MMIC amps: Input impedance

Well, this particular MMIC is not specified as "a generic 50 ohm building block" as some others are. I guess the dBm simply refers to power level in general, not usual RF dBm(50) or telco dBm(600). That would also mean that you might need more input voltage level to reach the same input power in dBm than you would if the preceeding stage is designed for 50 ohm load.

But even a BGA616 which IS specified as "Cascadable 50 ohm gain block" does not always behave as close to 50 ohm in/out in my simulations (SPICE is available). Almost a factor of two off can take place depending on biasing and input impedance.

Joop

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That came out wrong. I mean the input/output impedance varies with the impedance of the stage on the other side of the device. Supply biasing through resistor only or resistor/inductor combi has influence as well.

Hmm, I also looked at the demo board description.

That seems to suggest both input and output would be about 50 ohm (with SWR 1:1.5).

Joerg, they want you to AC couple anyway. So from the reverse biased photodiode an inductor to GND and a cap to the input.

Rene

Thanks, Joop. I'll use a resistor/inductor series combo since the needed frequency range is wide and there is that dreaded inductor series resonance.

The output also looks a bit high-Z-ish so I'll have to spring for a BFS17 follower there. The BGA616 looks better but not perfect either and its noise figure isn't so hot.

Yes, Rene, that is about how I plan to do it. Looking at the output chart S22 it seems that this MMIC (and lots of others) doesn't really like to drive a 50ohm load. More like several hundred ohms. Guess I'll spring for the usual BFS17 follower. If it was a really high volume product I'd also do the MMIC part with a couple transistors but not in this case.

Looks nasty. It's not unconditionally stable, and has weird impedances vs frequency. Whenever you see a mmic with a nf much below 3 dB, you can be certain that it's for a narrowband, tuned application. It looks sort of like a transistor with a bias generator.

The datasheet says that the nf is measured at optimum match, which I interpret as "tuned."

The Sirenza SGA-3586 SiGe part is about the lowest-noise, wideband, "true 50 ohm mmic" around.

John

Check this one out cross-ref to sga-3586, specs look a little better too.

I think that's what it is, a transistor with a bias circuit. I hope it's stable if I provide a nice 200ohm load. The plan was to follow it with a buffer so whatever else is connected (not under my control) won't upset the apple cart.

But how could they call it MMIC then? Marketeers at work?

It looks nice but the S11 and S22 also take a nose dive around 500MHz. I'll be staying under 100MHz. What amazes me is how cheap SiGe parts are now. There goes the art of designing our own amps...

Indeed. Thanks. But that poor little thing has to dissipate over 400mW. Probably causes a good blister if anyone touches it ;-)

Eventually nasty people (like a certain JT who I won't identify) integrate anything that's fun, so it's not worth designing ourselves any more. So we have to move up the abstraction stack.

John

The Sirenza is a lower power part and the NF is a bit better. But Hittite is making some kickass parts lately, like their '465 20 GHz distributed amps and things.

I like the Sirenza because it's really 50 ohms in, and in fact you can tune the input impedance by varying the bias current, so it's nice for time-domain signals at the end of a coax. It's a very nice medium-speed mmic, although is is a bit fragile as regards input zaps.

John

A bit less power would be great, I don't like stuff in my designs that get too hot to the touch. Except, of course, tubes :-)

Here the only way to create a zap would be a really bright flash. And when that one comes then I guess the performance of this amp will be of secondary concern...

There seem to be some more good analog chip guys out there. But even 30c a pop is too much at times. Not in this app but when it's a mass product I'll always be designing around a 5c RF transistor. Especially in cases when, as you had mentioned, the MMIC isn't much more than a glorified transistor with a bias. You can get a real noise figure of under 1.5dB for less. Just like with burgers, if you make your own from ground beef they cost less and taste better (we don't do them any other way).

I like a cheap Mini-circuit's ERA-33SM when I need to boost an rf signal.

I'll check that out, Max. Haven't looked at Mini-Circuits much because they are often single-sourced. But then again they have never disappointed me in my whole career and always delivered (mixers etc.). Unlike another company where the name also starts with "M" ;-)

The ERA-series parts are fine. They are absolutely stable and fairly rugged for an exotic part. I use them as fast pulse amps and as sub-100 ps photodiode amps, although I do have a fiber-coupled optical trigger pulse that's measured in watts! Their input impedances tend to run low, in the 30's maybe, when the output is matched. And they do run hot, although InGaAs melts at a higher temp than silicon!

All the classic darlington mmics have a Zin that depends strongly on output loading. They are very handy parts.

Mini-Circuits now has some faster, 8-GHz range, parts, and some nice higher-power SOT-89's. W-J makes nice SOT-89 mmics, too, probably the highest power ones around.

John

Sub-100psec? Wow, that is performance. I just don't like semiconductor stuff that gets really hot.

A little higher Z-in would be good in my case. Although I am afraid I am going to be flooded with DFB noise but that's what I can't know until the circuit runneth.

Have you seen pathologies or instabilities with the darlingtons? The old BGA2001 still looks enticing with it's high Z-in. Intercept is a bit marginal because it's a low current device.

Some of the SOT-89 parts I looked at appeared to be able to unsolder themselves...

My observation is that if you want a NF below 3 dB, from a 50 ohm signal, you've gotta tune the input. Some of the NEC phemts get below

I just had a burger with Mo at the Zuni Cafe, and I'd be astonished if any mere mortal can make their own burger that good. They serve it on fresh focaccia with homemade pickles and pickled onions and homemade aioli. $12.

John