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Design >> Analog Design >> Impedance matching question
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Message started by vivkr on Jul 26^th, 2005, 4:55am

Title: Impedance matching question
Post by vivkr on Jul 26^th, 2005, 4:55am

Hi,

I was looking through Tom Lee's book on RF design (T.H. Lee-The Design of CMOS RF ICs), and I ran into this statement which I dont understand. Perhaps, someone could throw some light on it.

The author says that impedance matching becomes more critical at higher frequencies because there is limited power gain available in this region, while at low frequencies, there is virutally unlimited power gain available. I dont understand what this really means. What exactly is this power gain and why is it plentiful at low frequencies and not at RF?

As far as I know, one tries to match impedances wherever feasible, even for low frequencies. For example, all the measurement equipment and oscilloscopes come with matched terminations. From what I see, it is usually not possible to match impedances in low-frequency ICs owing to the large passive components that would be needed to accomplish this goal. But it seems that I was wrong in assuming this.

Your feedback is most welcome.

Thanks
Vivek

Title: Re: Impedance matching question
Post by Eugene on Jul 26^th, 2005, 10:21am

I believe the author means that it is easier to create near-ideal voltage and current sources at low frequencies. An ideal source delivers as much power as the load requires. For example, a car battery can deliver enough DC current to destroy a DC load if the load malfunctioned. However, if a malfunctioning load tried to draw current straight from the battery at 1GHz, the internal impedance of the battery alone might protect the load. At high frequencies, it becomes harder to find devices/circuits that can deliver "zero" or "infinite" output impedance. Given that one must deal with a finite output impedance ( a non-ideal source), the best one can do is to match the load to the source.

Title: Re: Impedance matching question
Post by Ken Kundert on Jul 26^th, 2005, 10:18pm

Test equipment comes with matched terminations for a somewhat different reason. With things like receivers, the circuits are very narrowband, and the goal behind impedance matching is to maximize the power transferred. Here the matching is done with reactive components (capacitors and inductors). Impedance matching with lossless components effectively gives noise-free gain, and one would naturally want to maximize that. However, test equipment is necessarily broadband. As such, scopes and spectrum analyzers and the like provide a pure resistive termination so that you can achieve a broadband match. If you could not achieve a broadband match, then the spectrum of the signal displayed on the test equipment would be colored by the frequency response of the match.

-Ken