LM 380 Power Audio Amplifier

LM 380 Power Audio Amplifier

LM 380 is a popular power audio amplifier produced by National Semiconductor. It is capable of delivering 2.5 W(r.m.s) minimum to 8 Q load. Hence it is very much ideal for consumer applications. It requires minimum number of external components. In order to keep the cost to a minimum, its gain is internally fixed to 50 i.e. 34 dB.

1. Features of LM 380 Audio Amplifier

The LM 380 audio amplifier has following features :

1. Internally fixed gain (34 dB).

2. Wide supply voltage range (5 to 22 V).

3. Output is automatically self centreing to one half of the supply voltage.

4. Output is short circuit proof with internal thermal limiting.

5. A unique input stage allows input to be ground referenced or a.c. coupled.

6. Low quiescent power drain.

7. High peak current capability (1.3 A maximum).

8. High input impedance (150 kΩ).

9. Low total harmonic distortion, THD (0.2 %).

10. Standard dual in line package.

11. A bandwidth of 100 kHz typically at an output power of 2 W and load of 8 Ω.

2. Pin Diagram of LM 380

The Fig. 5.15.1 shows the pin diagram of LM 380 power audio amplifier. A copper lead frame is used with the centre three pins on either side (3, 4, 5, 10, 11 and 12) of the DIP package comprises a heat sink. Hence there is no need to use a separate external heat sink for the audio amplifier.

Symbolically LM 380 audio amplifier is represented as shown in the Fig. 5.15.2.

3. internal circuit diagram of LM 380

The LM 380 audio power amplifier consists of four stages

1. PNP emitter follower

2. Darlington compound differential amplifier

3. Common emitter       

4. Quasi-Complementary emitter follower.

The internal circuit diagram is shown in the Fig. 5.15.3.

The PNP transistors Q₁ and Q₂ form an emitter follower input stage. This drives PNP Q₃ -Q₄, the differential amplifier. Because of PNP transistors the input can be referenced to ground i.e. the input can be direct coupled to either inverting (6) or non-inverting (2) terminals of the amplifier. The differential amplifier is biased by R₃ and R₅.

The current in the PNP differential pair Q₃ - Q₄ is decided by the Q₇, R₃ and + V_CC. The current mirror of Q₇ and Q₈ and the associated resistors then establishes current through collector of Q 9. The transistors Q₅ - Q₆ acts as active load for the PNP differential pair.

The output of the differential pair is taken at the junction of Q₄ and Q₆ and is applied as an input to the common emitter voltage gain stage.

The transistor Q₉ with diodes D₁ and D₂ form the common emitter voltage gain stage where Q8 acts as a current source load. The capacitor is a feedback capacitor and is used for the frequency compensation to stabilise the amplifier against any type of oscillations. It also establishes the upper cut-off frequency off 100 kHz at 2 W for 8 Ω loads. The diodes D₁ and D₂ are used to develop a small prebias voltage across the base emitter junctions of Q₁₀ and Q₁₁, so as to minimize the cross over distortion. The resistances R₆ and R₇ are used for current limiting.

The output stage is a quasi-complementary pair emitter follower using NPN transistors Q₁₀ and Q₁₂. The quiescent output voltage level is centered at approximately one half the supply voltage by resistor ratio R₃ and R₅. This allows the maximum peak to peak output voltage swing and therefore maximum a.c. output voltage.

The resistances R₁ and R₂ provide a d.c. return path for the input bias current so that the amplifier can be operated with either input terminal open.

To decouple the input stage from the supply voltage +V_CC, a bypass capacitor of the order of microfarads should be connected between the bypass terminal (1) and the ground (7). The pins 3, 4, 5, 10, 11, 12 are ground heat sink pins.

The overall internal voltage gain of the amplifier is fixed at 50.

