No Ringing Zone

The problem

Build a non-inverting amplifier with a gain of 2 (6 dB): a resistor R1 from “out” to “inn” and R2 from “inn” to ground, with the signal source “inp” on the non-inverting input. The op amp has a modest 500 Ω output resistance and drives a 100 pF load, which together with a second pole inside the amplifier threaten the stability of the loop.

The feedback factor is 1/2 and the loop gain crosses 0 dB near 5 MHz, close to the second pole: the closed-loop response peaks. A capacitor Cf in parallel with R1 makes the feedback factor rise with frequency (a lead network) and restores the margin.

The test bench

• The test bench supplies the op amp as a ready-made block with terminals on the nets named “inp”, “inn” and “out”: DC gain 80 dB, poles at 1 kHz and 10 MHz, output resistance 500 Ω.
• The test bench hangs a 100 pF load capacitor on “out”.
• The test bench applies the AC stimulus to “inp”.

Numbers to hit

• ◆Closed-loop gain between 5.7 dB and 6.3 dB
• ◆Bandwidth (−3 dB) at least 3 MHz
• ◆Peaking at most 2 dB