I've a simple buffer amp (AD8044 video amp, 150MHz, high slew rate) set up to buffer a signal (NTSC monochrome video) as a basic follower.
The circuit is the most basic one and of course includes large and small (>10uF, 0.1, 0.01) bypass caps.
With a lower slew-rate amp (MCP6294) the circuit works as expected, but when the AD8044 is inserted in its place, the output goes wild by swinging between rails with great instability.
Adding a cap (0.05uF) to the output terminal almost eliminates the instability, but there's still a little bit of noise on the signal.
1) Why is the '8044 camp unstable without the output cap? I can see how it "slows down" the frequency response of the feedback loop (essentially filtering), but shouldn't the amp be stable without it? The capacitive load is just a 10x scope probe.
2) Why is there a little noise injected with the faster amp, and how can I remove it?
The circuit is the most basic one and of course includes large and small (>10uF, 0.1, 0.01) bypass caps.
With a lower slew-rate amp (MCP6294) the circuit works as expected, but when the AD8044 is inserted in its place, the output goes wild by swinging between rails with great instability.
Adding a cap (0.05uF) to the output terminal almost eliminates the instability, but there's still a little bit of noise on the signal.
1) Why is the '8044 camp unstable without the output cap? I can see how it "slows down" the frequency response of the feedback loop (essentially filtering), but shouldn't the amp be stable without it? The capacitive load is just a 10x scope probe.
2) Why is there a little noise injected with the faster amp, and how can I remove it?