranges of Frequency  
• Another way to describe amplifiers is by stating their frequency  
range. For in-stance, an audio amplifier refers to an amplifier that  
operates in the range of 20 Hz to 20 kHz. On the other hand, a  
radio-frequency (RF) amplifier is one that amplifies frequencies  
above 20 kHz, usually much higher. For instance, the RF amplifiers  
in AM radios amplify frequencies between 535 and 1605 kHz, and  
the RF amplifiers in FM radios amplify frequencies between 88  
and 108 MHz.  
• Amplifiers are also classified as narrowband or wideband. A  
narrow-band amplifier works over a small frequency range like  
450 to 460 kHz. A wide-band amplifier operates over a large  
frequency range like 0 to 1 MHz.  
• Narrowband amplifiers are usually tuned RF amplifiers, which  
means that their ac load is a high-Q resonant tank tuned to a radio  
station or television channel. Wideband amplifiers are usually  
untuned; that is, their ac load is resistive.  
5
Amplifier Terms  
Ranges of Frequency  
6
Amplifier Terms  
Ranges of Frequency  
• Another way to describe amplifiers is by stating their frequency  
range. For in-stance, an audio amplifier refers to an amplifier that  
operates in the range of 20 Hz to 20 kHz. On the other hand, a  
radio-frequency (RF) amplifier is one that amplifies frequencies  
above 20 kHz, usually much higher. For instance, the RF amplifiers  
in AM radios amplify frequencies between 535 and 1605 kHz, and  
the RF amplifiers in FM radios amplify frequencies between 88  
and 108 MHz.  
• Amplifiers are also classified as narrowband or wideband. A  
narrow-band amplifier works over a small frequency range like  
450 to 460 kHz. A wide-band amplifier operates over a large  
frequency range like 0 to 1 MHz.  
• Narrowband amplifiers are usually tuned RF amplifiers, which  
means that their ac load is a high-Q resonant tank tuned to a radio  
station or television channel. Wideband amplifiers are usually  
untuned; that is, their ac load is resistive.  
7
Amplifier Terms  
Class A Operation  
8
Amplifier Terms  
Class B Operation  
9
Class C Amplifier  
Class C Operation  
10  
Class C Amplifier  
Class C Operation  
11  
Class C Amplifier  
Class C Operation  
12  
Class C Amplifier  
Resonant Frequency  
• Class C amplifiers almost always have a circuit Q that is greater than 10.  
This means that the bandwidth is less than 10 percent of the resonant  
frequency. For this reason, class C amplifiers are narrowband amplifiers.  
13  
Class C Amplifier  
AC Collector Resistance  
• Any inductor has a series resistance RS, as indicated in Fig. 12-30a. The Q of the  
inductor is defined as:  
• Remember that this is the Q of the coil only. The overall circuit has a lower Q  
because it includes the effect of load resistance as well as coil resistance.  
• The series resistance of the inductor can be replaced by a parallel resistance Rp.  
When Q is greater than 10, this equivalent resistance is given by:  
=
RP QL /XL.  
• The ac resistance seen by the collector at resonance is  
rc = RP//RL  
• The Q of the overall circuit is given by: Q = rC / XL.  
• This circuit Q is lower than QL, the coil Q. In practical class C amplifiers, the Q of  
the coil is typically 50 or more, and the Q of the circuit is 10 or more. Since the  
overall Q is 10 or more, the operation is narrowband  
14  
Class C Amplifier  
Class C amplifier Design  
15  
Class C Amplifier  
Frequency Multiplication  
• Frequency multipliers are often used to multiply  
the frequency of the master oscillator or to  
increase the modulation index in the case of  
phase or frequency modulation.  
• The Class C frequency multiplier has the same  
schematic as the Class C power amplifier and  
operates in much the same way. The only  
difference is that the collector resonant circuit is  
tuned to the desired harmonic, suppressing all  
other harmonics.  
16  
Class C Amplifier  
Frequency Multiplication  
• The collector current can be written:  
• The amplitudes of the harmonics as afunction of the conductionangle  
17  
Class C Amplifier  
Frequency Multiplication  
18  
QUESTIONS?  
19  
APPENDIX  
20  
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