# Biased Voltage/Data Sheet Question

#### abu1985

Joined Oct 18, 2015
74
Looking at the data sheet for a TI - TPA3110D2, the audio input pins 3 & 4 and 11 & 12 are said to be biased at 3 volts.

Does that mean I need to supply an input signal of at least 3 volts (assuming peak to peak) just to start the amplifier's active region?

Here is a link to the data sheet. Page 4 is the pin configuration.

Thank you!

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#### crutschow

Joined Mar 14, 2008
27,241
Does that mean I need to supply an input signal of at least 3 volts
Nope.
It just means there is a DC bias voltage at those inputs, so you need to couple the audio signal in through a 1μF capacitor (as shown below) to block the DC but let through the AC.

Joined Mar 10, 2018
4,057
Of level shift your input in case you want DC coupling. That would
mitigate low frequency cutoff due to high pass filter effect of a C being
used in input circuit.

If you stick with cap choose its value based on considerations page 22 -

Regards, Dana.

#### Audioguru

Joined Dec 20, 2007
11,249
Why would anybody use an amplifier that cuts off signals less than 3V peak or peak-to-peak?

#### abu1985

Joined Oct 18, 2015
74
Why would anybody use an amplifier that cuts off signals less than 3V peak or peak-to-peak?
I’m not sure, why would they?

#### crutschow

Joined Mar 14, 2008
27,241
Of level shift your input in case you want DC coupling.
That would be problematic since the input DC has a tolerance and the level shift would likely need to be that precise value to avoid disrupting the input DC circuit bias levels.
You would also have to track any possible change in the level with temperature.

If you want a lower frequency input cutoff then just use a larger capacitor.

#### Audioguru

Joined Dec 20, 2007
11,249
The TPA3110 amplifier has a minimum voltage gain of 20dB which is 10 times. When its output is 15W into 8 ohms when its supply is 16V then its output voltage is almost 11V RMS which is almost 31V p-p. Then its input level is 3.1V p-p.
Of course it will still work when its input is less than 3V p-p or any lower level.

Joined Mar 10, 2018
4,057
That would be problematic since the input DC has a tolerance and the level shift would likely need to be that precise value to avoid disrupting the input DC circuit bias levels.
You would also have to track any possible change in the level with temperature.

If you want a lower frequency input cutoff then just use a larger capacitor.
That would be problematic since the input DC has a tolerance and the level shift would likely need to be that precise value to avoid disrupting the input DC circuit bias levels.
You would also have to track any possible change in the level with temperature.

If you want a lower frequency input cutoff then just use a larger capacitor.
Good point. The notes also discuss bias/leakage considerations at high G. And
no specs on Vos at inputs. So makes one wonder, w/o adequate input specs and
characterization, when AC coupled, high G, elevated T, what does happen. I guess
being an audio circuit no point in designing for worst case.....

Regards, Dana.

#### abu1985

Joined Oct 18, 2015
74
Nope.
It just means there is a DC bias voltage at those inputs, so you need to couple the audio signal in through a 1μF capacitor (as shown below) to block the DC but let through the AC.

View attachment 184844
I put together a PCB and the circuit is pretty close to the 'typical application' circuit shown in the data sheet. But I'm having an issue with it running. Either I have soldering a few things incorrectly (first time SMD soldering experience) or the chip is faulted and shutting down.. or both. My input is a solid 3v pp from an iPad, but no output. I actually have a negative gain output on the output channel negative output pin, oddly enough. It's my input sine wave, but at about 100mV..