Matching and tuning audio amplifier output stability and sound performance.

Kenneth Lee

November 10, 2008

Kenneth Lee

There are generally two common types of audio input design: AC- or DC-coupled. The advantages of AC-coupled input are the following:

1) The DC-offset to the input of the amplifier from pre-amplifier, filter stage or codec stage is always zero.
2) There is no need to protect the DC faults by any DC servo circuitry in the amplifier.

On the other hand, the DC coupled input offers the following advantages:

1) No large and expensive A C-coupling capacitor is required.
2) Low frequency distortion generated by the AC-coupling capacitor is avoided.3) It reduces the noise coupled from the AC-coupling RC network.

Figure 2: By increasing the value of the capacitor between these two pins, the amount of compensation and the phase margin can be increased. The suggested range is from 10p to 100p.

Negative feedback configuration in the power amplifier can provide higher stability and linearity of the system. Using negative feedback can avoid the saturation on the amplifier DC offset voltage.

Generally, there is phase shift at high frequency in the amplifier; a higher negative feedback factor can reduce the high frequency instability and oscillation.

In a discrete amplifier system, high feedback factor will cause poor transient response or high frequency instability. LME49810 has a higher open loop gain.

Thus, it has lesser close-loop gain error and power supply rejection and can maximize the amount of negative feedback in the circuitry to maximize the linearity. A recommended range of voltage gain is 30-40dB. Negative feedback factor can be easily set by the R input and R feedback relationship: AV = RF/RI (V/V).