Help me establish the load of the LNA

[spbhu]

Hi, I was designing a LNA. I want to know what is the load of the LNA? I thought it is 50ohms. But my supervisor said there should be a buffer following the LNA for testing.
Can any one tell me why we should have this buffer? Is it for testing purpose only? In real system, is LNA directly connected to the LNA?
If it is a buffer, then how to determine the LNA's load, how to calculate the s22?
Many thanks

 

[BigBoss]

The load of the LNA?

Probably you will have to design a LNA differential pair cascode/or not so output impedance will be very high and load (here 50 ohm) will degrade the gain and therefore to prevent overloading effect of 50 Ohm, a buffere is followed by LNA to avoid it.

But you're right, output impedance will be low than 50Ohm so you will have to optimize it regarding to 50Ohm. It's obvious that optimization won't be easy and it needs many cut-and-try simulations...

 

[spbhu]

Re: The load of the LNA?

Yes, my LNA is a differential cascode one. The output I set is 10kohms to prevent the loading effect. Is that correct? Is the 10kohms load play the role of the buffer? Also, will the power taken by the buffer be considered as the power consumption of the LNA?
Another interesting things is that: if I put the load to 10koms, the gain is quite high, but the s21 is low. While if I put it to about 1Koms, the voltage gain is lowered, but s21 is increased, so I am confused, shall we consider s21 more or voltage gain more in the LNA? Thanks very much

 

[khouly]

Re: The load of the LNA?

i think the buffer may be for tetsing , in some designs if u have integrated recivier , u can load the LNA by the Mixer directly . and some times u need matching between them

khouly