RF and Power Amplifier PDF

Chapter (5) Power Amplifiers Chapter (1) Introduction to Electronics. Chapter (2) Diodes and Applications. Chapter (3) Bipolar Junction Transistor (BJT) Circuits. Chapter (4) The Field Effect Transistor (FET) Circuits. Chapter (5) Power Amplifiers. Chapter (6) Operational amplifiers. Ch...

Chapter (5) Power Amplifiers Chapter (1) Introduction to Electronics. Chapter (2) Diodes and Applications. Chapter (3) Bipolar Junction Transistor (BJT) Circuits. Chapter (4) The Field Effect Transistor (FET) Circuits. Chapter (5) Power Amplifiers. Chapter (6) Operational amplifiers. Chapter (7) Flip Flops Circuits. Electronics & Applications Dr. Ali M. Gaballa Electronics & Applications ENE4145 Course Description  Introduction to Mechatronics o Typical components of a mechatronics system  A power amplifier is one that is designed to deliver a large amount of power to a load 2 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers 3 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers  Power amplifiers are normally used as the final stage of a communications receiver or transmitter to provide signal power to speakers or to a transmitting antenna. BJTs are used to illustrate power amplifier principles. TV channels  The power amplifier output stage has the following functions and characteristics: 1. It deals with relatively large signals ( in the range of volts and watts ). 2. It provides a low output resistance (in order not to lose gain in coupling the amplifier output to a load). 3. The transistor used in a power amplifier is a power transistor capable of handling large power or current, while not providing much voltage gain ( β is small). 4. Small-signal models are not applicable in amplifier modeling. 5. Efficiency is the most challenging factor in order not to raise the transistor junction temperature ( 150 – 200 ͦC for the Si junction to be destroyed), or to prolong life of batteries. 6. Linearity or Total Harmonic Distortion (THD) is an important factor. 4 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Classification of Amplifiers 1. According to frequency capabilities. Amplifiers are classified as Audio Amplifiers , Radio Frequency Amplifiers o AF Amplifier are used to amplify the signals lying in the audio range (20 Hz to 20 kHz ) o RF Amplifiers are used to amplify signals having very high frequency. 2. According to coupling methods. o R- C coupled amplifiers, o Transformer coupled amplifiers o Direct Coupled 5 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Classification of Amplifiers 3. According to use. a. Voltage amplifiers o Amplify the input voltage, if possible with minimal current at the output. o The power gain of the voltage amplifier is low. o The main application is to strengthen the signal to make it less affected by noise and attenuation. o Ideal voltage amp. have infinite input impedance & zero output impedance. b. Power amplifiers. o Amplify the input power, if possible with minimal change in the output voltage o Power amp. are used in devices which require a large power across the loads. o In multi stage amplifiers, power amplification is made in the final stages  Audio amplifiers and RF amplifiers use it to deliver sufficient power the load.  Servo motor controllers use power it to drive the motors. 6 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Classification of Amplifiers Parameters Voltage amplifiers Power amplifiers Current Gain Low High Voltage Gain High Low Heat Dissipation Low High Cooling Mechanism Not Required Required Transistor Size Small Large to dissipate heat Base Width Small Wide to handle higher current Beta Usually High >100 Low < 25 o Construct a comparison table between the power amplifiers and the voltage amplifiers with respect to current gain, voltage gain, heat dissipation, cooling mechanism, transistor size, base width, and beta 7 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Classification of Amplifiers o Construct a comparison table between the power amplifiers and the voltage amplifiers with respect to current gain, voltage gain, heat dissipation, cooling mechanism, transistor size, base width, and beta 8 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  In small-signal amplifiers, the main factors are usually amplification linearity and magnitude of gain.  Large-signal or power amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or other power device, typically a few watts to tens of watts.  The main features of a large-signal amplifier are the circuit’s power efficiency, the maximum amount of power that the circuit is capable of handling, and the impedance matching to the output device.  