2022-10-22 10:45 Status: # Op Amp Compensation Biasing - resistors take DC current out of the opamp  Capacitors - more popular for IC design but fiddly bias conditions.    Beta = feedback factor which is determined by the raio of some passive components. Loop gain = -beta x A $\beta \leq 1$ ## Op Amp Two-Pole Model Simplest model to give realistic results.  Simplification: Assume that 1+s/w1 is just s/w1 and thus replace the model at high frequencies with the GBP; wta. Bode plots of loop gain (ignored 1+ for w1 means the horizontal part has been ignored)  Phase at unity loop gain frequency determines the stability. The margin to 0 degrees (or 180 degrees) is called the [phase margin](phase%20margin.md).   Second pole should be higher than the unity loop gain frequency. ## [Two-stage op-amp circuit](Two-stage%20op-amp%20circuit.md) - Small Signal  Dominant pole is the Miller capacitance of the compensation capacitor (since this is much larger than C1, the parasitic capacitance) times the output resistance of first stage.  At high frequencies, the compensation capacitor's impedance goes to zero. Thus the second pole is determined by parasitic capacitances.  If we do not put in the miller capacitance, we get a pole from the first stage which is (1/r01C1) and a pole from the second with is 1/(ro2 C2).  The compensation capacitor also creates a zero - if there is no output voltage (0V) there is no current flowing in each load impedance. So all current flowing into the compensation capacitor flows into the current source.  But this is RHP - drops the phase by 90 degrees (even if it flattens the amplitude). Unstable! ## [Lead Compensation](Lead%20Compensation.md) Better idea: put a resistance in series with the compensation capacitor:  Typically you'd choose Rc = 1/gm7 (zero moves to infinite frequencies) and thus it isn't in the way any more. ## Choosing Rc and Cc - Use a simulator to choose the compensation capacitor large enough so that w2 > wta (amplifier GBP) - Move zero to infinity by choosing Rc - Use a simulator (AC simulations) as an aid - hand calcs can be too complex! - Check it holds for all process corners. [^1] --- # References [^1]: [vr-4602-wk05-sc03-opampcompensation](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W5/vr-4602-wk05-sc03-opampcompensation.mp4)