Why Circuits Oscillate Spuriously, Part 3: Amplifier
Loading
by Dennis L. Feucht
Innovatia Laboratories
In this third, and final part, we continue this TechNote with a difficult
aspect of amplifier design: instability due to reactive output loading of
a feedback amplifier. In Part 2, amplifier hf-region oscillation due to
reactive loading was considered. Reactive loading can also cause feedback-amplifier
loop instability. The design equations needed to isolate the amplifier output
from the load and achieve an acceptable dynamic response are developed and
explained here.
Output Load Isolation
In some feedback amplifiers the load impedance is highly reactive and
the amplifier has a significant output resistance Ro.
This combination can add a load-dependent output pole to the loop. A method
for isolating capacitive loads is shown below, where CL
is the load, and Ro is the amplifier open-loop output
resistance.
...download complete article here (68kb PDF file)
...download complete Part II here (312kb PDF file)
...download complete Part I here (127kb PDF file)
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