15: AUDIO AMPLIFIER I. INTRODUCTION

Transcription

1 I. INTRODUCTION 15: AUDIO AMPLIFIER A few weeks ago you saw that the properties of an amplifying circuit using an opamp depend primarily on the characteristics of the feedback network rather than on those of the opamp itself. Since a typical feedback network consists of resistors and capacitors, the accuracy and stability of circuits using them is very high, because these components are available in conditions of high precision and low drift. Audio receiver systems consist of a number of amplifying stages since the gain of each stage has to be kept low due to the frequency range required for audio signals. Because of thermal drift, component tolerances and variations, a DC level is produced. In order to prevent the amplification of such DC levels, coupling capacitors must be used between the stages. The coupling capacitor not only blocks the DC voltage but also sets a lowfrequency cutoff limit; it acts as a filter. Such an amplifier with a coupling capacitor is called an AC amplifier. The following figure shows a simple amplifier using an opamp as a smallsignal AC amplifier. The input signal comes from the voltage output of a microphone. Usually microphones have a very high internal electrical impedance and therefore must be connected to a highimpedance circuit in order to maximize to power transfer to this circuit. The high input impedance of the opamp (several MegOhms) is ideal for such an application. 10 µf 10 kž 22 kž input R (200 kž max) (from generator or microphone) kž 10 µf 10 kž B 15 V C 2N1725 E Ideally, the opamp output would drive the audio speaker directly so that the signal from the microphone, amplified by the opamp, would be audible The speaker used, however, has a very low impedance (8 Ohms) in the audio frequency range (like most HIFI speakers), while the opamp output impedance is approximately ten times this value. Therefore, the direct connection of the speaker to the opamp output constitutes an excessive load, and the opamp output signal will be heavily attenuated. Thus, a power P108 Lab 15: page 1

2 transistor driver (connected in a unitygain emitterfollower configuration) is interposed between the opamp and the speaker. The opamp sees the severalkiloohm input impedance of the driver circuit, while the speaker is placed directly in the verylowimpedance output of the driver transistor. Wire up the above circuit and make sure it is working (note the polarities of the electrolytic capacitors, which have values greater than 1 µf). For this purpose use the 2002 Function Generator. Now replace the function generator by the microphone. 1. Set the gain of the opamp stage at its maximum and observe the input and output wave forms on the oscilloscope while whistling a tone into the microphone. What is the approximate maximum amplitude of the microphone output voltage (observed at the input to the opamp stage)? 2. What is the function of the variable feedback resistor R in the amplifier stage? Now replace the microphone with the function generator and make a few gain measurements on the amplifier at some midrange frequency (lets say f = 1 khz) for the following settings of the variable resistor: maximum, minimum and one intermediate setting. Does the measured range of gains of the amplifier agree with the theoretical gains using the minimum and maximum values of the feedback resistance R 22k? What are the maximum and minimum theoretical gains? Observe the maximum undistorted output voltage delivered to the speaker by increasing the input voltage. What is it? P108 Lab 15: page 2

3 BANDWIDTH The bandwidth of an amplifier is defined as the range (band) of frequencies for which the gain remains essentially constant. To fix the frequency boundaries, (V out /V in ) mid was chosen to be the gain cutoff level. The corresponding frequencies f 1 and f 2 are generally called the cutoff, band, or halfpower frequencies. The bandwidth is then f 2 f 1. The multiplier = 1/v2 was chosen because at this level the output power is half the midband power output, that is, at midfrequencies. Measure the frequency band width of the amplifier for a gain setting of 10. Remember that it is not necessary to take a lot of data in the region where the response is flat. Does this circuit satisfy the minimum requirements (20 Hz 20 khz) of a hifidelity audio amplifier? Gain versus frequency for 741 plus transistor frequency V in V out db = 20 log V out /V in P108 Lab 15: page 3

4 MONOLITHIC AUDIO AMPLIFIER In the previous circuit of an audio amplifier a general purpose opamp is connected in series to a power booster (transistor connected in an emitterfollower configuration). Power amplifiers are also commercially available in monolithic form. Although power amplifiers differ from generalpurpose opamps in delivering various amounts of power, they are nearly as compact. In the following we will use the ECG 704 power audio amplifier. The 14 pins on the chip are hooked as follows: open 15 V 1 14 Inputs Output 7 3,4,5,10,11,12 Ground The following figure shows the simplest and most basic application of the ECG704 as an audio power amplifier. 15 V 0.1 µf 10 µf input (from generator or microphone) 1 Mž , µf This amplifier requires very few external components (and absolutely no 10 k? resistors!). Although the gain of the LM380 is internally fixed at 50, it can be changed with the use of external components. In the present configuration, variable gains up to 50 are obtained with the use of the potentiometer across the two input terminals. P108 Lab 15: page 4

5 Measure the frequency band width of the amplifier for a gain setting of 10 and 40. If necessary, reduce input voltage to avoid distortions. Gain versus frequency for ECG704 audio amplifier frequency V in V out db = 20 log V out /V in Compare it with the previous results. How does the monolithic stack up against your homebrewed amp? Observe the maximum undistorted output voltage delivered to the speaker by increasing the input voltage. What is it? P108 Lab 15: page 5