Matching and tuning audio amplifier output stability and sound performance.
This "Product How-To" article focuses how to use a certain product in an embedded system and is written by a company representative.
Audio amplifier products are becoming more and more advanced in terms of product outlook, style, system control and sound quality. Experienced electronic engineers are using different sorts of circuitry in their audio amplifier designs.
Many traditional high-power audio amplifiers with over 100W/ channel are using discrete components circuitry. High-fidelity audio amplifier even needs to take extra effort on matching and tuning the output stability and sound performance.
Considerations
There are different approaches to developing an audio system. National
Semiconductor's LME49810 is optimal for high-end consumer and
professional audio applications that span powered studio monitors and
subwoofers, A/V receivers, commercial sound reinforcement, after-market
audio, professional mixers, distributed audio and guitar amplifiers. It
is also well-suited for a wide range of industrial applications, where
high voltage and low distortion are required.
Input stage design is a critical consideration in the amplifier. It generates error signal by subtracting the feedback signal from the input and then drive to the output. The error signal is assumed to be small to provide high linearity for the amplifier. LME49810 is a bipolar input amplifier.
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| Figure 1: To further reduce the output offset voltage, reducing the impedance both on feedback and input signal path is a way to do. |
The input impedance matching is important. Due to the bias current from the positive and negative input port, input impedance mismatch leads to input offset voltage. The input offset voltage will amplify with close loop gain and appear on the output.
While LME49810 has a very low input bias current, the offset voltage that will appear on output will be insignificant for general uses. For example, a typical LME49810 bias current is 100nA and 1K-ohms input impedance mismatch. Voffset = Ibias x Rmismatch = 100nA x 1K-ohms = 0.1mV. For a typical 30x close loop gain in the amplifier, Voutputoffset = 0.1mV x 30.
Due to very low input bias current, the offset voltage that appears on output will be insignificant in general uses. To further reduce the output offset voltage, reducing the impedance both on feedback and input signal path is a way to do. The offset due to input bias current imbalance can be reduced. But we need to make sure that there is enough driving capability from the front stage.
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