The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> LNA Design Unable to get any gain from the circuit
https://designers-guide.org/forum/YaBB.pl?num=1518002506

Message started by Sagar Juneja on Feb 7^th, 2018, 3:21am

Title: LNA Design Unable to get any gain from the circuit
Post by Sagar Juneja on Feb 7^th, 2018, 3:21am

I am designing LNA for GPS application (1.57GHz) using cadence virtuoso (IC616) and Spectre (MMSIM131) tool. Circuit is attached.

My topology is inductive source degeneration. I am able to match the input and output of LNA to 50 Ohms input and output port respectively with S11 = -18dB and S22 = -10dB. My noise figure is 2.4dB at the frequency of interest.

But the problem is I am not been able to get any gain out of the circuit. My S21 is always coming equal to S22 and it is -2.5dB. Please help me understand what could possibly be wrong with the circuit.

DC parameters/bias points for my gain transistor are as follows:-
Gate Over Drive Voltage - 0.12V, Id = 5.69A, Vgs = .608V, Vds = .605V, Vth = .558V, gm = 81m, gds = 2.14m, Cgs = 548Ff

Title: Re: LNA Design Unable to get any gain from the circuit
Post by santosh31013 on Feb 23^rd, 2018, 3:59am

Hello,

I have few comments.
1. Id = 5.69A is very high. (it should be in mA)
2. Vds = 0.605V and Vgs = 0.608V (pls check the bias point in DC-IV characteristics).
3. matching (< -10dB) and Noise Figure (2.4dB) are acceptable. so you should get required gain!! (Pls recheck the circuit topology, Bias and High value resistors.)

Thanks,
Santosh ;)

Title: Re: LNA Design Unable to get any gain from the circuit
Post by Jacki on Mar 20^th, 2018, 1:48am

Check your output port resistance, do you use 50Ohm? If you don't need output impedance matching, don't use the port. If you need the output impedance matching, you need a buffer.