The Novice Writer: LM380 – (Audio Power Amplifier)

LM380 – (Audio Power Amplifier)

Key Features:

• Wide supply voltage range: 10V to 22V.

• Low Quiescent Power Drain : 0.13W (Vs = 18V)

• Voltage gain fixed at 50.

• High peak current capability up to 1.3A.

• Input reference to ground.

• Low distortion.

• High input impedance.

• Output is short circuit proof, with an internal thermal limiting package.

• Output automatically self-centers to half the supply voltage.

14 pin IC:

1. By pass

2. Non-inverting terminal

3. Ground

4. Ground

5. Ground

6. Inverting terminal

7. Ground

8. Output voltage

9. Not connected

10. Ground

11. Ground

12. Ground

13. Not connected

14. Supply voltage.

8 pin IC:

1. Not connected

2. Non-inverting terminal

3. Inverting terminal

4. Ground

5. Ground

6. Output voltage

7. Supply voltage

8. By pass

Internal Circuitry of LM380:

Working of LM380:

• The LM380 is a variation on an operational amplifier, but with higher power output and an input circuit designed to operate at the negative rail (ground) rather than in the middle of a bipolar power supply.

• The differential input:

o Compares the input to the output, attenuates by the feedback network.

o If the feedback network attenuates the output by a factor of 10, the amplifier will produce an output which is 10 times the input so that the negative feedback matches the input.

• The voltage gain:

o Produces a voltage based on the differential amplifier output current.

o This is also where pole splitting frequency compensation is applied, that is the most common method to stabilize the feedback and prevent oscillations.

• The output buffer that is current gain:

o Is an emitter follower which provides the current required to apply the output voltage to the load.

Specifications of LM380:

Parameters Absolute Maximum

Supply Voltage 22 V

Peak Current 1.3 A

Input Voltage +/- 0.5 V

Operating Temperature 0`C to 70`C

Junction Temperature 150`C

Lead Temperature 260`C

Output Power RMS 2.5 W

Gain 50

Output Voltage Swing 14 Vpp

Bandwidth 100 kHz

Bias Current 100 nA

Application of LM380:

• Phase Shift Oscillator:

o Diagram:

o Working:

 Output of the circuit is a sine wave.

 Consist of an inverting amplifier element such as an operational amplifier in its inverting mode with the output feedback to its input through a phase shift circuit which is a series of RC combination that reduces or increases the phase shift according to the value of R and C used for the feedback.

 This ladder circuit of resistors and capacitors produces the desired output which would have just been an oscillating waveform if it wasn’t present.

• Intercom Circuit:

o Diagram:

o Working:

 Main role of LM380, in this case, is Audio Amplifier which would amplify the input that it gets from the microphone end of the overall circuit.

 The circuit is directly connected to mains.

 The 12VA transformer reduces the voltage to a level in which circuits can safely work.

 A 1A fuse is connected to the transformer to prevent the circuit against high voltage transition.

 Bridge rectifier and RC filter (Pi Filter) is used to rectify and filter the input signal to stabilize it.

 Microphone used is a three terminal electronic condenser type and connected to the microphone preamplifier stage formed by two direct coupled BC104C transistors.

 The gain is very high so 10Kohms pot is used to adjust it so that only desired signal is passed on to the other room.

 Advantage you don’t need to be near the microphone as the sensitivity is high due to this circuitry.

 LM380 audio amplifier IC can deliver up to 2W into 8 ohms load with a supply current of only 50mA.

 The relay circuitry at the speaker end completes the circuit forming a sensitive intercom system.

~Jay Mehta