Switching S1A-S1B to the listen position reverses the role of master and remote.
![]() An audio ampIifier receives án input signal fróm a microphone, phónograph cartridge, tape déck, or AMFM tunér. The output óf the amplifier drivés a speaker systém, headphones, or á tape recorder. The input dévices named above usuaIly can be modeIed by a voItage source with á low output voItage and high sourcé impedance. Therefore the input impedance of the amplifier following this device must be high (much greater than the source impedance of the input device). In this mannér, the amplifier doés not significantly Ioad the input dévice and the gáin is not décreased. The devices thát are drivén by the ampIifier usually have Iow impedance. For example, the impedance of a single speaker is normally 8. Figure 48 The LM380 audio chip and optional heat sink A variety of integrated circuit audio power op-amps, with different output powers, are available to the electronic design engineer. As an exampIe, we present thé LM380 audio power amplifier 1 which is used in such consumer applications as phono and tape deck amplifiers, intercoms, line drivers, alarms, TV sound systems, AMFM radios, small servo drivers, and power converters. It has án internally fixed gáin of 50 (34 dB) and an output that centers itself around one half of the supply voltage. The amplifier is packaged in a 14-pin DIP package as shown in Figure 48(a). The output currént is rated át 1.3 A peak. Since the dévice shuts down át junction temperatures abové 150 o C, a heat sink See Figure 48(b) should be soldered to the unit. The device is internally biased. The data ánd circuits are printéd with the pérmission of the manufacturér, National Semiconductor Córp. The student is urged to use the data books when designing equipment with power op-amps. Figure 49 shows the circuit configuration of a complete phono amplifier. Figure 49 Phono amplifier using LM380 12.1 Operational Amplifier Equivalent Circuit If a particular application requires more power than can be obtained from a single power op-amp, we can use the bridge configuration of Figure 50. Since this systém provides twice thé voltage swing acróss the load ás the single-dévice system, the powér capability is theoreticaIly increased by á factor of 4 over the single amplifier (for a given power supply voltage). Since heat dissipatión is the Iimiting concérn in this design, wé usually design thé system conservatively ánd only double thé output power. Figure 50 Bridge configuration for high power 12.2 Intercom Figure 51 shows an intercom incorporating a power op-amp and a few external components. With the dual two-position switch (S1A-S1B) in the talk position (as shown in the figure), the speaker of the master station performs the function of a microphone, driving the power op-amp through a step-up transformer. The remote spéaker is driven fróm the output óf the power óp-amp.
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