1 dB compression, PSAT, IIP3, OIP3 for a RF power amplifier revisited

A quick look at the high input power specifications and linearity specifications that are usually used in RF Power amplifier specifications. These quantities generally are: 1 dB compression, PSAT, IIP3 and OIP3. These are defined and described below.

1.0 1 dB compression: In the linear region of a RFPA operation, as the input power is increased, the output power also increases linearly. However, if the input power is continuously increased a point is reached where the increase in output power does not occur linearly. The output power starts compressing. This means that even though the input power is increased the output power does not respond to that increase. When the output power falls 1 dB below what was expected of a linear amplifier, that point is the 1 dB compression point of the amplifier. In many cases the 1 dB compression point is assumed to be the end of linear operation for the amplifier although in reality the amplifier starts compressing much before this point.

2.0 PSAT: PSAT stands for saturated output power of the amplifier. This means that as the power input to the amplifier is increased even beyond the 1 dB compression point a point will be reached where the gain of the amplifier will become 0 dB i.e. the amplifier is saturated at that point. Sometimes a 3 dB saturation point is also defined as a saturation point of the amplifier for more definition for the user.

3.0 IIP3: As the RFPA starts coming closer to compressing and gain starts changing from linear to compressed ( and in some cases even earlier, second and third order distortion products of the input signal start appearing in its output. A way to quantify this type of linearity distortion is to use quantities such as IIP3. As stated above as the input signal is continuously increased the output power also increases. If the third order distortion product is monitored, it will be found to increase at three times the rate of the fundamental component. If the third order product power is extrapolated it will intersect with the fundamental output power characteristic at a point where the input power is IIP3. It is a limiting power point that is used to understand the linearity of an amplifier as well as the limits of operation of the amplifier.

4.0 OIP3: OIP3 is simply the power being delivered by the RFPA when the input power is IIP3. Different users and manufacturers use IIP3 and OIP3 or both when specifying their amplifiers.

For more information on these topics and others of interest please visit the Signal Processing Group Inc., website.http://www.signalpro.biz

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