RF2469 datasheet(9/18 Pages) RFMD

9 / 18 page

Preliminary

8-41

RF2469

Rev A5 010717

LNA P1dB Compression Point

For large signal input, this type of LNA will not have a fixed DC bias current. The LNA will tend to self-bias when the input

signal level starts increasing above small signal conditions. This particular characteristic will move the DC bias current to

a higher DC bias current. Obviously, increasing the bias current will increase the linearity of the LNA.

To accurately measure the P1dB, the measurement technique must force the bias current in the LNA to be a constant,

while preserving the collector output voltage of the LNA. In order to due this, a separate supply voltage must be used for

the bias voltage of the LNA (pin 3) and the open collector supply (pin 1). As the input signal level is increased, the bias

voltage must be dropped while monitoring the DC current in the LNA to ensure that it remains constant. Incidentally, the

P1dB compression measured with this technique is consistent with the standard approximation relating P1dB to IIP3

(i.e., Input P1dB= IIP3(dBm)-10). Since the IIP3 measurements are done under small signal conditions (the input tones

are low power levels), this approximation provides a good figure for P1dB under a constant DC bias condition. For the

RF2469, with an IIP3 of approximately +8dBm, the Input P1dB is approximately -2dBm.

However, for many applications, forcing the bias current in the LNA to be constant is not a practical solution. Leaving the

LNA to self-bias will not produce any damage to the part and the P1dB performance under this condition will be:

Frequency

(MHz)

Gain

(dB)

Input

P1dB

Output

P1dB

LNA Current

(mA)

2140

10.5

5.25

14.92

~23

RF2469 Datasheet(PDF) 9 Page - RF Micro Devices