dB Compression Point

The 1 dB compression point is a figure of merit commonly used to characterize the power capabilities of PAs along with their linearity. In the Pout vs. Pin characteristic, the 1 dB compression point is the one for which the actual output power of the amplifier is 1 dB lower than what it would have been if the amplifier was linear (and having a gain equal to its small signal gain). This definition is illustrated graphically in Figure 1.10, which reports the Pout vs. Pin characteristics of the actual and ideal amplifier. The ideal amplifier characteristic represents the extrapolated version of the linear portion of the actual amplifier’s Pout vs. Pin characteristic. This figure shows that the 1 dB compression point can be defined either with respect to the input power (P1dB,in in Figure 1.10) or with reference to the output power (P1dB,out in Figure 1.10).

Sample AM/PM characteristic of a power amplifier

Figure 1.9 Sample AM/PM characteristic of a power amplifier

Though, the 1 dB compression point is commonly reported with respect to the output power of the device.

Similarly, the 1 dB compression point can be defined from the AM/AM characteristic. In this case, it corresponds to the power level for which the gain of the amplifier is 1 dB lower than its small signal linear value. From the AM/AM characteristic, the P1dB,m can be graphically determined as illustrated in Figure 1.11. If the small signal gain of the amplifier is denoted as GSS, then the 1 dB compression point output power (P1dB,out) can be obtained according to:

For a given DUT, the 1 dB compression point can vary depending on the test signal (CW versus modulated signals). The 1 dB compression point concept can be extended to the X-dB compression point. The X-dB compression point is defined in a way similar to that of the 1-dB compression point but for a gain compression of X-dB rather than 1 dB. Thus, the 3-dB compression point is the point of the Pout vs. Pin characteristic for which the actual output power of the amplifier is 3 dB less than what it would have been if the amplifier was linear; it is also the point of the AM/AM characteristic for which the gain of the device is 3 dB lower than its small signal value. The X-dB compression point can be used for the system level design of power amplification stages as well as building equation-based behavioral models in simulation software.

Graphical definition of the 1 dB compression point from Pout vs. Pin characteristic

Figure 1.10 Graphical definition of the 1 dB compression point from Pout vs. Pin characteristic

Graphical definition of the 1 dB compression point from the AM/AM characteristic

Figure 1.11 Graphical definition of the 1 dB compression point from the AM/AM characteristic

 
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