E7.1.OBJECTIVEEXPERIMENT E7 Audio Amplifiers E7.1. OBJECTIVE The purpose of the present experiment is to introduce the student to audio amplifiers through the hands-on process of building one using the popular audio amplifier integrated circuit (IC) LM386 and then measuring the amplifier performance, including its frequency response. E7.2. INTRODUCTION The traditional method of designing and building an audio amplifier was based on the use of discrete transistors. However, this task is enormously simplified if one uses an audio amplifier IC such as the LM386 used in the present experiment, which is universally popular around the world. The LM386 IC is manufactured by the National Semiconductor Corporation whose web site www.national.com you may want to visit for more information. The LM386 IC makes the design and implementation of an audio amplifier very easy. Additionally, the design of the LM386 IC is optimized to a level that is not readily duplicated if one starts designing an audio amplifier using discrete transistors. The use of the LM386 IC provides a black-box approach, which is particularly suitable to students who are beginners to electrical engineers. In a nutshell, the student will simply put together the specified circuit on the breadboard and then make terminal measurements of input and output voltages. The LM386 IC is a low voltage audio power amplifier with a default voltage gain of 20, which can, however, be increased to any value between 20 and 200 by simply placing an external series RC circuit between pins 1 and 8. In the amplifier circuit, the output AC power is clearly larger than the input AC power reflecting the voltage, and thus power, gain. The increase in AC power in going from the input terminals of the amplifier to the output terminals is provided by the DC voltage source biasing the circuit. In the case of 1the LM386 IC, the operating DC power source must have a voltage between 4V and 12V and supply a current that is typically 4 mA. These values of DC voltage and current are readily provided by a battery so that an audio amplifier based on the LM386 IC is very much amenable to battery operation. The applications of the LM386 audio amplifier include: • AM and FM radio amplifiers, • Portable tape player amplifiers, • Intercoms, • TV sound systems, • Ultrasonic drivers, and • Small servo drivers. E7.3. PIN DIAGRAM FOR LM386 IC The LM386 integrated circuit uses the Dual-In-Line Package housing with 8 pins labeled 1 to 8 as shown in the top view of the IC in Fig. E7.1. Fig. E7.1. Top view of the LM 386 IC 2Notice the semicircular indentation at the top of the schematic in Fig. E7.1, which provides a reference for the identification of the eight pin #s. There are two input terminals, i.e., pin # 2 (- or inverting input terminal) and pin # 3 (+ or non-inverting terminal). In most applications including the present audio amplifier application, the inverting terminal (pin # 2) grounded, i.e., it is connected to pin # 4, which is a permanent ground point. The positive terminal of the DC voltage source is connected to pin #6 (labeled VSS) while its negative terminal is grounded, i.e., connected to pin # 4. Pin # 5 provides the output signal. If pins 1 and 8 are the gain control pins. If nothing is connected to pins 1 and 8, i.e., they are left open-circuited, the voltage gain of the amplifier is 20. If one wants a voltage gain of any value lying between 20 and 200, an appropriate series RC circuit must be connected between pins 1 and 8. In the present experiment, pin # 7 is unused. The actual circuit inside the LM386 IC contains 10 transistors and is too complex to be discussed. The circuit may be seen at the web site of the National Semiconductor Corporation where detailed specifications of the IC as well as various application notes are given. E7.4. PRELAB Before coming to the lab, show how you would connect up the audio amplifier circuit of Fig. E7.2 on a schematic of the breadboard. According to the manufacturer’s specifications, this circuit has a voltage gain of 20 if the switch S1 is open and 200 if the switch S1 is closed. For any value of voltage gain between 20 and 200, a resistor of appropriate resistance is inserted between the switch S1 and the 10µF electrolytic capacitor. 3E7.5. EXPERIMENT 1. In the lab, connect up the circuit of Fig. E7.2 without the speaker and without the 10µF capacitor so that the voltage gain of the amplifier should be 20. Using the oscilloscope, measure the frequency response of the voltage gain function Vo/Vi over the audio frequency band. Is the voltage gain in agreement with manufacturer’s specification? Can you comment on the bandwidth of the amplifier? 2. Check if, by connecting the 10µF capacitor, between pin #1 and pin # 8, the voltage gain does go up to 200. 3. With the speaker connected to the output of the circuit (Fig. E7.2) and switch S1 closed, apply the following inputs and state qualitatively what you hear on the speaker for each case: a. A sinusoidal voltage signal of different frequencies in the audio band b. A square wave voltage signal c. Your voice on a microphone connected to the input terminals of the amplifier (OPTIONAL). Fig. E7.2. Audio amplifier circuit employing the LM386 IC. 4Fig. E7.3. Schematic of breadboard 5IMPROVED AUDIO AMPLIFIER CIRCUIT The original circuit of the audio amplifier was kept as simple as possible in terms of parts count. While the circuit works, the output signal may be found to be rather noisy or “hummy.” The following modified circuit significantly reduces the noise. Fig. E7.5. Improved audio amplifier circuit Brief remarks on the changes are given below for student’s benefit but it is not expected that the student may appreciate the reasons for the changes. The AC signal generator available in the lab cannot go down enough to prevent saturation effects. Hence a potential divider is used to further reduce the signal level of the input into the amplifier. 6The 100nF capacitor connected between VSS and ground simply provides AC decoupling of the DC voltage source, i.e., any AC signal picked up by DC source is shorted to ground by the capacitor. The high-frequency decoupling circuit at the output end also helps in reducing noise in the output signal. When doing the frequency response, the student is to generate is a plot of voltage gain as a function of frequency. The phase response