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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.