Amplifier classes represent the amount the output signal varies over one cycle of operation for a full cycle of input signal. Amplifier Classes Class A Class AB Class B Class C 9 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Class A Amplifier  Class B Amplifier Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Class AB Amplifier  Class C Amplifier Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Amplifier Classes Amplifiers are classified into classes according to their construction and operating characteristics Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Amplifier Classes Collector-current waveforms for transistors operating in (a) class A, (b) class B, (c) class AB, and (d) class C amplifier stages. Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Amplifier Classes Amplifiers are classified into classes according to their construction and operating characteristics Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Class A Amplifier: o The output signal varies for a full 𝟑𝟔𝟎° of the input signal. o Bias at the half of the supply. o The transistor conducts for a full 𝟑𝟔𝟎° of the cycle of input signal. o This requires the Q-point to be biased at a level so that 𝑰𝑪𝑸 > 𝑰𝒔 max o The efficiency is low ( maximum efficiency is 𝟐𝟓% ) 15 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Class B Amplifier : o provides an output signal varying over one-half the input signal cycle, or for 180° of signal. o The transistor conducts over only one-half the input signal cycle ( conduction angle is 𝟏𝟖𝟎°). o The dc bias point is at 0 V ( 0 dc bias current). o The efficiency is high ( maximum efficiency is 𝟕𝟖. 𝟓% ) 16 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Class AB Amplifier: o An amplifier may be biased at a dc level above the zero-base-current level of class B and above one-half the supply voltage level of class A. o The transistor conducts over a little bit greater than one-half the input signal cycle ( conduction angle is greater than 180°). o The dc bias point is greater than 0 V o Two transistors are used in a push-pull connection to restore the original signal. 17 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Class C Amplifier: o The output of a class C amplifier is biased for operation at less than 180° of the cycle and will operate only with a tuned (resonant) circuit, which provides a full cycle of operation for the tuned or resonant frequency. o The transistor is biased for operation at a small portion of the input signal cycle ( conduction angle 𝟗𝟎% ) 18 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Conduction Angle 19 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Amplifier Efficiency o The power efficiency of an amplifier, defined as the ratio of power output to power input, improves (gets higher) going from class A to class D. Class A Class AB Class B Class C Operating cycle 360° 180° to 360° 180° Less than 180° Power efficiency 25% to 50% Between 25% 78.5% > 90% (50%) and 78.5% 20 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Amplifier Classes  Compare between the following power amplifiers: class A, class B and Class C from point of view of: basic structure, operating point, output waveform and the conduction angle. o Output Waveform o Conduction Angle. 𝟑𝟔𝟎° 𝟏𝟖𝟎° 𝟗𝟎° 21 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Basic class A amplifier operation. o A class A power amplifier operates entirely in the linear region of the transistor’s characteristic curves. o The transistor conducts during the full of the input cycle. o The Q-point must be centered on the load line for maximum class A output signal swing. o The maximum efficiency of a class A power amplifier is 25 percent. o Output is shown 180° out of phase with the input (inverted). 22 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Basic class A amplifier operation. o Maximum class A output occurs when the Q-point is centered on the ac load line 23 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Basic class A amplifier operation. 24 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Basic class A amplifier operation. 25 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Series-fed Class A Amplifier o Fig. below shows a typical class-A amplifier and its input and output waveforms. In 𝑽 this case, the transistor is biased at 𝑽𝑪𝑬 = 𝑪𝑪 , which is midway between saturation 𝟐 and cutoff, and which permits maximum output voltage swing. The output can vary through (approximately) a full VCC volts, peak-to-peak. The output is in the transistor's active region during a full cycle (360º) of the input sine wave. 26 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Series-fed Class A Amplifier o DC Bias Operation  The dc base-bias current 𝑰𝑩 = 𝑽𝑪𝑪 − 𝟎. 