# In distributed amplifiers, is it total input capacitance of the gain stage or Cgs

1. ## In distributed amplifiers, is it total input capacitance of the gain stage or Cgs

Question:In distributed amplifiers, is it the total input capacitance of the gain stage or just the Cgs of the input transistor that gets absorbed into the artificial transmission line?

Hi, so far I was thinking that I should find the total capacitance of the gain stage to get the appropriate inductance value. But if I see any book or papers, it is just mentioned gate to source capacitance. Also same regarding output side. Can someone confirm what exactly it is that I should consider. Thanks.

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2. ## Re: In distributed amplifiers, is it total input capacitance of the gain stage or Cgs

Originally Posted by circuitslave
In distributed amplifiers,
is it the total input capacitance of the gain stage
or just the Cgs of the input transistor
that gets absorbed into the artificial transmission line?
What do you mean by “the artificial transmission line” ?

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3. ## Re: In distributed amplifiers, is it total input capacitance of the gain stage or Cgs

Originally Posted by pancho_hideboo
What do you mean by “the artificial transmission line” ?
If the line is modeled with a discrete set of inductors and capacitors, then it is referred to as an artificial transmission line.

Please refer Radio Frequency Integrated Circuit Design, Second Edition by John W. M. Rogers Calvin Plett (Page # 231)

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4. ## Re: In distributed amplifiers, is it total input capacitance of the gain stage or Cgs

No.
It is a LC ladder or an approximation model of line.

5. ## Re: In distributed amplifiers, is it total input capacitance of the gain stage or Cgs

Seriously, does the discussion about terms help in any way to answer the original question?

Does the term "artificial transmission line" clarify anything compared to "LC delay line" (my favorite designation) or "approximated transmission line"? In any case you can calculate an equivalent characteristic impedance Z0 = √(L/C). If L is the series inductance between two amplifier stages, then C is the total stage input/output capacitance.

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