Graph of P1dB, IP2, IP3, and Saturation
See cascade calculations for NF, IP2,
IP3, and P1dB.

When operating within the linear region of a component, gain through that component is constant for a given frequency.
As the input signal is increased in power, a point is reached where the power of the signal at the output is not
amplified by the same amount as the smaller signal. At the point where the input signal is amplified by an amount
1 dB less than the small signal gain, the 1 dB Compression Point has been reached. A rapid decrease in gain will
be experienced after the 1 dB compression point is reached. If the input power is increased to an extreme value,
the component will be destroyed.
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.
Click here to view an example of a cascaded system. 
Calculating the cascaded values for 1 dB compression point (P1dB) for the system
budget requires use of ratios for gain and power levels for P1dB (do not use dB and dBm values,
respectively). The standard format for indicating decibel values is to use upper case letters; i.e.,
P1dB for units of dBm. The standard format for indicating power values is to use lower case letters; i.e.,
p1db for units of mW.Conversions: p1db = 10^{P1dB/10} ↔ P1dB
(dB) = 10 * log_{10} (p1db)
where p1db has units of mW and P1dB has units of dBm
A Typical Chain of Cascaded Components

Combining 2 Stages at a Time for Calculations

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.
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 
