I'm trying to understand how this circuit (a low-noise amplifier) works from this paper. According to my understanding, the first stage is a class AB amplifier and the second stage is a simple follower. Capacitive coupling is used to block low-frequency noise, and DC biasing is added through the feedback of an opamp. The 4 capacitors are also used as negative feedback to set the gain.
I built the first stage in Cadence and tried to simulate it. However, I got almost nothing from the output (tens of micro-volt signal). gm1 and gm2 are both about 8uS, and the output impedance of M1 and M2 are about 50M Ohm. So I was expecting a gain of 400x.
I ran a transient simulation and found that the signal at the gate of M1 and M2 are in opposite phases. I was wondering if this was the reason of the problem, as M1 and M2 should receive nearly identical inputs to work properly? How can I fix it?
Best Answer
This circuit works OK when built from junkbox parts, so if the simulation doesn't work, it's the simulation's problem. I've built it with a 12 V supply, discrete small signal MOSFETs, and LM334s set to 1 mA as current sources. Worked on first try, with a series resistor on the output of the op-amp.
The mistake you made in simulation is not using a low-conductance-output discrete op-amp model. The circuit was drawn without a series resistor on the op-amp's output because it's easy to control output resistance of an op-amp with device geometry when you're designing an IC. The on-chip op-amp's output would be inherently weak. You used a generic op-amp model that doesn't represent any particular op-amp that could be built in the process node you chose.