Download Active Circuit Stability Analysis

Active Circuit Stability Analysis
Active circuits can be divided into an active part and frequency determining part (example in LNA
design the transistor input may be viewed as active and input passives set matching frequencies).
Assumption: in a series circuit (see above), the current entering the active part in steady state is
approximately sinusoidal (in a parallel case the voltage across the division node is sinusoidal).
Instability criteria:
In series cases involves impedances of the two circuit parts as seen from the chosen node (see
below)
In parallel analysis, the criteria involves admittances of the two circuit parts
The above conditions for starting oscillations can be tested on any node of the circuit by “looking” at
impedances into both sides of the node and checking. The conditions are derived by solving the
“Characteristic Equation” of the circuit:
Z S (S ) Z L (S )
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