There is a noninverting amplification circuit whose DC gain is 4.3. Due to the large and fixed output capacitor (C10: 10n~100n), the loop is not stable. The open loop bode is attached. besides, the way that adding an additional output resistor (R5: 0~10k) has also tried but no use. is there any other solution for this large capacitive load condition?
Hi Steve
Adding 150ohm R5 resistor and 5pF feedback cap stabilizes the circuit for 100nF load with phase margin above 45 degrees - see below.
Transient simulation confirms stable operation - see below.
Below I have attached Tina-TI schematics for your convenience.
Hi Marek,
Thanks a lot for the help. I find that the Transient stability model that you built is a little different from mine - seen below. Is it because my model does not consider the differential/common input capacitance/resistance that the simulation results are completely different? Didn't these parameters exist in the OPA2188-TINA model？ if so, Is the way I used to test AC stability inaccurate or even wrong?
Hi Steve,
There is nothing wrong with your simulation method - there are several equivalent ways to conduct AC stability analysis and yours is a valid one. The only difference is that your circuit (with R5=0 and C1=0) was NOT compensated to drive 10nF load. If I simulate your circuit with my method, I get exactly the same results of -54 degrees phase margin you got and the circuit is unstable - see below.
