Oscillation
Understanding Unwanted Oscillation
Oscillation is one of the most common failure modes in RF amplifier design. When the Barkhausen criteria are met (loop gain >= 1 and phase = 0 or 360 degrees), an amplifier becomes an oscillator. This can happen at the operating frequency, at out-of-band frequencies, or even at VHF/UHF where transistors have high gain.
Causes of Oscillation
- Inadequate stability margin: K-factor < 1 indicates potential instability.
- Poor grounding: Inductive ground paths create feedback.
- Output-to-input coupling: Physical proximity, shared ground paths, or radiated coupling.
- Improper bias decoupling: Bias supply lines provide feedback paths.
Prevention
- Stability analysis: Check K-factor and mu-factor across all frequencies.
- Stabilization resistors: Series or shunt resistors at input/output reduce gain at problematic frequencies.
- Decoupling: Multiple bypass capacitors on every bias line.
- Isolation: Shielding between input and output.
|T(f)| >= 1 (loop gain)
angle T(f) = n x 360 deg (phase)
Rollett stability factor:
K = (1 - |S11|^2 - |S22|^2 + |Delta|^2) / (2|S12 S21|)
K > 1 and |Delta| < 1: unconditionally stable
mu factor (Edwards-Sinsky):
mu = (1 - |S11|^2) / (|S22 - Delta x S11*| + |S12 S21|)
mu > 1: unconditionally stable
Frequently Asked Questions
What causes unwanted oscillation?
Oscillation occurs when amplifier output feeds back to the input with enough gain and the right phase to sustain itself. Common causes include poor grounding, inadequate bias decoupling, output-to-input coupling, and operating the amplifier with mismatched terminations.
How do you check for oscillation risk?
Calculate the Rollett stability factor (K) from S-parameters across all frequencies. K > 1 with |Delta| < 1 means unconditionally stable. If K < 1 at any frequency, the amplifier can oscillate with certain load/source impedances at that frequency.
How do you stop oscillation?
Add stabilization resistors (series at input for low-freq stability, shunt at output for in-band). Improve grounding with more vias. Add decoupling capacitors on bias lines. Increase physical separation between input and output. Use absorptive filtering at out-of-band frequencies.