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P1dB_{output} = P1dB_{input} + (Gain  1) dBm
Passive, nonlinear components such as diodes also exhibit 1 dB compression points. Indeed, it is the nonlinear active transistors that cause the 1 dB compression point to exist in amplifiers. Of course, a power level can be reached in any device that will eventually destroy it.
A common rule of thumb for the relationship between the 3rdorder intercept point (IP3) and the 1 dB compression point (P1dB) is 10 to 12 dB. Many software packages allow the user to enter a fixed level for the P1dB to be below the IP3. For instance, if a fixed level of 12 dB below IP3 is used and the IP3 for the device is +30 dBm, then the P1dB would be +18 dBm.
In order to test the theory, IP3 and P1dB values from 53 randomly chosen amplifiers and mixers were entered into an Excel spreadsheet (see table below and resulting graph to the right). The parts represent a crosssection of silicon and GaAs, FETs, BJTs, and diodes, connectorized and surface mount devices. A mean average and standard deviation was calculated for the sample.
As it turns out, the mean is 11.7 dB with a standard deviation of 2.9 dB, so about 68% of the sample has P1dB values that fall between 8.8 dB and 14.6 dB below the IP3 values. What that means is that the longlived rule of thumb is a pretty good one. A more useful exercise might be to separate the samples into silicon and GaAs to obtain unique (or maybe not) means and standard deviations for each.
An interesting sidebar is that where available, the IP2 values were also noted. As can be seen in the chart, the relationship between IP2 and P1dB is not nearly as consistent.
Of equal motivation for the investigation was the desire to confirm or discredit the use of the noise figure and IP3 type of cascade formula for use in cascading component P1dB values. As discussed elsewhere, the equation for tracking a component from its linear operating region into its nonlinear region is highly dependent on the entire circuit structure, and one model is not sufficient to cover all instances. Indeed, the more sophisticated (pronounced “very expensive”) system simulators provide the ability to describe a polynomial equation that fits the curve of the measured device. Carrying the calculation through many stages is calculation intensive. Some simulators exploit the rule of thumb of IP3 versus P1dB tracking and simply apply the IP3 cascade equation to P1dB. As with other shortcuts, as long as the user is aware of the approximation and can live with it, it’s a beautiful thing.
Cascading P1dB Values in a Chain of Components
Click here to view an example of a cascaded system. 
Conversions: p1db = 10^{P1dB/10} ↔ P1dB (dB) = 10 * log_{10} (p1db)
where p1db has units of mW and P1dB has units of dBm
Cascading of 1 dB Compression points is not a straightforward process, since the curve followed from linear operation into saturation is dependent upon the circuit characteristics. A precise calculation requires knowing the equation of the input/output power transfer curve of each device, which is typically a highorder polynomial that would be very difficult both to ascertain and also to apply mathematically. A wellknown ruleofthumb is to subtract 10 to 15 dB to the IP3 value to estimate the P1dB value. To test that theory, I looked at the published values of IP3 and P1dB for some common devices and calculated the difference between IP3 and P1dB (see table below). A sample of 53 devices resulted in a mean difference of 11.7 dB, with a standard deviation of 2.9 dB. That is pretty good agreement with the ruleofthumb.
Accordingly, a reasonable estimate of the cascaded P1dB value is to either apply the cascaded IP3 equation directly to each device's P1dB value, or to simply calculate the actual cascaded IP3 and subtract 10 to 15 dB to the result and declare that to be the cascaded P1dB. Note that this estimate only holds when none of the stages in the cascade are normally operating outside of the linear region.
This equation gives the method for calculating cascaded output p1db (op1db) values based on the equation for oip3 and gain of each stage. When using the formula in a software program or in a spreadsheet, it is more convenient and efficient to calculate each successive cascaded stage with the one preceding it using the following format, per the drawing (aboveright).
