Amplifiers and return loss...

I've noticed that most InGaP MMICS have rotten S11s, typically running around 30 ohms wideband Zin. I'm guessing they go for better noise figures at the expense of matching. Some of the SiGe parts are very close to 50 ohms... maybe that's because their NFs are better there.

John

I think you need to determine first if you really care about input or output return loss. There are reasons to not worrying about it, and other reasons why you would want to worry about it. We make one piece of equipment that has a guaranteed 46dB input return loss, as I recall. I remember thinking it's not much fun to have to test that and guarantee it over some range of temperatures, etc. It certainly IS possible to design a circuit to present specific input and output impedances, at least at certain frequencies.

But as John L. pointed out, the best noise figure is generally not for an impedance matched input in a simple amplifier; and a power amplifier may be designed to operate to spec into a 50 ohm load, while itself presenting some much different source impedance. As an extreme example, consider a hifi audio amplifier that might present an 0.1 ohm or lower source resistance, but be designed to operate optimally into a

Cheers, Tom

In addition to Tom's excellent points, consider how S-parameters are defined and measured.

By definition, all ports are perfectly terminated in the system impedance and the reflection coefficients and transmission coefficients are measured one at a time.

If for example, s22 is measured and s12 is not zero then s22 is influenced by the termination on port 1. In the measurement system, s11 is zero but in actual operation it is something else, thus s22 is also something else.

So in the usage environment unless all of the external terminations are perfect, the s-parameters, while not useless, are deceptive. Gain and stability can be predicted using them, but only if -all- of them are considered simultaneously.

In other words, why can't the input/output return losses of real amplifiers be, let's say, better than -20 or -30dB (within a certain narrow frequency band),

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Bill,

yes it is not hard to design a matching circuit over a narrow range of frequencies if that were needed.

It is more difficult to match over a broad range of frequencies.

The greater the transformation needed, the more difficult it is to make it broadband.

Matching at a single frequency is usually easy.

Mark

Hello Mark,

Not always. Sometimes you either need narrow tolerance caps (meaning expensive) or hand tuning in production (very much frowned upon). PCB material can become quite expensive as well if you want a tightly toleranced dielectric coefficient.

Regards, Joerg

Some of the Sirenza SiGe wideband mmic amps are very close to 50 ohms from DC to several GHz. In fact, you can adjust their operating current to tweak them to exactly 50, or anywhere from about 40 to 60 ohms, never mind that Sirenza said it wouldn't work. I have some graphs around here somewhere...

If you can stand a 2-3 dB noise figure, it's hard to beat a 99-cent MMIC as a gain element.

John