MT-052| Application Note

MT-052 TUTORIAL
Op Amp Noise Figure: Don't Be Mislead

INTRODUCTION Op amp noise is generally specified in terms of input current and voltage noise, as previously discussed in MT-047, MT-048, MT-049, MT-050, and MT-051. In communications systems, however, noise is often specified in terms of noise figure (NF)--see Figure 1 below. This can lead to confusion, especially when op amps are used as gain blocks, and the noise figure of the op amp is not specified for the specific circuit conditions. In order to understand how to apply noise figure to op amps, we will first review the basic theory behind noise figure.
NF is usually specified for matched input/output conditions, but this is not always a system requirement Noise Figure is a popular figure of merit in RF applications: LNAs, Mixers, etc. Difficulties arise when applying NF to op amps. NF is dependent on Impedance levels Feedback network Closed loop gain Other difficulties arise due to different definitions of NF as found in various textbooks We will start with the basics and work up to the op amp issues

Figure 1: Noise Figure (NF) in Communications Applications AVAILABLE NOISE POWER The first concept is that of available power from a source. The available power of a source is the maximum power that can be drawn from the source. Figure 2 below shows a resistor of value R as the noise source. The thermal noise of this source is (4kTBR). The maximum noise that can be transferred to an ideal noiseless load occurs when the load resistance is also equal to R. Under these conditions, the maximum available noise power from the source reduces to kTB, where k is Boltzmann's constant, T is the absolute temperature, and B is the noise bandwidth. Note that this power is independent of the value of the source resistance, R.

Rev.0, 10/08, WK

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