These formulas are used to convert back and forth between input and outputreferenced P1dB values:
P1dB_{Output} = P1dB_{Input} + (Gain  1) dBm
P1dB_{Input} = P1dB_{Output}  (Gain  1) dBm
The following table of values was used to create the chart shown near the top of the page.
Table of IP3, IP2, and P1db Values from Vendor Datasheets  
Type  Mfg  Model  IP2  IP3  P1dB  P1dBIP2  P1dBIP3 
Amp  Amplifonix  2001  36  32  17  19  15 
Amp  Amplifonix  8701  47  35  25  22  10 
Amp  Amplifonix  5404  43  33  22  21  11 
Amp  Couger/Teledyne  A2C5119  46  33  19  27  14 
Amp  Couger/Teledyne  A2C4110  54  34  21.5  32.5  12.5 
Amp  Couger/Teledyne  A2CP14225  54  40  28  26  12 
Mixer  Couger/Teledyne  MC1502  35  12  35  12  
Amp  Mimix Broadband  CMM4000  39  29.5  19  10.5  
Amp  Mimix Broadband  CMM1110  31  22  13  9  
Amp  M/ACOM  A101  64  36  23  41  13 
Amp  M/ACOM  A231  25  22  10  15  12 
Amp  M/ACOM  AM050005  55  37  23  32  14 
Amp  M/ACOM  SMA411  32  24  10  14  
Mixer  Polyphase  IRM0714B  67  15  7.6  7.4  
Mixer  Polyphase  IRM1925B  68  14  8  6  
Mixer  Amplifonix  M53T  13  3.5  9.5  
Amp  JCA  JCA01301  20  13  7  
Amp  JCA  JCS02332  33  23  10  
Amp  Mimix Broadband  XL1005  24  16  8  
Amp  Technology Distribution  06000007  25  10  15  
Amp  Technology Distribution  06000025  20  15  5  
Amp  Technology Distribution  06000024A  30  12  18  
Amp  Stealth Microwave  SM343634HS  47  34  13  
Amp  Stealth Microwave  SM192533  47  33  14  
Amp  M/ACOM  MAALSS0045  32  20  12  
Mixer  M/ACOM  CSM110  19  6  13  
Mixer  M/ACOM  M5T  18  7  11  
Mixer  Marki Microwave  M10204L  12  2  10  
Mixer  Marki Microwave  M1R0726M  15  5  10  
Mixer  Polyphase  SSB2425A  19  8  11  
Amp  Triquint  TGA2512SM  16  6  10  
Mixer  Triquint  CMY 210  24  14  10  
Amp  Miteq  AFS30050020027PCT6  38  27  11  
Amp  Milliwave  TMT40601803510P2  20  10  10  
Amp  Milliwave  TMT65007501005P5  14  5  9  
Amp  Milliwave  AMT40601804010P1  22  15  7  
Amp  Skyworks  SKY6501370LF  29  14  15  
Amp  Skyworks  SKY6501592LF  35  18  17  
Mixer  Synergy  FSM2  40  23  17  
Mixer  Synergy  SGM217  18  10  8  
Amp  Microwave Technology  MwTA989  39  24  15  
Amp  Hittite  HMC376LP3  36  21.5  14.5  
Amp  Hittite  HMC564  24  12  12  
Mixer  Hittite  HMC399MS8  34  24  10  
Amp  RFIC  RFISLNA01  24  14  10  
Amp  RFMD  NBB302  23.5  13.7  9.8  
Amp  RFMD  RF2878  29  14.4  14.6  
Amp  NuWaves  NILNAGPS  31  17  14  
Amp  MCL  AMP15  22  8  14  
Amp  MCL  ZFL500HLN  30  16  14  
Amp  MCL  ZQL900LNW  35  21  14  
Mixer  MCL  MCA19FLH  25  10  15  
Mixer  MCL  MCA112GL  9  1  8  
Mean  27.1  11.7  
StdDev  8.1  2.9  
Samples  10  53 