I am currently working on a multi-stage wideband amplifier (80-110 GHz) design with the goal of having 5-6 dB for each stage. The transistor that I am using has the transit frequency around 300 GHz (so we are not limited by the device). My objective is to use a capacitance and a phase inverter for the feedback to widen the bandwidth, something like this:

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(Image source - Figure 5 from paper: Katayama, Kosuke et al. “An 80–106 GHz CMOS amplifier with 0.5V supply voltage.2017 IEEE Radio Frequency Integrated Circuits Symposium (RFIC) (2017): 308-311.)

Without feedback, the maximum gain that I could get was around 3 dB (+- 1.5dB) around the desired frequency range (in other frequencies less than 80 GHz, the gain was around 6-8 dB). Now, when I tried to add a feedback with a capacitor (calculated to be 38fF using the formulas and technique described here: https://www.researchgate.net/publication/280204342_Theory_of_gain_and_stability_of_small-signal_amplifiers_with_lossless_reciprocal_feedback

Then, I tried to add the phase inverter with a simple 90 degree transmission line (microstrip line) that I got with Linecalc tool from ADS (with Zo as 50 ohms), the gain curve almost flat around 80-110 GHz but the gain (S21) is negative (around -5 or -4 dB).

I tried the following:

  1. I noticed that S11 was far away (around 5-j*x) and the feedback signal is not flowing because of the impedance mismatch so, we tried to improve the matching at the input (S11) to 50 ohms by adding a series inductor (around 400-1000 pH) - it did not improve the performance.
  2. Then I tuned the Width, length of the phase inverter (Microstrip line) and the capacitor values. (almost flat around gain around 4-5 dB) but I don't know the logic when I tune.

Could you please suggest some ideas to improve the gain in wideband (without tuning) with this technique?


Your transistor needs to be able to DRIVE that feedback stripline. You don't want 90% or 99% or even 50% of your output energy to be used in generating feedback.

Thus a highZ, high Zo, stripline is appropriate.

In your step#2, you probably ended up with a highZ piece of metal.

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