𝟕 Τ𝑹𝑩  The collector current 𝑰𝑪 = 𝜷 𝑰𝑩  The collector–emitter voltage 𝑽𝑪𝑬 = 𝑽𝑪𝑪 − 𝑰𝑪 𝑹𝑪 27 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Series-fed Class A Amplifier o AC Operation 28 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Series-fed Class A Amplifier o Power Considerations  The power then drawn from the supply is  The ac power delivered to the load (RC) 29 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Series-fed Class A Amplifier o Power Considerations  Efficiency and Maximum Efficiency 30 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Series-fed Class A Amplifier o Advantages of Class A Amplifiers The advantages of Class A power amplifier are as follows :  The current flows for complete input cycle  It can amplify small signals  The output is same as input  No distortion is present o Disadvantages of Class A Amplifiers The advantages of Class A power amplifier are as follows :  Low power output  Low collector efficiency 31 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Example : Calculate the input power, output power, and efficiency of the amplifier circuit in Fig. below for an input voltage that results in a base current of 10 mA peak. Solution 32 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Capacitor-Coupled Class-A Power Amplifiers Fig. below shows the output side of a class-A amplifier with capacitor-coupled load 𝑅𝐿. Also shown are the dc and ac load lines that result. 33 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Capacitor-Coupled Class-A Power Amplifiers o In this case, the average power delivered to the load is 𝟏 𝑷𝑳 = 𝑰𝟐𝑷𝑳 𝑹𝑳 , where 𝑰𝑷𝑳 is the peak ac load current 𝟐 o The average power from the dc source is computed in the same way as for the series-fed amplifier 𝑷𝑺 = 𝑽𝑪𝑪 𝑰𝑸 o The efficiency is 𝑷𝑳 𝑰𝟐𝑷𝑳𝑹𝑳 𝜼= = 𝑷𝑺 𝟐𝑽𝑪𝑪 𝑰𝑸 o Recall that maximum output swing can be achieved by setting the Q-point in the center of the ac load line, at 𝑽 𝑰𝑸 = 𝑪𝑪 𝑹𝑪 +𝒓𝑳 o The peak collector current under those circumstances is 𝑽𝑪𝑪 𝑹𝑪 +𝒓𝑳 o Neglecting the transistor output resistance, the portion of the collector current that flows in 𝑹𝑳 is, by the current-divider rule, 𝑽𝑪𝑪 𝑹𝑪 𝑰𝑷𝑳 = 𝑹𝑪 +𝒓𝑳 𝑹𝑪 +𝒓𝑳 34 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Capacitor-Coupled Class-A Power Amplifiers o The average ac power in the load resistance 𝑅𝐿 is then 2 1 2 1 𝑉𝐶𝐶 𝑅𝐶 𝑃𝐿 = 𝐼𝑃𝐿 𝑅𝐿 = 𝑅𝐿 2 2 𝑅𝐶 + 𝑟𝐿 𝑅𝐶 + 𝑟𝐿 o The average power supplied from the dc source is 2 𝑉𝐶𝐶 𝑃𝑆 = 𝑉𝐶𝐶 𝐼𝑄 = 𝑅𝐶 +𝑅𝐿 o Therefore, the efficiency under the conditions of maximum possible undistorted output is o The efficiency depends on both 𝑅𝐶 and 𝑅𝐿. In practice, 𝑅𝐿 is a fixed and known value of load resistance, will the value of 𝑅𝐶 is selected by the designer. 𝑑𝜂  =0 → 𝑅𝐶 = 2 𝑅𝐿 → 𝜂 𝑚𝑎𝑥 = 0.0858 𝑑 𝑅𝐶 𝑅  𝑅𝐶 = 𝑅𝐿 = 𝑅 , 𝑟𝐿 = → 𝜂 max 𝑝𝑜𝑤𝑒𝑟 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 = 0.0833 2 35 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Exercise : The class-A amplifier shown in figure below is biased at 𝑽𝑪𝑬 = 𝟏𝟐 𝑽. The output voltage is the maximum possible without distortion. Find (a) the average power from the dc supply, (b) the average power delivered to the load, (c) the efficiency, and (d) the collector efficiency. [Answers: (a) 5.76 W, (b) 0.36 W, (c) 0.0625, (d) 0.125] 36 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: A form of class A amplifier having maximum efficiency of 50% uses a transformer to couple the output signal to the load as shown in Fig. below o Transformer Action A transformer can increase or decrease voltage or current levels according to the turns ratio, as explained below. In addition, the impedance connected to one side of a transformer can be made to appear either larger or smaller (step up or step down) at the other side of the transformer, depending on the square of the transformer winding turns ratio. The following discussion assumes ideal (100%) power transfer from primary to secondary, that is, no power losses are considered. 38 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: o Voltage Transformation 𝑽𝟐 𝑵𝟐 = 𝑽𝟏 𝑵𝟏 o Current Transformation 𝑰𝟐 𝑵𝟏 = 𝑰𝟏 𝑵𝟐 39 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: o Impedance Transformation 𝟐 𝑹′𝑳 𝑹𝟏 𝑵𝟏 𝑵𝟏 = = ,𝒂 = → 𝑹′𝑳 = 𝒂𝟐 𝑹𝑳 𝑹𝑳 𝑹𝟐 𝑵𝟐 𝑵𝟐 o The reflected impedance (𝑹′𝑳 ) is related directly to the square of the turns ratio. If the number of turns of the secondary is smaller than that of the primary, the impedance seen looking into the primary is larger than that of the secondary by the square of the turns ratio. 