LM4862 Amplifier Circuit – A Better LM386 Alternative

The LM386 based amplifier is still very popular as one of the smallest sized amplifier chips. However, the LM386 is not perfect and has a few drawbacks and limitations.

As shown below, the LM386 works with large electrolytic capacitors, which makes it bulkier and costlier, and causes it to be prone to distortions with age.

Another flaw with LM386 is its input impedance which seems to be very high, allowing the chip to be much vulnerable to oscillations if the inputs are not sufficiently isolated from the output.

Its voltage gain of 20 (or 200 by inserting an additional capacitor) looks quite high for line input level (1 V RMS) and this results in further oscillation issues.

On the other hand, the LM4862 IC is more advanced and slightly more powerful compared to LM386 and it works without any electrolytic capacitor.

Main Features of LM4862

It is designed to deliver 0.675 watt into an 8-ohm speaker with a total harmonic distortion of 1%.

When operated at slightly lower power levels, the distortion is reduced to negligible limits.

Another great feature of the IC LM4862 is its automatic thermal shutdown which protects the chip from damage even if the output is overloaded or short circuited.

This circuit requires just a single 5 V supply for the operations. The input impedance of LM4862 is comparatively low and can be suitably adjusted externally, which ensues that the oscillation issue is kept to minimal.

Internal Layout

The following figure shows the internal structure of the chip LM4862. The output of the IC LM4862 drives the speaker in a differential mode, which involves opposite push pull waveforms driving the speaker across the two output terminals. This differential topology is commonly recognized as BTL (bridge -tied load).

How the LM4862 Works

In the BTL operation the two terminals of the speaker are alternately toggled with a +5V and a 0V depending on the music frequency. This means, the amplifier is able to generate a total of 10 volts swing across the speaker from a 5 volt supply. This is just enough to create a impressive volume of music volume over 4 inch full range speaker.

The chip will work with supply voltage ranging from 2.7 V to 5.5 V. Which means the LM4862 can be powered from two or three 1.5 V AAA cells or a from a computer 5 V USB, or simply from your mobile phone charger.

But remember, the supply must not exceed 5.5 V, and therefore even a 6 V supply can permanently damage the chip.

Total current consumption of the chip can be expected to be around ranges 5 mA in the absence of a music input. and to around 250 mA when it is operated at its maximum volume limit.

Power supply ripple rejection is superb, which is greater than 50 dB when C2 = 1µF.

How to Make an Amplifier using LM4862

A general LM4862 based amplifier circuit can be in the following figure.

It looks very simple without the use of any electrolytic capacitors; which allows it to be cheap and yet have a high-fidelity audio output.

Basically, C2, works like the bias bypass capacitor, which can be a tantalum electrolytic; blocks the audio signal through it.

If possible a 100 µF electrolytic could be added parallel with the C3 to increase stability when the IC is operated with batteries or a poorly regulated power supply. The voltage gain is determined by 2(R2 /R1) that must not be exceed the value of 20.

You can expect the sound quality to be the best when R2 = R1 and the gain is 2. This is exactly what must be followed to drive a speaker when the input is 1 volt from a line input or a headphone jack 3.5 mm.

In case the gain is made higher than 5, it may be necessary to add a bypass capacitor C4 across R2 to prevent oscillation. This can be a 5 pF capacitor, although up to 22 pF capacitor can also be used. But higher values than this may cause problems.

Typically, smaller value resistors can be used like R1 = 4.7K and R2 = 4.7K to 47K, when the input is fed from a low impedance supply. The following image shows us the component values for a few typical typical amplifier set ups.

Note that, the design of the amplifier becomes more efficient in terms of cost and power saving when the bass response is kept at its minimum, although that would also mean the absence of the heavier low frequency notes.

The LM4862 is specified to work with at least a 8 ohm speaker, lower ohms might also work such as 16 ohm, 32 ohm, or 64 ohm speaker, but that may cause the power output to be significantly less.

If you wish to operate the speaker as a single ended output by grounding its one end, you might need to add a series capacitor with the other end of the speaker which is connected with the IC output as shown below:

But single ended operation may drastically reduce the power output from the speaker compared to the differential mode.

Using the Shutdown Pin

Normally, the shutdown pin#1 is connected with the ground line in normal. However, this specific pin can be configured with a button to implement a "mute" function without the need of putting a switch directly on the signal line.

Using the Bias Pin

The bias pin#2 is terminated as the output from an internal voltage divider which is used for maintaining the positive inputs of both the op amps at half the supply voltage so that it becomes possible to power the circuit with a single supply.

The bias pin2 may be further used to bias a couple of more op amps as shown indicated in the following figure.

It may be necessary to bypass the bias pin to ground using any capacitor from 0.1 and 10 µF for improving ripple rejection response and also to suppress the "thump" sound each time the amplifier is switched on.

LM4862 Application Circuits

This little amplifier circuit can be actually used for all applications that require a small audio signal to be amplified to a reasonably high audible level.

AM Radio

A radio receiver circuit is one of these examples, as shown below using a tiny ZN414 AM receiver. Nevertheless, you can use the section of LM4862 stage after the R3 volume control for any similar small audio amplification purpose.

This simple radio will receive all the local AM stations loud and clear over the attached loudspeaker

Square Wave Oscillator

The IC can be also effectively applied as a simple square wave oscillator circuit as shown below:

Bidirectional Motor Control

Although the IC LM4862 is designed to work like an audio amplifier it can well be applied as a full bridge motor driver stage, and the direction of the motor can be changed simply by changing the input logic signals, as demonstrated in the following diagram.

Reference: http://www.ti.com/lit/ds/symlink/lm4862.pdf