Power Audio Amplifier IC LM 380:
Small signal amplifiers are essentially voltage amplifier that supplies their loads with larger amplifier signal voltage.
On the other hand, large signal or power amplifier supply a large signal current to current operated loads such as speakers & motors.
In audio applications, however, the amplifier called upon to deliver much higher current than that supplied by general purpose op-amps. This means that loads such as speakers & motors requiring substantial currents cannot be driven directly by the output of general purpose op-amps. To handle it following is done
· To use discrete or monolithic power transistors called power boosters at the output of the op-amp
· To use specialized ICs designed as power amplifiers like LM 380.
1. Internally fixed gain of 50 (34dB)
2. Output is automatically self centering to one half of the supply voltage.
3. Output is short circuit proof with internal thermal limiting.
4. Input stage allows the input to be ground referenced or ac
5. Wide supply voltage range (5 to 22V).
6. High peak current capability.
7. High impedance.
It is connected of 4 stages,
(i) PNP emitter follower
(ii) Different amplifier
(iii) Common emitter
(iv) Emitter follower
· The input stage is emitter follower composed of PNP transistors Q1 & Q2 which drives the PNP Q3-Q4 differential pair.
· The choice of PNP input transistors Q1 & Q2 allows the input to be referenced to ground i.e., the input can be direct coupled to either the inverting & non-inverting terminals of the amplifier.
· The current in the PNP differential pair Q3-Q4 is established by Q7, R3 & +V.
· The current mirror formed by transistor Q7, Q8 & associated resistors then establishes the collector current of Q9.
· Transistor Q5 & Q6 constitute of collector loads for the PNP differential pair.
· The output of the differential amplifier is taken at the junction of Q4 & Q6 transistors & is applied as an input to the common emitter voltage gain.
· Common Emitter amplifier stage is formed by transistor Q9 with D1, D2 & Q8 as a current source load.
· The capacitor C between the base & collector of Q9 provides internal compensation & helps to establish the upper cutoff frequency of 100 KHz.
· Since Q7 & Q8 form a current mirror, the current through D1 & D2 is approximately the same as the current through R3.
· D1 & D2 are temperature compensating diodes for transistors Q10 & Q11 in that D1 & D2 have the same characteristics as the base-emitter junctions of Q11. Therefore the current through Q10 & (Q11-Q12) is approximately equal to the current through diodes D1 & D2.
· Emitter follower formed by NPN transistor Q10 & Q11. The combination of PNP transistor Q11 & NPN transistor Q12 has the power capability of NPN transistors but the characteristics of a PNP transistor.
· The negative dc feedback applied through R5 balances the differential amplifier so that the dc output voltage is stabilized at +V/2;
· To decouple the input stage from the supply voltage +V, by pass capacitor in order of micro farad should be connected between the bypass terminal (pin 1) & ground (pin 7).
· The overall internal gain of the amplifier is fixed at 50. However gain can be increased by using positive feedback.
· Amplifier requires very few external components because of the internal biasing, compensation & fixed gain.
· When the power amplifier is used in the non inverting configuration, the inverting terminal may be either shorted to ground, connected to ground through resistors & capacitors.
· Similarly when the power amplifier is used in the inverting mode, the non inverting terminal may be either shorted to ground or returned to ground through resistor or capacitor.
· Usually a capacitor is connected between the inverting terminal & ground if the input has a high internal impedance.
· As a precautionary measure, an RC combination should be used at the output terminal (pin 8) to eliminate 5-to-10 MHz oscillation.
· C1 is coupling capacitor which couples the output of the amplifier to the 8 ohms loud speaker which acts as a load. The amplifier will amplify the Vin applied at the non-inverting terminal.
· The gain of LM380 is internally fixed at 50. But it can be increased by using the external components.
· The increase in gain is possible due to the use of positive feedback, this setup to obtain a gain 200.
· Instead of getting a fixed gain of 50, it is possible to obtain a variable gain up to 50 by connecting a potentiometer between the input terminals.
· If a certain application requires more power than what is provided by a single LM380 amplifier, then 2 LM380 chips can be used in the bridge configuration.
· With this arrangement we get an output voltage swing which is twice that of a single LM380 amplifier.
· As the voltage is doubled, power output will increase by four times that of a single LM380 amplifier. The pot R4 is used to balance the output offset voltages of the two chips.
· When the switch is in Talk mode position, the master speaker acts as a microphone.
· When the switch is in Listen position, the remote speaker acts as a microphone.
· In either phone the overall gain of the circuit is the same depends on the turns of transformer T.
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