40 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: o Operation of Amplifier Stage 41 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: o Signal Swing and Output AC Power o The ac power developed across the transformer primary can then be calculated using 42 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: o For the ideal transformer, the voltage delivered to the load is 𝑵 𝑽𝑳 = 𝑽 𝟐 = 𝟏 𝑽𝟏 𝑵𝟐 o The power across the load can then be expressed as 𝑉𝐿2(𝑟𝑚𝑠) 𝑃𝑆 = 𝑅𝐿 o The load current is 𝑵𝟐 𝑰𝑳 = 𝑰𝟐 = 𝑰 𝑵𝟏 𝑪 o The output ac power then calculated using 𝑃𝐿 = 𝐼𝐿2 (𝑟𝑚𝑠) o Efficiency 43 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: Example : Calculate the ac power delivered to the 𝟖𝛀 speaker for the circuit of Fig. below. The circuit component values result in a dc base current of 6 mA, and the input signal (𝑽𝒊 ) results in a peak base current swing of 4 mA. 44 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: Solution : Dc base current of 𝟔 𝒎𝑨, and the input signal (𝑽𝒊 ) results in a peak base current swing of 𝟒 𝒎𝑨. 45 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: Solution 46 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class A Amplifiers  Transformer-Coupled Class-A Power Amplifiers: Solution 47 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B and Class AB Push – Pull amplifiers  A class B amplifier operates in the linear region for half of the input cycle and it is in cutoff for the other half.  The Q-point is at cutoff for class B operation..  The dc bias point is at 0 V ( 0 dc bias current).  Class B amplifiers are normally operated in a push-pull configuration in order to produce an output that is a replica of the input.  The maximum efficiency of a class B amplifier is 79%. 48 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B and Class AB Push – Pull amplifiers  Class B amplifier: When an amplifier is biased at cutoff so that it operates in the linear region for 180o of the input cycle and is in cutoff for 𝟏𝟖𝟎𝒐  Class AB amplifiers: are biased to conduct for slightly more than 𝟏𝟖𝟎𝒐  Both are more efficient than a class A amplifier;  A disadvantage of class B or class AB is that it is more difficult to implement the circuit in order to get a linear reproduction of the input waveform.  The term push-pull refers to a common type of class B or class AB amplifier circuit in which two transistors are used on alternating half-cycles to reproduce the input waveform at the output. 49 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Operation o The class B amplifier is biased at the cutoff point so that  𝑰𝑩 = 𝟎 , 𝑰𝑪𝑸 = 𝟎 , 𝑽𝑪𝑬𝑸 = 𝑽𝑪𝑬(𝒄𝒖𝒕𝒐𝒇𝒇) = 𝑽𝑪𝑪 50 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Operation o It is brought out of cutoff and operates in its linear region when the input signal drives the transistor into conduction. o The circuit only conducts for the positive half of the cycle. o Can not amplify the entire cycle Emitter Follower Circuit 51 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Push – Pull Operation o To amplify the entire cycle, it is necessary to add a second class B amplifier that operates on the negative half of the cycle. o The combination of two class B amplifiers working together is called push-pull operation 52 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Push – Pull Operation o There are two common approaches for using push-pull amplifiers to reproduce the entire waveform. 53 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Push – Pull Operation o Transformer Coupling The input transformer thus converts the input signal to two out-of-phase signals for the two 𝒏𝒑𝒏 transistors. o The output transformer combines the signals by permitting current in both directions, even though one transistor is always cut off. 54 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Push – Pull Operation o Transformer Coupling 55 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Push – Pull Operation o Complementary Symmetry Transistors  The figure shows one of the most popular types of push-pull class B amplifiers using two emitter-followers and both positive and negative power supplies.  This is a complementary amplifier because one emitter-follower uses an 𝒏𝒑𝒏 transistor and the other a 𝒑𝒏𝒑 , which conduct on opposite alternations of the input cycle. 56 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class B Amplifiers  Class B Push – Pull Operation o Crossover Distortion  When the dc base voltage is zero, both transistors are off and the input signal voltage must exceed VBE before a transistor conducts.  Because of this, there is a time interval between the positive and negative alternations of the input when neither transistor is conducting, as shown in Figure.  The resulting distortion in the output waveform is called crossover distortion. 57 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class AB Amplifiers  A class AB amplifier is biased slightly above cutoff and operates in the linear region for slightly more than of the input cycle.  