Help with uni assignment - Page 1
Forum section: electronics / rf-microwave

author: faust1002
author_rank: Contributor
timestamp: November 08, 2021, 07:25:43 pm
content: Hello guys,I have a few minor questions regarding to my uni assignment. The main problem is that I don't fully understand what my lecturer. Furthermore, he is not keen on explaining the questions (actually, here I fully get his point). So I decided to ask for here.The first question ask"Based on the expected maximum output power from the data sheet and the corresponding input power, determine the operational bandwidth of your amplifier setting suitable criteria for the transistor used. Comment also on the consistency of the gain within your chosen bandwidth."The amplifier I designed has about 16dB of gain. If I understand the datasheet correctly, 1dB compression point is about 42.5dBm. Assuming the world is perfect, and the gain is always linear regardless of input power, the maximum input power is 26.5dBm, right? If yes, does s11/ s22in order of -20dB make sense. Or should I target s22of -30dB.The second question"You will notice from the data sheet that this transistor is designed to operate with a bias suitable for class AB operation. Suppose you were to re-bias the transistor to work for class A operation. Determine the changes you would have to make from what is recommended on the data sheet and would the matching circuit you had designed in this lab be any longer valid? Justify your answer."I can find the quiescent point quite easily, but I wonder if the matching network should be redesign. On one hand, I don't see any reason why do this. On the other hand, I guess that stray capacitance / inductance or other RF black magic can have impact here.Kind regards

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: November 09, 2021, 02:22:17 am
content: Quote from: faust1002 on November 08, 2021, 07:25:43 pmNope. Compression implies the output is NOT linear (if it were linear, it would not compress!).Quote from: faust1002 on November 08, 2021, 07:25:43 pmIf you compute the change in S21 vs. input mismatch, it will be quite small for input matches less than -15 dB in fact. But mismatch ripple causing gain ripple are a bigger issue so you can't really say what match you should shoot for if you don't know the driving source match or the load match of the next stage. In fact, if the next state is perfectly match, the match of this amplifier almost doesn't matter.Quote from: faust1002 on November 08, 2021, 07:25:43 pmThis is a bit tricky because the biasing will affect the output impedance of the device and presumably the S22. I didn't check carefully your data sheet (after all it's your homework, not mine) but if a change in bias affects S22 you would want to change the matching network. Might be the data sheet doesn't have the proper information.But a more compelling reason is that in real life, the S22 is not constant with power but changes in complex ways (see "X-parameters") that are difficult to infer. You can usually be confident that an amplifier that is properly matched near the compression point must be mismatched in the linear operating regions. this is the complicated part of amplifier design and often not discussed. The old way of figuring this out was simply doing a loadpull on the particular chip you are using. Modern methods include X-parameter analysis and simulation.

author: faust1002
author_rank: Contributor
timestamp: November 09, 2021, 07:47:58 am
content: Hello Joel,Thanks for your answer. However, I am afraid that you got the wrong end of the stick.This is uni assignment- no practical life considerations, no X paramaters, etc.Let's take this step by step - what do you understand by "Based on the expected maximum output power from the data sheet and the corresponding input power, determine the operational bandwidth of your amplifier setting suitable criteria for the transistor used."Kind regards

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: November 10, 2021, 05:01:14 am
content: Quote from: faust1002 on November 09, 2021, 07:47:58 amYeah that confused me a bit and I don't have a good answer. If you say the max output power of 43 dBm (at 1 dB compression) means you need 33 dB of drive (based on a gain of 11 dB power gain presumably before compression so 10 dB at compression) and you created a matching network with a single element or 2 element match at the input and the output, you could use the provided S-parameters, matching at the mideband (e.g. 2.3 GHz for the middle device) and find the 3 dB corner points of the gain, giving you an operational BW under this given matching condition. But do write back with what the grader says is the proper answer.

author: faust1002
author_rank: Contributor
timestamp: November 10, 2021, 04:46:30 pm
content: Sure, thanks for further explanation. I guess I will limit myself to the region when gain remains linear. Based on that, I will figure out the maximum possible input power. Then, based on this I will select frequency range, where s11and22are "reasonably low" (whatever this means). As for s21, surprisingly 3dB criterium was not discussed during lectures as all, but I will this to my report.Thanks for your help.

author: rfeecs
author_rank: Frequent Contributor
timestamp: November 10, 2021, 06:07:20 pm
content: I have no idea what the instructor intended, but I will point out that the FLL200IB-1, -2, -3 are internally matched.All the data presented is with 50 ohms in / 50 ohms out. Typically there is no matching circuit used with these.Also, all the data is at 60% Idss bias. This is class A bias. Although it says performance in class AB bias is ideal (blah blah blah), all the data is under class A bias.