A stable circuit is a circuit that does not oscillate.
Oscillations can take the form of either spurious signal generation or
noise generation, and are commonly accompanied by changes in DC
operating point (bias level). They can be brought on by changes in
source or load impedance (reflection oscillator), changes in bias, or
improper grounding.
An unconditionally stable device has the property that it will not
oscillate as a consequence of changes in source or load impedance.
Unconditional stability is frequency and bias dependent.
Mathematically, it corresponds to having k>1 and a nonnegative
determinant, where
k is Rollett's stability factor,
k = { 1  S11^2  S22^2 + D^2} / {2 S12 S21 }
and D is the Determinant,
D = S11 S22  S12 S21
Many Agilent Technologies gain block ICs are
unconditionally stable; and the k factor is often shown with the
Sparameter table. Unconditional stability is important because it
implies that a device can be used in cascades and with filters without
worrying about oscillations.
A conditionally stable device (one with k<1) has the property that there
are some source or load impedances that will cause the device to
oscillate. Care must be taken to avoid incorrectly loading such a
device.
Any device with gain can be made to oscillate if feedback is added.
Feedback may be either shunt (outputtoinput) or series (inserted
between the circuit and ground). Since poor grounding adds series
feedback, it can cause even an unconditionally stable device to
oscillate. Poor grounding is the most common cause of oscillation in
RF components.
