Question about LNA Design!

[dopradar]

Hi All,

When I am tunning LNA, I met a problem:Because this LNA needs high linearity, I added an inductor between transistor emitter and GND. When I increase the value of inductor,the gain meets such problem.as I increase the input power, the gain firstly become higher and then lower down.The increase range is 5dB. So what's the reason?

Thanks!
B&R

 

[_RFIC_]

What is the power supply and current consumption? I guess it is because your output impedance deviate from 50Ohm too much when input power is high. Do you use ADS? You can use LSSP simulation to check the input and output impedance for high power input. Before doing this, keep in mind that your LNA is stable.

 

[BigBoss]

The LNA may be very close to unstable region or may be conditional unstable due to emitter inductance.

 

[flatulent]

You are driving the transistor into a large signal mode. The S parameters are different here.

It is always best to examine a circuit with a general purpose instrument. Look at the output with a spectrum analyzer to check for spurious oscillations and harmonics. Specialized instruments such as network analyzers are narrow band and do not indicate the spurious oscillations and harmonics.

 

[toonafishy]

At higher drive levels, you may actually be shifting the DC bias point of the amplifier. Check the DC terminal voltages at low and high drive levels.

 

[sgperzoid]

I think -13dBm is not too much high for the LNA

In my opinion, the phenomenon may arise due to the stability of the LNA

Source inductive degeneration is not so stable at high input power.

btw, I think you need the emitter inductor more for matching than improving linearity, right?