
Cascaded 1 dB Compression Point (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). 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
Calculating the cascaded values for P1dB for the system budget requires the following operation based upon
ratios for gain and P1dB (do not use decibel values). The standard format for indicating decibel values is to
use upper case letters, i.e., OP1dB for units of dBm. The standard format for indicating watt values is to use
lower case letters, i.e., op1dB for units of mW or W.
A Typical Chain of Cascaded Components
This equation gives
the method for calculating cascaded output p1dB
(op1dB) values based on the oip3 and gain of each stage. When using the formula in a software program or in a
spreadsheet, it is more convenience and efficient to calculate each successive cascaded stage with the one
preceding it using the following format:
These formulas are used to convert back and forth between input and out P1dB values:
P1dB_{output} = P1dB_{input} + (Gain  1) dBm P1dB_{input} = P1dB_{output}
 (Gain  1) dBm

Table of
IP3, IP2, and P1db Values from Vendor Datasheets 
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 
27.05 
11.6566 





StdDev 
8.1 
2.9 





Samples 
10 
53 

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.
RF Workbench uses an approximation to model the transition from linear operation through saturation. 
Click here to view an example cascaded system.