4. Applications of LM 380 Audio Amplifier

As the number of external components required for LM 380 is very less, it is mostly used for consumer applications. Some of such applications are discussed below.

a. LM 380 as Audio Power Amplifier

The simplest and most basic application of LM 380 is as an audio power amplifier. The IC can be used in inverting as well as non-inverting configuration. When used in the non-inverting mode the inverting terminal can be either shorted to ground, left open or returned to ground through resistor or capacitor. Similarly, when it is used in the inverting configuration, the non-inverting terminal may be either shorted to ground or returned to ground through resistor or capacitor. In both the configurations, the supply voltage must be decoupled by connecting a capacitor between the terminal 14 and ground. As a precautionary measure a lag compensating RC network must be connected at the output to ground, to eliminate 5 to 10 MHz oscillations. The LM 380 as audio power amplifier in non-inverting mode is shown in the Fig. 5.15.4.

b. LM 380 as High Gain Audio Amplifier

As mentioned earlier, LM 380 is a fixed gain audio amplifier whose gain is internally fixed to 50 (34 dB). But with the help of positive feedback its gain can be increased upto 300. The Fig. 5.15.5 shows the use of LM 380 as an audio amplifier with gain 200, using positive feedback.

c. LM 380 with a Variable Gain

The circuits discussed above are the amplifiers designed for fixed gain values. But variable gain upto 50 can be obtained with the use of a potentiometer across the two input terminals. This is shown in the Fig. 5.15.6. 

This circuit can be used for volume control.

d. Bridge Configuration using LM 380

If the load requires more power than a single LM 380 can deliver, the bridge amplifier using LM 380 can be used. The bridge power amplifier uses two LM 380 ICs. The bridge amplifier provides the maximum output voltage swing twice as that provided by single LM 380 amplifier. Due to this the power delivered to the load is four times that delivered by single LM 380 amplifier. Fig. 5.15.7 shows the bridge power amplifier using LM 380 ICs. The resistance R₄ is used to balance the output offset voltages of LM 380 ICs.

e. Phono Amplifier using LM 380

A phono amplifier with volume and tone control can be obtained using LM 380. The output is given to the 8 Ω loud speaker. The resistances R (2.7 Ω) and C (0.1 µF) are used for circuit stability from the undesirable oscillations.

The port K_v is used for the volume control with a pot of 25 k Ω while the port K_T is used for the tune control with a pot of 10 k Ω. The phonoamplifier using LM 380 is shown in the Fig. 5.15.8. The crystal cartridge is used at the input side. 

f. Intercom System using LM 380

Fig. 5.15.9. (a) shows the intercom system using LM 380, in talk mode.

The speakers used are of permanent magnet type and hence they act as microphones also. Depending upon the role of the master speaker action, the system can be operated in talk mode or listen mode.

When the switch SW is in talk mode, the master speaker acts as microphone accepting the voice in, with the help of step up transformer T. The overall gain of the circuit is constant and depends on the turns ratio of the transformer T as well as internal gain of LM 380. But generally the internal gain of LM 380 is kept controllable with the help of pot at R_v port as shown.

When the switch SW is in listen state, the circuit acts in listen mode as shown in the Fig. 5.15.9 (b). The roles of remote speaker and the master speaker get interchanged. 

g. Siren / Alarm using LM 380 Power Amplifier

The certain conditions may be monitored by blowing siren or alarm in a complex system. The undesirable conditions can be recognised by producing variety of sounds using sirens or burglar alarms.

Such a siren or alarm system can be designed using following basic components :

i) Audio amplifier LM 380

ii) Dual op-amp MC 1458

iii) 1 W speaker.

The dual op-amp is to be used as a signal generator which can generate triangular, square or pulse waveform. While the IC LM 380 is used as an audio amplifier which is discussed earlier. The entire siren or alarm system is shown in the Fig. 5.15.10.

The MC 1458 is dual op-amp consisting op-amps A₁ and A₂. Both together composed of a wave form generator. The output of A1 is a square wave while output of A₂ is triangular or sawtooth waveform. The potentiometer R 2 not only controls the frequency but also decides the type of the output waveform of A1 and A₂. The switch SW1 connects the outputs of A₁ and A₂ to the LM 380 audio amplifier. Only one output at a time, either of A₁ or A ₂ gets connected to LM 380 with the help of switch SW1. The LM 380 drives the 8Ω , 1 W loudspeaker. A pot R_v connected at the input terminals of LM 380 controls the sound level. The sound level depends on the switch position, R₂ position and capacitor C_f. So varying sound intensities can be obtained.

Instead of op-amp signal generator, the waveform generator using 555 also can be used.

Review Questions

1. Draw the internal circuit schematic of LM 380 power amplifier and briefly discuss its salient features.

2. What are features of LM 380 power amplifier ? With a schematic, explain its application as high gain audio power amplifier.

3. With necessary diagrams, explain the applications of LM 380 IC.

May-08, Dec.-15, Marks 10

4. Write a note on LM380 power amplifier.