YEDITEPE UNIVERSITY DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EE333 ANALOG ELECTRONIC CIRCUITS LABORATORY FALL

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1 YEDITEPE UNIVERSITY DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EE333 ANALOG ELECTRONIC CIRCUITS LABORATORY FALL EXPERIMENT 7: POWER AMPLIFIERS Objective: Studying some power amplifier classes and their design considerations. Equipment: Oscilloscope Function Generator DC Power Supply BC 547 NPN transistor; BC 557 PNP transistor. 1N4001 Diode Resistor and Capacitor series. General Information: An amplifier receives a signal from some pick up transducer or other input source and provides a larger version of the signal to some input device or to another amplifier stage. An input transducer signal is generally small (a few millivolts from a cassette or CD input, or a few microvolts from an antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-handling device). Power amplifiers, primarily, provide sufficient power to an output load to drive a speaker or other power device, typically a few watts to tens of watts. One method of categorizing amplifiers is by classes, which depends on their conduction angles. The efficiency of a power amplifier depends on its class of operation. Briefly; Class A: The transistor in class A conducts for the entire cycle of the input signal; that is the conduction angle is 360 o. As shown in Fig. 7.1a, the Q-point is chosen at the mid-way between Vcc and VCEsat. Class B: The transistor in class B conducts for only half of the cycle of the input sine wave, resulting in a conduction angle of 180 o as shown in Fig. 7.1b. Therefore the class B amplifier is biased at zero DC current. It has to be used as a push-pull stage for linear amplification. Class AB: It is an intermediate class between A and B. For class AB operation the resulting conduction angle is between 180 o and 360 o. It has to be used as a push-pull stage for linear amplification. Class C: The transistor in class C conducts for an interval shorter than a half-cycle; that is the conduction angle is less than 180 o. It will operate only with a tuned circuit which provides a full cycle of

2 operation for the tuned or resonant frequency. Class C amplifiers are used for radio-frequency (RF) power amplification. Class D: This operating class is a form of amplifier operation using pulse signals. The signal to be amplified pulse-width modulates (PWM) a periodic pulse train before being applied to the class-d amplifier. This high-frequency pulse train is suppressed at the output of the class-d amplifier. Figure 1 Amplifier operating classes Figure 2 Comparison of amplifier classes Figure 3 Power considerations for different amplifier classes For efficiency of a power amplifier is a measure of the amount of ac power delivered to load. η = P O AC P DC. 100 [%]

3 Pre-Laboratory Work: 1. Design a class AB amplifier with efficiency not less than 60%. The voltage gain should be at least 50. In your design use VCC=12V and the DC bias current for voltage amplifier stage is ICQ=3mA. You can choose transistor parameters arbitrarily. For the circuit you designed, find the efficiency and voltage gain. Explain shortly how you designed the circuit. 2. Find the maximum input voltage that does not cause distortion for the circuit you have designed. 3. Simulate your design on computer and find the desired values and results in steps 1 and 2. Procedure: Figure 4 Class A Amplifier R1=120 kω; R2=10 kω; RC=820 Ω; RL= 1.2 kω; RE=22 Ω CS=1 μf; CE=4.7 μf VCC=12 V Vin = 100mVp-p sine wave with a frequency of 1kHz 1. Build the circuit in Fig. 4. Draw the input and output signals you see on the oscilloscope screen. 2. Find the voltage gain of the circuit and find the maximum input voltage that does not cause distortion at the output. 3. Find the maximum efficiency of the circuit.

4 Figure 5 Complementary symmetry class B push-pull circuit RL= 1.2 kω ; CS= 1μF VCC= 8V, frequency= 1 khz 4. Build the circuit in Fig. 5. Note that you should apply an input signal (Vin) having an amplitude of more than the cut-in voltages of the transistors. Draw the input and output signals you see on the oscilloscope screen and observe the crossover distortion. Find the maximum efficiency of the circuit. What is the minimum amplitude of the signal that must be applied at the input? Figure 6 Pin connection diagram for BC 547B and BC557B

5 Figure 7 AB class push-pull amplifier R1=120 kω; R2=10 kω; RC=820 Ω; RL=1.2 kω; RE=22 Ω CS=1 μf; C2=1 μf Vin = 100 mvp-p sine wave with a frequency of 1 khz 5. Build the circuit in Fig Observe the input and output signals. Find the maximum efficiency of the circuit. Comment on the efficiency values of each type of power amplifier schemes.