Impedance Matching (A Match made in heaven)

Hi all, I'm browned off from reading unhelpful literatuire about impedance matching networks.

I have two questions that need answering if possible. Q1. I have a CE Amp with Rc=576ohms so Zout =576 ohms. The CE Amps a CC Buffer with Zin=15Kohms. How do I match these two circuits for transferring the full and propper RF energy.

Q2. How do you know if the input or output impedance has reactive components in it. If the CE amplifier above has capacitor on the output does its Zout become

Regards, Kevin.

PS the avtice devices I'm using are the mps 5179 npn Transistor ft=900Mhz.

matching

Theres been a few of these questions lately.

With Zout Q2. How do you know if the input or output impedance has reactive components

If its a largish decoupling capacitor in series with the output it can probably be ignored, the stray capacitances, inductances and transister charecteristics become an issue at high rf frequencies, like a capacitor in parallel with the Rc it will add a reactive component.

Colin =^.^=

You don't always need to. Impedance matching is typically required between:

1. antennas and recivers;
2. transmitters and antennas;
3. high-power stages in a transmitter

See the recent thread "How come its OK to mix impedances in a radio system??" in s.e.b.

Judging by the impedance levels you mention, those are low-power stages. They can be voltage driven. Between higher power stages, the matching network is designed to present the driver with just the right dynamic collector load to suck out the correct amount of power needed to drive the following stage. You've probably seen examples using P = Vpk^2/2R to calculate collector load.

At the other end of the power range - in receivers - impedance matching is important for totally different reasons: to preserve precious pico-watts of energy, maximising the signal-to-noise ratio. At low and intermediate power levels, it doesn't matter.

You can work out how much drive a stage needs by subtracting the transistor manufacturer's stated gain from the desired output power. Or you can do it by trial and error. It's good to have a way to adjust drive.

If the manufacturer's datasheet doesn't tell you, you can always include trimmers in your circuit to allow for some latitude of adjustment.

Not if the capacitor and resistor are (effectively) in parallel.

I dont think they do. Its usally a case of a smart arse proffessor trying to be clever with a bit of calculas. You know, the bit about diferentiating and setting to zero. Then the dumb s*it students pay no attention at all as to the conditions of the derivation.

The max power theorem is when, given a *fixed* source impedance, and the load is varied, then value of load gives max power when RL=RS. If the load is fixed and we want to find the max power into it, by varying Rs, the max occurs when Rs=0!

Kevin Aylward snipped-for-privacy@anasoft.co.uk

matching

components

Kevin.

Time to get a book or two.

Solid State Design for the Radio Amateur - published by ARRL

RF Circuit Design - Chris Bowick - published by SAMS.

Introduction to RF Design - Wes Haywood - Prentice Hall

Flipping thru Bowick's book I see a chapter on Small Signal RF Amplifiers and one on Power Amplifiers - nice discussion on the difference. Also a chapter on Impedance Matching". Clear worked examples. You can build transmitters with this information! ;) Recommended.

Perhaps this thought will help. Mostly, we are not impedance *matching* - we are impedance *transforming*. The same networks and maths is used for both. Mostly, we transform the impedance of the load into the impedance required by the amplifier. The impedance required by an amplifier depends on its output voltage swing and the power it can safely deliver. In the rarer cases where we do impedance match, we use transform the load to the correct value - often 50 ohms - stamped on the amplifier output etc.

Roger

Maybe, just seemed to me there does seem to be a surprising number of people who think this though. I cant remember how they taught it at uni, maybe I wasnt paying attention !

yes thats a better way of explaining the situation actualy, youl get least smoke too ! :D

Colin =^.^=

Hi there, The circuit I have is in the FM Band 88-108. Yes I think I need to get a good book on the topic. I remember my lecturer in college using the system of transfer of max power for every bit of theory we did "Thanks Michael!" It looked great and and clean cut on the board but now I'm finding it doesn't work in the real work. I am getting some good help here though, thanks!

Kevin.