11: AUDIO AMPLIFIER I. INTRODUCTION

Transcription

1 11: AUDIO AMPLIFIER I. INTRODUCTION The properties of an amplifying circuit using an op-amp depend primarily on the characteristics of the feedback network rather than on those of the op-amp itself. A typical feedback network consists of resistors and capacitors. Because these components are available in conditions of high precision and low drift, the accuracy and stability of circuits using them is very high. 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 tolerance and variation, 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 op-amp as a small-signal AC amplifier. The input signal comes from the voltage output of a microphone. Usually microphones have very high internal electrical impedance and therefore must be connected to a highimpedance circuit in order to maximize to power transfer to this circuit, and the high input impedance of the op-amp (several Meg-Ohms) is ideal for such an application. The op-amp output could drive the audio speaker directly so that the signal from the microphone, amplified by the op-amp, would be audible. However, the speaker has very Acoustic Lab 11: page 1

2 low impedance (8 Ohms) in the audio frequency range (like most HI-FI speakers), while the op-amp output impedance is approximately ten times this value. Therefore, the direct connection of the speaker to the op-amp output constitutes an excessive load, and the opamp output signal will be heavily attenuated. Thus, a push-pull (connected in a unity-gain emitter-follower configuration) is interposed between the op-amp and the speaker. The op-amp sees the several-kilo-ohm input impedance of the driver circuit, while the speaker is placed directly in the very-low-impedance output of the driver transistor. Wire up the above circuit and make sure it is working. Make a few gain measurements on the amplifier at some mid-range frequency (say f = 1 khz) for the following settings of the variable resistor: maximum, minimum and one intermediate setting. 1. The variable 10 kω feedback from the pre-amplifier can also come from the preamplifier output port #6. Try it, and write down your comment here: 2. From the result of your lab#10, what are the minimum and maximum theoretical gains of this circuit with feedback resistance 10kΩ? 3. What is the range of the measured gain of the amplifier? Does the measured gain agree with the theoretical gain calculated above? 4. Observe the maximum undistorted output voltage delivered to the speaker, or observed on the oscilloscope by increasing the input voltage. What is it? Acoustic Lab 11: 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 half-power frequencies. The bandwidth is then f 2 f 1. The multiplier was chosen because at this level the output power is half the mid-band power output, that is, at mid-frequency. Measure the frequency band width of the amplifier for a gain setting of 10kΩ. Remember that it is not necessary to take a lot of data in the region where the response is flat. Gain versus frequency for 741 plus transistor Frequency (Hz) V in V out db = 20 log V out /V in Plot your data on graph paper and find the bandwidth. Note that if you plot (Vout/Vin) vs frequency, you should use log-log graph paper, and if you plot db vs frequency, you should use the semi-log graph paper! Does this circuit satisfy the minimum requirements (20 hz 20 khz) of a hi-fidelity audio amplifier? Acoustic Lab 11: page 3

4 MONOLITHIC AUDIO AMPLIFIER In the previous circuit of an audio amplifier a general purpose op-amp is connected in series to a power booster (transistor connected in an emitter-follower configuration). Power amplifiers are also commercially available in monolithic form. Although power amplifiers differ from general-purpose op-amps in delivering various amounts of power, they are nearly as compact. In the following we will use the ECG704 or LM380 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 or LM380 as an audio power amplifier. This amplifier requires very few external components (and absolutely no 10 kω 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. Note the polarities of the electrolytic capacitors having values greater than 1 µf. Most electrolytic capacitors are polarized and may catastrophically fail if voltage is incorrectly applied. This is because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short Acoustic Lab 11: page 4

5 circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst. Measure the frequency band width of the amplifier for a gain setting of 10 and 40. Record your results in the Table below. If necessary, reduce input voltage to avoid distortions. Gain versus frequency for ECG704 or LM380 audio amplifier Frequency (Hz) V in V out db = 20 log V out /V in Plot your data on graph paper and find the bandwidth. Note that if you plot (Vout/Vin) vs frequency, you should use log-log graph paper, and if you plot db vs frequency, you should use the semi-log graph paper! 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? Acoustic Lab 11: page 5

6 (Plot db vs frequency, where frequency is in log scale, and db in linear scale) Acoustic Lab 11: page 6