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Broadband SiGe HBT Amplifier using CPW

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Puppet123

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Hello.

I am designing an amplifier in SiGe HBT for operation in broadband frequency ranges of 0-85GHz - it is a broadband LNA basically. Not a distributed amplifier, but a lumped one.

I am using CPW for the passive elements and not spiral inductors for better control of the gain over the whole frequency range.

My questions: how do I size the CPW inductors ? Do they have to be a certain wavelength of the frequency (lamda over 4, lamda over 2) and what frequency should I design them for - the 3db frequency of the amplifier - since it is broadband ?

Are there any resources on designing inductors in CPW for amplifier design that anyone can provide ?

Thank you.
 

i have designed CPWG TLs and not inductors. the rule of thumb for any lumped element in a design is for them to have self-resonance outside the band of operation. meaning your inductor SRF should be > 85GHz. is it possible ?

you should optimize the design with the lumped element and get the value of the lumped element 1st. after that you should do start converting the lumped element into CPW elements.
 

I agree with pragash that you should design with ideal lumped elements first, to see what values you need. Inductance implemented by transmission line is fine for narrow band use at very high frequency, where circular spiral geometry doesn't provide small enough values. But in general, a line is not a replacement for an inductor, because the wideband response is very different.

For your wideband use, I would expect that you need large inductance value -> long line -> behaves like inductance only at low frequency, transmission line behaviour (=distributed LC) at high frequency.
 

Not a type, this is a wideband amplifier for optical communications running at 100 Gb/s.

Most wideband amplifiers at this frequency I have seen using CPW lines as passive elements even in silicon (hajimiri has been doing it since fifteen years ago in silicon) so it is possible.

So use an ideal inductor first as baseline then size an equivalent CPW line - is that correct?
 

An inductor or a capacitance which you will use will have a parasitic model(with parasitic L and C's depending on model)

Meaning: Inductor itself will turn into a RLC circuit by nature, and go into self-resonance at some point. Since you are planning on building a wideband amplifier; first you should be sure that your components will work as expected.

I built VCO's in UHF range using TL as inductors in the past. Using TL as inductor means you need to use it in narrowband, or you need some kind of switching of TL's to go into wideband.
 

Puppet123 said:
So use an ideal inductor first as baseline then size an equivalent CPW line - is that correct?
Click to expand...

The basic question is if your design requires an inductance response or a transmission line response. You can convert exactly only at one frequency, not broadband.

We don't know your circuit design, but you mentioned "lumped amplifier, not distributed" so yes, start from an ideal inductor and see what values you need.
 

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