Hey everyone,
I'm an ECE undergrad working on my first 5-transistor OTA in LTspice (using a TSMC 180nm process), and I've hit a wall with my AC analysis.
Getting the DC biasing right was a journey. I quickly learned that square-law hand calcs don't really survive contact with actual BSIM models! After some tuning, my DC operating point is finally solid: my tail current is sitting at ~19.6µA, it splits nicely to ~9.8µA per branch, and all transistors are well into saturation.
The problem is my AC sweep (1 Hz to 100 MHz). It is completely flat at around 36 dB.
I’m trying to find the unity-gain frequency (UGF) using a standard 1pF load capacitor at the output, but the plot just refuses to roll off. The capacitance acts like it is completely negligible. Through pure trial and error, I found that the gain only drops down to cross 0 dB if I slap a ridiculously massive 791.25 µF capacitor on the output. Obviously, that ruins the actual circuit characteristics.
Things I've already tried:
- Set up a true differential AC input (AC 0.5 phase 0 on one side, AC 0.5 phase 180 on the other).
- Plotted the differential gain explicitly instead of just testing
V(out).
Even with all that, the 1pF response is still just a flat horizontal line at 36 dB all the way to 100 MHz.
Has anyone run into this? Is LTspice somehow ignoring my 1pF cap, or is there a known quirk with how the output pole behaves in these TSMC 180nm models under normal pF loads?