Matching and tuning audio amplifier output stability and sound performance.

Kenneth Lee

November 10, 2008

Kenneth Lee

Compensation
The compensation of the amplifier is to adjust the open loop gain and phase performance to stabilize the system when the feedback loop is closed. Generally speaking, the amount of compensation is as high as possible to obtain a higher stability. However, it has a tradeoff on reducing the bandwidth and slew rate of the audio chips.

Lower slew rate will provide a softer sound nature, while higher slew rate can provide a clearer and harder sound nature in general. In LME49810, Miller compensation is implemented by inserting a single capacitor between the pin Comp and BiasM.

Figure 3: It is not recommended to exceed 200µA current through the mute pin.

By increasing the value of the capacitor between these two pins, the amount of compensation and the phase margin can be increased. However, it is not recommended to be too high or too low.

The suggested range is from 10p to 100p.The compensation capacitor should be low equivalent series resistance (ESR); it can avoid introducing a potential zero by the ESR of the capacitor. Usually, a ceramic capacitor is better than an electrolytic capacitor.

The mute pin is controlled by the current flow into the mute pin. This is done by connecting a reference voltage to the mute pin through a resistor to control the current from 50 to 100 microAmps as "play" mode and less than 50µA as "mute" mode. It can be calculated by a default formula: IMUTE = (VMUTE - 0.7V) / (RM + 10k-ohms) (A). It is not recommended to exceed 200 microAmps current through the mute pin.

Figure 4: LME49810 has two dedicated pins (BiasP and BiasM) for Bias setup and provides a certain output bias current.

Output
LME49810 has two dedicated pins (BiasP and BiasM) for Bias setup and provides a certain output bias current. The R pot (variable resistor) is for tuning the bias current for the output stage. Lowering the R pot + Rb1 will result in a higher bias voltage.

The QMULT (multiplier) compensates the bias voltage to prevent the thermal runaway of the bipolar output transistors.

The QMULT must be attached with the same heat sink as the output transistors. When the temperature gets higher, the Vbe is decreased to lower the bias voltage - the higher the bias current provided the less crossover distortion from the output, but higher current consumption.

The most common output stage in audio power amplifier is emitter-follower (Figure 5 below). It is usually called double emitter- follower or Darlington pair configuration, where the first follower acts as driver to the output device.

Figure 5: The most common output stage in audio power amplifier is emitter follower.

The large signal's linearity of the emitter-follower mainly depends on the loading. As the loading increases (i.e. loading resistance decreases), the output current increases and the bipolar junction transistor current gain reduces due to the RE (emitter de-generator) and the beta rolloff at high current density.

It may reduce the linearity and increase the distortion at the output stage. For higher power, multiple stages output is recommended to maintain the high current and better linearity. LME49810 audio driver has around 50mA output current. It may be configured as single Darlington pairs or parallel transistors output, depending on the application requirement.