Hi, please see attached op-amp testbench.
I need 10MHz GBW, so I have got this in unity gain configuration (ie. output shorted to input, Rfb=0). I used a +/- 1.0V DC sources in place of the Rfb to make sure the output is at maximum slew, where because of reduced Vds over current sources, they have lower Rout (than when at common mode output of 1.65V). This way, I think I can simulate worst-case open-loop gain-bandwidth of amplifier.
But when using amplifier in feedback configuration, eg, with Rin=1k and Rfb=1k, amplifier is still in unity gain mode but GBW drops in half, and Av0 drops by 10dB! Why?! Then, I try increasing Rfb (but I must also decrease Vin, ie. Vin_new=Vin*(Rin/Rfb), else output will saturate), and as Rfb increases, GBW and Av0 keep decreasing. This is still all open-loop simulation, ie. results from stb analysis using cmdmprobe to break all loops.
windknows points out that I need to include feedback factor - I kept doubling Rfb so the feedback factor keeps halfing ie. 1, 0.5, 0.25, 0.125. So should be -6dB/step decrease in Av0 as Rfb is increased 1k/2k/4k/8k. This doesn't match table above though...
I try simulating closed loop gain-bandwidth as well, by using AC sim with AC=0.5 source in series with INP and AC=-0.5 source in series with INN, but do not understand results: Closed loop UGF increases with increasing Rfb! Have added this to the table in second post, please see last column for closed loop gain-bandwidth (GBW_CL).