Output Backoff (OPBO) vs. Input Backoff (IPBO) in RF Amplifiers

Both Output Backoff (OPBO) and Input Backoff (IPBO) are crucial for determining the appropriate operating power levels in satellite transponder TWTAs (Traveling Wave Tube Amplifiers) or RF Power Amplifiers to ensure they function within their linear region.

Output Back-Off (OPBO)

Output Back-Off (OPBO) represents the power level at the RF amplifier’s output relative to its maximum achievable output power.

Example:

• Maximum output level: +40 dBm
• Measured output level of RF Amplifier: +34 dBm
• Output Backoff (OPBO): 6 dB

Input Back-Off (IPBO)

Input Back-Off (IPBO) signifies the power level at the RF amplifier’s input relative to the input power required to produce maximum output power.

Example:

• Input power level causing max. output power: -20 dBm
• Actual input power level: -25 dBm
• Input backoff (IPBO): 5 dB

OPBO is also instrumental in calculating inter-modulation distortion power levels at the output of TWTAs or SSPAs (Solid State Power Amplifiers). For SSPAs, the output backoff is equivalent to the 1 dB compression point, while for TWTAs, it corresponds to the saturated output power.

In satellite link calculations, input backoff is determined by taking the difference between the actual flux density and the saturated flux density. Additionally, the calculated output backoff aids in determining the actual EIRP (Effective Isotropic Radiated Power) of the satellite.

Typical Backoff Requirements

The following table outlines typical backoff requirements in SCPT (Single Carrier Per Transponder) and MCPT (Multiple Carrier Per Transponder) operations.