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LNA Design Unable to get any gain from the circuit (Read 809 times)
Sagar Juneja
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Posts: 3

LNA Design Unable to get any gain from the circuit
Feb 07th, 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
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Re: LNA Design Unable to get any gain from the circuit
Reply #1 - Feb 23rd, 2018, 3:59am

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.)

Santosh  ;)
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Re: LNA Design Unable to get any gain from the circuit
Reply #2 - Mar 20th, 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.
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