Why Circuits Oscillate Spuriously, Part 1: BJT
Circuits
by Dennis L. Feucht
Innovatia Laboratories
One of the more troubling aspects of analog circuit design is amplifiers
that oscillate when they are not supposed to. Engineers commonly respond
to spurious oscillations with empirical efforts to defeat the instability,
by throwing small-value resistors, ferrite beads, and bypass capacitors
at the problem (whether in a hardware prototype or circuit simulation),
hoping it will go away. Perhaps these kludged circuits stop oscillating,
but then the performance has been degraded. Or perhaps occasional production
units oscillate. What is a circuit designer to do?
The answer, of course, is to understand why the circuit oscillates. There
is always a rational, underlying cause for annoyingly unwanted sinusoids,
and finding it can lead not only to a stable circuit, but also a performance-optimized
circuit. Not only can the oscillation be modeled and its compensation designed,
the margin of stability can also be estimated so that you can know how far
from oscillation the circuit design is.
...download complete article here (127kb PDF file)
Contact the author