Source resistance and noise considerations in amplifiers

In many applications it is critical to design for low noise. Different types of sensors, filters, and audio designs are common examples where low noise is critical. These applications can be modeled as a source resistance in series with a signal source.

The source resistance has thermal noise and also converts current noise into voltage noise, increasing the amplifier’s total output-voltage noise.

A common question is how to choose an amplifier that minimizes total output-voltage noise, even when a source resistance is modeled. This question is relevant because amplifiers can be fabricated in either bipolar or CMOS technology. Bipolar amplifiers have significant current noise but often have lower voltage noise than CMOS amplifiers for a given quiescent current. Current noise is most problematic when the source resistance is high.

This article demonstrates how a CMOS amplifier is the best choice when a high source resistance is used and noise is the only concern. Knowing these facts along with the voltage-noise specifications of the amplifier is instrumental in making the right choice. For this analysis, it is assumed that the bipolar and CMOS amplifiers have comparable bandwidth, power, and intrinsic voltage noise. The trade-offs between the two amplifiers are also examined.

You can read the application note in the EE Times Technical Papers section here.

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