Linearity is an essential standard for a power amplifier(PA) determined by the transistor's property. It can directly impact PA and decide if it can work adequately in RF systems. Of course, it will have a specific requirement in different projects and strategies and, finally, reflect on the amplifier's output ability and characteristic performance.
There are three definitions to describe an amplifier: P1dB, Psat and Po. The description as following:
P1dB: the output power at amplifier's 1dB point of compression - When input a small signal into the power amplifier step by step, meanwhile the output power should increase the same value if it is linear. Then cumulatively increase the power in the same method until the output power shows increased (N-1)dBm when input NdBm. At this time, it is the P1dB power of this amplifier.
Psat: Means saturation power. In general, keep inputting 5dBm power when PA is at its P1dB compression point, the output power of this time is the Psat, it usually greater than P1dB 1-3dBm.
Po: The linear output power. It is the PA's actual max power, usually 3dBm lower than P1dB.
According to the operational purpose or the actual application, there are several parameters for the users that can help them to evaluate the amplifier's linearity:
1- IMD3: (About 5% of linear relative bandwidth) When input two CW signal f1, f2 with same carrier level, calculate with formula Pf1-P(2*f1-f2) and Pf2-P(2*f2-f1) (P means output power), the biggest one between the two values is IMD3.
For the systems like Tetra, GSM and DCS which with 1-4 narrow channels and low data transmit rate:
Normally require: 36dBc to 40dBc
For micro BTS: 45dBc to 50dBc
2- ACPR - Adjacent Channel Power Ratio: (About 5% of linear relative bandwidth) for the Broad channel systems like 3G, 4G, LTE, OFD and Wifi which with high data transmit rate systems:
Basic require: 32dBc
Higher standard: 40dBc to 45dBc
3- Broadband working system:
For the broadband system like 20-500MHz and 500-2500MHz frequency bands, linearity usually means harmonics, reference value:
Odd harmonics >12dB
4- Pulse working system, especially for radars:
Naturally, without the specific requirements, we will only consider the rising and falling time, reference value:<100ns