Power Amplifier ICs

Introduction

Low power amplifier ICs are very useful for simple amplifier applications having a low component count and a wide range of power supply requirements. The following circuits are typical for some of the most common ICs but are not the only possible circuits.

TBA820

Features & Notes:

The TBA 820M Audio amplifier chip is a general purpose amplifier with a variable gain and bandwidth
The device is suitable for driving a small 8Ω speaker or headphones
The IC operates from a single supply rail and a wide range of supply voltages making it convenient in battery powered devices
The device has a low quiescent current making it suitable for battery operated devices (4mA)
The chip is supplied in a standard 8 pin DIL plastic package
Take care that the chip has sufficient space as it may become warm in use!!

Specifications:

Supply Voltage - 3 to 16 volts
Input impedance - 5MΩ
Voltage gain- up to 180 - set using external components
Output power - up to 2W depednding on supply voltage and the type of speaker used. With an 8Ω speaker the output power is 2W with a 12V supply but only 1.2W with a 9V supply.
Bandwidth - up to 20kHz

Assuming an 8Ω speaker is being used: when R2 is set to the lowest reasonable value of 33Ω the voltage gain is x180, with R2 set to a typical value of 120Ω the voltage gain is x50. Increasing R2 decreases the gain.
Assuming an 8Ω speaker is used: with C6 set to a value of 220pF the bandwidth is 25Hz to 20kHz - suitable for audio/music. The bandwidth reduces as C6 is increased. With C6 at a maximum reasonable value of 680pF the bandwidth is 25Hz to 7kHz - suitable for speech e.g. on the telephone.

Typical Circuit and Component Values

R1 = 10kΩ minimum
R2 = typically 100Ω - this sets the gain
R3 = 1Ω
R4 = 56Ω
C1 = 100µF electrolytic, 10V minimum
C2 = 100µF electrolytic, 16V minimum
C3 = 100nF
C4 = 47µF electrolytic, 10V minimum
C5 = 220nF
C6 = typically 220pF - sets bandwidth
C7 = 100µF electrolytic, 16V minimum
C8 = 220µF or larger, electrolytic, 16V minimum
Spk = 2W, 4Ω - 16Ω speaker

LM386

Features & Notes:

The LM386 Audio amplifier chip is a general purpose amplifier with a variable gain and a high bandwidth
The device is suitable for driving a small 8Ω speaker
It operates from a single supply rail making it convenient in battery powered devices
The device has a low quiescent current making it suitable for battery operated devices (4mA)
The chip is supplied in a standard 8 pin DIL plastic package
Take care that the chip has suficient space as it may become warm in use!!

Specifications:

Supply Voltage: 4 to 12 volts
Input impedance: 50kΩ
Voltage gain: 20 to 200 - set using external components
Output power: 300mW to 500mW depending on supply voltage and type of speaker used
Bandwidth: 300kHz

With pins 1 and 8 not connected, the gain is set to x20. The addition of the capacitor and resistor between pins 1 and 8 sets the voltage gain to anything between x20 and x200. For example, a 1.2kΩ resistor in series with a 10µF capacitor between pins 1 and 8 will set the gain to x50. If the 10µF capacitor is used on its own, without the resistor, between pins 1 and 8 then the gain is set to x200.

Typical Circuit and Component Values

R2 = 10Ω
C1 = 10µF electrolytic, 16V (can often be omitted)
C2 = 100µF electrolytic, 16V
C3 = 100nF
C4 = 47nF
C5 = 220µF or larger, electrolytic, 16V
C6 = 10µF - sets voltage gain
Spk = 0.5W, 4Ω to 16Ω speaker

LM380

Features & Notes:

The LM380 Audio amplifier chip is a general purpose amplifier with a set gain and a set bandwidth and a high current output. It can be used with very few external components
The device is suitable for driving a small 8Ω speaker
It operates from a single supply rail making it convenient in battery powered devices but the minimum supply voltage means it cannot be used with a standard 9V battery. The quiescent output voltage is exactly half the supply voltage
The chip is supplied in a standard 14 pin DIL plastic package
The center pins (3-5, 10-12) on the chip are used as a heatsink and so should be connected to a suitable area of PCB and are typically connected to 0V. In use as a power amplifier these pins would be soldered to the printed circuit board and, ideally, to a reasonable area of copper on the board. Pins 9 and 13 are not used.

Specifications:

Supply Voltage: 10 to 22 volts
Input impedance: 150kΩ
Voltage gain: x50
Output power: 2W
Bandwidth: 100kHz
Peak Current: 1.3A

Typical Circuit and Component Values

R1 = 2.7Ω (only needed with C4 for high current applications)
C1 = 470nF (electrolytic or non-electrolytic)
C2 = 100µF electrolytic, 25V
C3 = 100nF
C4 = 100nF (only needed with R1 for high current applications)
C5 = 220µF or larger, electrolytic, 25V
Spk = 2W, 4Ω to 16Ω speaker

Volume Control

With all of the amplifiers shown above it may be necessary or desirable to provide a simple volume control. It is also good practice to block any unwanted d.c. signals using a small capacitor. The input circuit shown below can be used with any of the amplifier circuits.

C = 470nF
VR = 47kΩ logarithmic potentiometer