N a t i o n a l s e n i c o n d u c to r August 2000 Feature

N a t i o n a l s e n i c o n d u c to r August 2000 Features The LM386 is a power amplifier designed for use in low volt-R Battery operation age consumer applications. The gain is internally set to 20 to a minimum external parts keep external part count low, but the addition of an external a Wide supply voltage range: 4V-12V or 5V-18V resistor and capacitor between pins 1 and 8 will increase the a Low quiescent current drain: 4mA gain to any value from 20 to 200. E Voltage gains from 20 to 200 The inputs are ground referenced while the output automati - - Ground referenced input cally biases to one-half the supply voltage. The quiescent , - Selfcentering output quiescent voltage power drain is only 24 milliwatts when operating from a 6 volt n Low distortion: 0.2% (Av = 20, Vs = 6V, RL = 82, P. = supply, making the LM386 ideal for battery operation. 125mW, f = 1kHz) E Available in 8 pin MSOP package LM386 Low D Voltage Audio Power Amplifier General Description Applications I AM-FM radio amplifiers A Portable tape player amplifiers Intercoms a TV sound systems Line drivers Ultrasonic drivers a Small servo drivers n Power Converters Equivalent Schematic and Connection Diagrams O Small Outline, Molded Mini Small Outline, and Dual-In-Line Packages YPASS GAIN GAIN GAIN GAIN -HU INPUT BYPASS WHITE 1.35k +INPUT INPUT INPUT GND DS006976-2 GND DSO06976-1 Top View Order Number LM386M-1 LM386MM-1, LM386N-1, LM386N-3 or LM386N-4 See NS Package Number M08A, MUA08A or NO8E © 2000 National Semiconductor Corporation DS006976 www.national.com

Solution

3    a)   Given the resitance of the output i.e.speaker = 8 ohms

   operating voltage = 9v

               current = V/I = 9/8 = 1.125 A

                  power = I²R = 10 watt

     b)    To calculate input power

             given current = 0.13A,

                          voltage = 9v

                          resistance = V/I = 9/0.13 =69ohm

                          power = I² R = 1.166 watt

                      efficiency = output power / input power   = 1.0 /1.100 = 8.5

C) From the graph in datasheet we can get the voltage across the 8 ohm speaker is

               V_rms = peak voltage * 0.7071 = 6.36 volts

               Vpk or V_pp (peak to peak voltage) = 9 volts

d)    In the above case the power dissipation is 1.5 watt

e) Max power = V_rms² / R = 0.79 watt            

f)        However the resistance is reduced the voltage drop across the collector to emitter is same and the

        peak to peak volatge is same,however the heat dissipation will be more as the voltage drop at                   collector emitter junction is more.