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Design >> RF Design >> Output matching for cascode LNA
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Message started by engrvip on Nov 21^st, 2016, 7:47am

Title: Output matching for cascode LNA
Post by engrvip on Nov 21^st, 2016, 7:47am

I am designing a single stage source degenerated cascode LNA. I have understood most of it theory regarding input matching but dont understand how to do output matching of thi LNA ?

Hope someone can explain it to me.

Title: Re: Output matching for cascode LNA
Post by tzg6sa on Nov 30^th, 2016, 11:06am

With matching network? Any impedance can be transformed into any another by two reactive elements.

Title: Re: Output matching for cascode LNA
Post by dog1 on Dec 1^st, 2016, 4:20am

Hello,

It seems to me that the degenerated LNA doesn't guarantee matching at the output in itself. If you want output impedance matching, then you have to use matching network at the output unless by coincidence your output is impedance matched.

Title: Re: Output matching for cascode LNA
Post by engrvip on Dec 13^th, 2016, 8:06pm

Why is output matching not important between integration of on chip blocks in RFIC like between LNA and mixer ?

Title: Re: Output matching for cascode LNA
Post by deba on Dec 14^th, 2016, 10:29am

Matching is more important for discrete RF design where one has to deal with external components.

In RFIC, matching is only important for blocks exposed to outside world. Like LNA and PA.

Title: Re: Output matching for cascode LNA
Post by dog1 on Dec 21^st, 2016, 2:24am

Hello,

In order to answer this question we need to understand why we want to do impedance matching.

From what I heard and understand, there are two main reasons:

1. from transmission line point of view, the frequency response of transmission line and filter changes with the load (and the source) impedance. Also, when mismatch, there will be reflection and standing waves on the transmission line, which can cause problems (say, reflection into the free space through the antenna at the input of the receiver) and distributed V and I (deviation from standard circuit theory, thus making the circuit analysis invalid)

2. from circuit theory point of view, for maxim power transfer.

Note that the latter is based on circuit theory only and requires no distributed wave analysis.

when designing IC, say at the output of the LNA, if the mixer is place close to the LNA, the voltage and current in the interconnect is not much distributed over the line, and that the reflection in itself is not a problem. So reason 1 is not valid.

Additionally, reason 2 is valid only under the circumstance of drawing 1 in the picture attached, where you can have lossless matching network. However
1) on-chip inductor is lossy and costly.
2) in IC design we are very often interested in stable voltage and current, while by using matching network we are essentially creating an resonating network. Since in IC design, the load is very often purely capacitive, and the loss due to packaging and interconnect is small or non-exsiting, this often require large and almost purely inductive matching network. It is hard to build those inductors and the resonance gives large ringing on the line with pulsed input (long settling time and not suitable for digital and switching circuit). Say, in drawing (b), the input of the next stage is almost purely Cgs, thus the matching network will be almost just an L, and this LC network give a resonance and ringing.

Title: Re: Output matching for cascode LNA
Post by DER19 on Oct 7^th, 2017, 1:21am

although it is an old post but may help anyone.....we know that output matching is usually not important for LNA but for simulation and measurement of the standalone LNA, it needs to be terminated with 50 ohm. Therefore, draw your LNA and place a Common Drain buffer at its output. Adust its current such that its 1/gm = 50 ohms. This will give 50 ohm at the output of LNA. Now use a 50 ohm port easily for simulataions or a 50 ohm instrument at the output for measurement

Title: Re: Output matching for cascode LNA
Post by Horror Vacui on Oct 22^nd, 2017, 12:12pm

DER19 wrote on Oct 7^th, 2017, 1:21am:

we know that output matching is usually not important for LNA

Do not say that in front of a professor or an RF engineer! The need for 50 Ohm mathcing has nothing to do with the circuit type, but the available gain, and the feasibility of a matching network. At 1kHz no one would consider an LC matching circuit, but it does not mean that it is not important in RF (I would say ~0.1-1GHz and beyond) circuits!