[LNA] What's wrong with its gain?

yolande_yj · Apr 22, 2005

Mazz said:
Just follow the impedance definition: V/I.
Probe the input current and define the Zin in data display.

For output, I think you have to inject a signal on output.

Understood.

Added after 16 minutes:

biff44 said:
Maybe you should care about the output match!

You do not define what you mean by gain, but if your "gain" is 20 log s21, then the gain is being calculated with a simulated 50 ohm load. If you have a poor output match to a 50 ohm load, you will have less overall gain. If you have a good output match to a 50 ohm load, you will have more overall gain. It does not matter if at some later time you want to hook this up to a different load. We are talking about the bump in "gain" that you showed in your plot, and we only care right now about the load that the computer software used during the simulation.

If, at -23 dBm input power, the output impedance shifts so that you have a much better match than when you have -30 dBm input power, that could explain the 3 db jump in gain.

This is just a guess, but a 3dB bump in gain is not that much, and could be a large signal impedance effect, especially if you consider a match to some harmonic generated at the higher output power.

The problem should be related to the model I think. But you are not wrong. Please see the graph below, it is a HB simulation using HICUM model, it shows a better output matching at around -23dbm and there is no gain ripple at this point. But I think this is reasonable as you can see the matching is below -15db, a better matching like -23db could not vary the gain much. For instance, if you get a -15db matchin and a 20db gain, do you expect there is a large gain increase when you finely tune the matching to perfect (-∞)? I don't think so according to my experience.

However, when I switch to GP model, ADS just can not finish the simulatoin and showing an error message all the time.

BTW, my simulation is HB with small-signal at 1kHz.
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Mazz · Apr 23, 2005

See what I've found on Gummel poon model on Support center:

ADS, BJT Model gives different results between ADS 2002C and ADS 2003A

Problem

I am using a BJT (Gummel Poon) mode in my circuit. I used to get good results in ADS 2002. Now I moved to ADS 2003A and I see very different results. I haven't tried this on 2002C.

Is this a bug in the software. Or was there any change in the code that can explain this?

Solution / Workaround

There could be more than one reason for the change in results. In this specific case, we discovered the following.

The BJT model contains the parameter Rbm=0. If you change it to a very small value, say 0.0001, you will get results consistent with the those produced by ADS 2002. It is not enough to leave Rbm blank, as its default value is also zero.

I hope it can help.
Mazz

yolande_yj · Apr 23, 2005

Mazz said:
See what I've found on Gummel poon model on Support center:

@DS, BJT Model gives different results between @DS 2002C and @DS 2003A

Problem

I am using a BJT (Gummel Poon) mode in my circuit. I used to get good results in @DS 2002. Now I moved to @DS 2003A and I see very different results. I haven't tried this on 2002C.

Is this a bug in the software. Or was there any change in the code that can explain this?

Solution / Workaround

There could be more than one reason for the change in results. In this specific case, we discovered the following.

The BJT model contains the parameter Rbm=0. If you change it to a very small value, say 0.0001, you will get results consistent with the those produced by @DS 2002. It is not enough to leave Rbm blank, as its default value is also zero.

I hope it can help.
Mazz

I just checked my ADS model, for the Gummel Poon model, the Rbm=0.05. I tried to change it to 0, the results are just the same. So I think the reason for my case is not Rbm. I have sent email to the foundary for help. Anyway thank you very much.

BTW, where is the "support center" you mentioned?

Mazz · Apr 23, 2005

Agilent support center, available through the web for customers.
Mazz

bigmac · May 24, 2005

Sorry didn't have time to read all the topic. But I got the same kind of problem between GP-like bipolar models and Hicum models, mainly because GP model is bad when the bipolar is too biased, which can lead to erroneous convergences or non CV.

What is your current density in your bipolars ? Did you check it against the Ft vs Ic curves ?

BTW, I strongly advise you not to use Hicum simulations in @ds with versions prior to 2003c.

[LNA] What's wrong with its gain?

yolande_yj

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