Power, Linearity, and Distortion Compression and Intercept Points Informational

What is the difference between input referred and output referred P1dB and IP3?

Q: What is the typical IP3 to P1dB ratio?

For most amplifiers: IIP3 ≈ IP1dB + 9.6 dB (theoretically, for a memoryless polynomial nonlinearity). In practice: IIP3 = IP1dB + 8 to 12 dB (depends on the amplifier technology and topology). GaAs pHEMT: IIP3 ≈ IP1dB + 10-12 dB. GaN HEMT: IIP3 ≈ IP1dB + 8-10 dB (GaN has a harder compression characteristic). CMOS: IIP3 ≈ IP1dB + 9-11 dB.

Q: Why do some datasheets only give one reference?

Convention varies by device type: LNA/receiver front end: IIP3 almost always (because engineers need it for the cascade formula). PA: OP1dB and OIP3 almost always (because the output power is the key specification). Mixer: both IIP3 and OIP3 are often given (because the conversion gain makes the relationship explicit). If only one is given: use the gain to convert to the other reference.

Q: How do I handle variable-gain amplifiers?

For a variable-gain amplifier (VGA) or AGC: the IP3 and P1dB change with the gain setting. High gain setting: IIP3 degrades (the amplifier compresses earlier because the internal signal level is higher). Low gain setting: IIP3 improves. The OIP3 remains approximately constant (it tracks the output stage capability, which does not change with gain setting). This is why AGC is typically placed after the LNA: adjusting the gain of a stage after the first amplifier changes the IIP3 of that stage without affecting the first-stage NF.

P1dB and IP3 can be specified as either input-referred (referenced to the input) or output-referred (referenced to the output). The difference is the gain of the device: (1) Relationship: OIP3 = IIP3 + Gain (in dB). OP1dB = IP1dB + Gain (in dB, where Gain is the linear gain, not the compressed gain). Example: an amplifier with IIP3 = +5 dBm and 20 dB gain: OIP3 = +5 + 20 = +25 dBm. An amplifier with IP1dB = -5 dBm and 20 dB gain: OP1dB = -5 + 20 = +15 dBm. (2) When to use input-referred: for receivers and cascaded systems: use IIP3 and IP1dB. The input-referred values are convenient for the Friis cascade linearity formula: 1/IIP3_cascade = 1/IIP3_1 + G1/IIP3_2 + G1×G2/IIP3_3 + ... (in linear power units, not dB). The front-end stages dominate the cascade IIP3 when they have high gain. (3) When to use output-referred: for transmitters and power amplifiers: use OIP3 and OP1dB. The output-referred values directly relate to the output power capability: OP1dB determines the maximum useful output power. OIP3 determines the maximum output while meeting distortion specifications. For mixers: OIP3 is commonly specified because the mixer gain (conversion loss) varies with LO drive and frequency. (4) For passive devices (filters, attenuators, cables): IIP3 = OIP3 (the gain is 0 dB or negative, so input and output referred are the same or differ only by the loss). Passive IP3 is typically very high (> +60 dBm) for well-designed components. Passive intermodulation (PIM) can limit the effective IP3 of connectors and cables (PIM is a separate mechanism from active device nonlinearity).

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Amplifiers, Mixers, Attenuators

Input vs Output Referred Linearity

Understanding the difference between input-referred and output-referred linearity metrics is essential for correctly cascading component specifications in a system design.

Common Data Sheet Presentations

(1) LNA datasheets typically specify: IIP3 (because the LNA is used at the input of the receiver, and the input-referred value is needed for the cascade calculation). IP1dB (input-referred). NF, Gain, S-parameters. (2) PA datasheets typically specify: OP1dB (the maximum useful output power). OIP3 (or the IM3 level at a specified output power, e.g., "IM3 < -30 dBc at Pout = +27 dBm"). Output power at a given PAE (power added efficiency). (3) Mixer datasheets: may specify both IIP3 and OIP3 (because the conversion gain/loss varies). IIP3 for the RF port and OIP3 for the IF port are related by the conversion gain.

Referred Point Relationships

OIP3 = IIP3 + Gain (dB)
OP1dB = IP1dB + Gain (dB)
Receivers: use IIP3 for cascade
Transmitters: use OIP3 for output spec
Passive: IIP3 ≈ OIP3 (very high)

Common Questions

Frequently Asked Questions

What is the typical IP3 to P1dB ratio?

For most amplifiers: IIP3 ≈ IP1dB + 9.6 dB (theoretically, for a memoryless polynomial nonlinearity). In practice: IIP3 = IP1dB + 8 to 12 dB (depends on the amplifier technology and topology). GaAs pHEMT: IIP3 ≈ IP1dB + 10-12 dB. GaN HEMT: IIP3 ≈ IP1dB + 8-10 dB (GaN has a harder compression characteristic). CMOS: IIP3 ≈ IP1dB + 9-11 dB.

Why do some datasheets only give one reference?

Convention varies by device type: LNA/receiver front end: IIP3 almost always (because engineers need it for the cascade formula). PA: OP1dB and OIP3 almost always (because the output power is the key specification). Mixer: both IIP3 and OIP3 are often given (because the conversion gain makes the relationship explicit). If only one is given: use the gain to convert to the other reference.

How do I handle variable-gain amplifiers?

For a variable-gain amplifier (VGA) or AGC: the IP3 and P1dB change with the gain setting. High gain setting: IIP3 degrades (the amplifier compresses earlier because the internal signal level is higher). Low gain setting: IIP3 improves. The OIP3 remains approximately constant (it tracks the output stage capability, which does not change with gain setting). This is why AGC is typically placed after the LNA: adjusting the gain of a stage after the first amplifier changes the IIP3 of that stage without affecting the first-stage NF.

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