LM4871 audio amplifier BTL configuration question

by Bruce Hsu   Last Updated August 20, 2019 14:25 PM

I am reading the datasheet of LM4871 audio amplifier. The typical application looks like:

enter image description here

The datasheet says it is in BTL configuration. The 1st OPA inverts input signal and the 2nd OPA inverts again so we get a differential pair for driving a loudspeaker at output stage.

My question is:

  1. The path of Vo2 (2 OPA) is obviously longer than Vo1 (1 OPA). Why there is no synchronization problem between them? Are these OPAs fast enough so we can ignore the asynchronous?

  2. I also found an article of wikipedia which describes BTL. It looks like:

enter image description here

Why there are different design of BTL. What is the difference between them?

Tags : amplifier audio


Answers 1


The path of Vo2 (2 OPA) is obviously longer than Vo1 (1 OPA). Why there is no synchronization problem between them? Are these OPAs fast enough so we can ignore the asynchronous?

This IC is for audio signals so the opamps do not need to be that fast to be "fast enough". Look at the datasheet, page 8, figure 26 which shows the open loop gain over frequency. At 0 dB gain (Av = -1) the Bandwidth is more than 3 MHz. That means a delay of about (-3dB point at 3 MHz (T = 333 ns) so 90 degrees phase shift equals (333ns / 4= 83 ns) ) 83 ns.

The highest audio frequency is 20 kHz which has a period of 1/ 20 kHz = 50 us.

That 83 ns is only 83 ns / 50 us = 0.17 % time error, at 20 kHz. In my view, no one can hear that.

So yes, the opamp is fast enough for the signal delay not to be a problem. Since we're dealing with audio, it is not an issue.

Bimpelrekkie
Bimpelrekkie
August 20, 2019 14:24 PM

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