What is the difference between P1dB compression point and third order intercept point IP3?
Understanding P1dB
Every amplifier has a maximum output power determined by its supply voltage and bias current. As the input signal increases, the output eventually cannot grow proportionally, and the gain begins to decrease. The 1 dB compression point quantifies where this nonlinear behavior becomes significant.
P1dB is measured by sweeping the input power while monitoring the output power and small-signal gain. At low input levels, the gain is constant. As input increases, the gain decreases. The input level where the gain has dropped by 1 dB is the input P1dB; the corresponding output power is the output P1dB. Typically, output P1dB is 1 to 3 dB below the saturated output power.
In a system cascade, the component with the lowest input P1dB referred to its input determines where compression occurs first. The system P1dB can be calculated by referring each component's P1dB to a common reference point (usually the system input) using the gain preceding each component.
System P1dB (referred to input):
P1dBsys = P1dBn - ΣGain(stages 1 to n-1)
Frequently Asked Questions
How does P1dB relate to saturated power?
Saturated output power (Psat) is typically 2 to 4 dB above P1dB for most solid-state amplifiers. The exact relationship depends on the device technology and operating class.
Does P1dB change with frequency?
Yes. P1dB varies with frequency due to changes in gain, matching, and device performance. It is typically specified at the center frequency but should be characterized across the operating band.
What happens above P1dB?
Above P1dB, harmonics and intermodulation products increase rapidly. Gain continues to decrease. AM-PM conversion increases. Signal quality degrades significantly. Operating above P1dB is acceptable for saturated modes (Class C, FM) but not for linear modulations (QAM, OFDM).