Class AB eliminates crossover distortion found in pure class B. 58 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class AB Amplifiers  Biasing the Push-Pull Amplifier for Class AB Operation o To overcome crossover distortion, the biasing is adjusted to just overcome the VBE of the transistors o In class AB operation, the push-pull stages are biased into slight conduction, even when no input signal is present. o This can be done with a voltage-divider and diode arrangement, as shown o Using equal values of R1 and R2 the positive and negative supply voltages forces the voltage at point A to equal 0 V and eliminates the need for an input coupling capacitor o When the diode characteristics of D1 and D2 are closely matched to the characteristics of the transistor BE junctions, the current in the diodes and the current in the transistors are the same; ((current mirror.)) o The diode current will be the same as 𝑰𝑪𝑸 𝑽𝑪𝑪 −𝟎.𝟕 𝑽 𝑰𝑪𝑸 = 𝑹𝟏 59 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class AB Amplifiers  AC Operation o The Q-point is slightly above cutoff. (In a true class B amplifier, the Q-point is at cutoff.) o The ac cutoff voltage is at VCC 𝑽 o The ac saturation current is: 𝑰𝐜(𝐬𝐚𝐭) = 𝑪𝑪 𝑹𝑳 o In class A , the Q-point is near the middle and there is significant current in the transistors even with no signal. o In class B , when there is no signal, the transistors have only a very small current and therefore dissipate very little power. o Thus, the efficiency of a class B amplifier can be much higher than a class A amplifier. The ideal maximum peak output voltage is 𝑽𝒐𝒖𝒕(𝒑𝒆𝒂𝒌) ≅ 𝑽𝑪𝑬𝑸 ≅ 𝑽𝑪𝑪 𝑽𝑪𝑪 The ideal maximum peak current is 𝑰𝒐𝒖𝒕(𝒑𝒆𝒂𝒌) ≅ 𝑰𝑪(𝒔𝒂𝒕) ≅ 𝑹𝑳 60 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class AB Amplifiers  Single-Supply Push-Pull Amplifier o The circuit operation is the same as that described previously, except the bias is set to force the output emitter voltage to be 𝑽𝑪𝑪 𝑽𝑪𝑬𝑸 𝑽𝒐𝒖𝒕(𝒑𝒆𝒂𝒌) ≅ 𝑽𝑪𝑬𝑸 = 𝑰𝒐𝒖𝒕(𝒑𝒆𝒂𝒌) ≅ 𝑰𝑪(𝒔𝒂𝒕) = 𝟐 𝑹𝑳 61 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class AB Amplifiers  Maximum Output Power 62 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class AB Amplifiers  DC Input Power  Efficiency 63 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class C Amplifiers  Class C Amplifier: o The output of a class C amplifier is biased for operation at less than 180° of the cycle and will operate only with a tuned (resonant) circuit, which provides a full cycle of operation for the tuned or resonant frequency. o The transistor is biased for operation at a small portion of the input signal cycle ( conduction angle 90% ) 64 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class C Amplifiers 65 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class C Amplifiers 66 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class C Amplifiers o Class C amplifiers are biased so that conduction occurs for much less than 180o o Class C amplifiers are more efficient than either class A , B, or AB o The output amplitude is a nonlinear function of the input, so class C amplifiers are not used for linear amplification. o They are generally used in radio frequency (RF) applications, including resonance circuits  Basic Class C Operation o A class C amplifier is normally operated with a resonant circuit load, so the resistive load is used only for the purpose of illustrating the concept. o The ac source voltage has a peak value that exceeds the barrier potential of the base- emitter junction for a short time near the positive peak of each cycle, 67 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class C Amplifiers  Basic Class C Operation o During this short interval, the transistor is turned on. o The power dissipation of the transistor in a class C amplifier is low because it is on for only a small percentage of the input cycle. o The power dissipation during the on time is 𝑃𝐷(𝑜𝑛) = 𝐼𝑐(𝑠𝑎𝑡) 𝑉𝑐𝑐(𝑠𝑎𝑡) o The power dissipation averaged over the entire cycle is 𝒕𝒐𝒏 𝒕𝒐𝒏 𝑷𝑫(𝒂𝒗𝒈) = 𝑷𝑫(𝒐𝒏) = 𝐼 𝑉 𝑻 𝑻 𝑐(𝑠𝑎𝑡) 𝑐𝑐(𝑠𝑎𝑡) 68 Electronics & Applications Dr. Ali M. Gaballa Chapter (5) Power Amplifiers Class C Amplifiers  Tuned Class C Operation o Because the collector voltage (output) is not a replica of the input, the resistively loaded class C amplifier alone is of no value in linear applications. o It is therefore necessary to use a class C amplifier with a parallel resonant circuit (tank), as shown o The short pulse of collector current on each cycle of the input initiates and sustains the oscillation of the tank circuit so that an output sinusoidal voltage is produced 69 Electronics & Applications Dr. Ali M. Gaballa