Trying to design input/output impedance match for amp with smith chart - Page 1
Forum section: electronics / rf-microwave

author: guitchess
author_rank: Regular Contributor
timestamp: April 05, 2022, 05:06:02 pm
content: Hello,After some inspiration from W2AEW, I decided to try to design an amp for my diy low power transmitter. I need some clarification, though. In the datasheet for the transistor, it shows a smith chart and data table for in/out impedance. Is the table normalized to 10ohms as the chart is? Am I completely misunderstanding that matching network design is the purpose of this data?I'm trying to use SimSmith to design the network. There is no doubt I'm out of my depth, but that's why I'm studying.Thanks.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 05, 2022, 05:10:49 pm
content: The center is 1.0 which is often considered as 50 ohms.

author: jimmc
author_rank: Frequent Contributor
timestamp: April 05, 2022, 06:01:46 pm
content: The table is absolute, the chart is normalised to 10ohms(Zo).If you look at the 470MHz figures, the real part is given as 2ohm in the table and 0.2 in the chart.RF Circuit Design by Chris Bowick gives a reasonable introduction to impedance matching using the smith chart.(A quick Google should turn up a pdf copy.)Jim

author: RoGeorge
author_rank: Super Contributor
timestamp: April 05, 2022, 06:46:53 pm
content: Quote from: guitchess on April 05, 2022, 05:06:02 pm1. Smith charts are normalized, the center usually is 50+j0 ohms, but yours seems to be normalized for Z0=10 ohms, IDK why 10. The table contains absolute values. In general, to math the impedance for maximum power transfer, you will need a circuit with the an impedance that is the complex conjugate (same numbers but with the sign for j flipped).No idea how that chart/table can help designing, or why Z0 is 10 ohms there, looks more like a measurement for a given circuit.2. I don't know what that snapshot is, without context it can be anything.3. You may want to try, too, something easier and more interactive, like for example this online tool.https://www.will-kelsey.com/smith_chart/To match an impedance, you'll need to add components until the line ends in the center of the smith chart.

author: RoV
author_rank: Regular Contributor
timestamp: April 05, 2022, 07:31:01 pm
content: I confirm what jimmc said. The Smith chart is just another way of showing the same data reported on the table. Only for the Smith, the 10 ohms normalization was chosen for readability: with a standard 50 ohm chart the curves would have been too close to the center-left part of the circle to be readable; this is universally done for power transistors.As regards impedance matching, you should carefully read the notes below the table: Zsource is defined as the impedance looking into the matching network from the transistor gate; Zload is defined as the impedance looking into the matching network looking from the drain.So, if e.g. your load is a standard 50 ohm antenna, you should design the load matching network adding elements while going backwards from the output, until you obtain the correct Zload. This Zload is inclusive of the inductor connecting the drain to the power supply, which is one of the elements.Same from Zsource: start from the 50 ohm input and add parts until you see Zsource, to be "seen by the gate".When this is done, the device will be optimally driven and loaded according to manufacturer suggestion at the frequency of choice. However, since there are many ways to accomplish this matching, impedances could be "strange" on other frequencies and the device may choose to oscillate... If you have no experience, you had better build something similar to the schematic suggested in the datasheet, adapting values.

author: guitchess
author_rank: Regular Contributor
timestamp: April 06, 2022, 02:11:12 pm
content: Thanks for the replies. You have confirmed the direction I was going. I had reasoned the Z0 = 10ohms chart was for legibility, but could find no way to confirm other than to ask the forum.Quote from: RoV on April 05, 2022, 07:31:01 pmThat's pretty much what I'm doing. However, as my intended freq is at the low end of the suggested range, I was trying to use SimSmith to calculate the necessary match network values.Also, since I forgot to mention in the first post, the part I was trying to use is AFT09MS007N. It's about the cheapest RF fet on DigiKey with an output in the range I was looking for. I've already blown two up. So, I thought I should get some input before wasting more parts/money.