Question 6A238

In multistage amplifiers, all stages typically share a common power supply. Dynamic current changes in one stage's plate circuit can cause voltage fluctuations on this shared power supply line due to its internal impedance. If these fluctuations are coupled back to an earlier stage with the correct phase, they can create unwanted positive feedback. This positive feedback can quickly lead to uncontrolled self-sustaining oscillations, often referred to as parasitic oscillations, which disrupt proper amplifier operation. Decoupling networks, usually consisting of a resistor and a capacitor (an RC filter), are placed in the power supply line feeding the plate circuit of each stage. The resistor isolates the stage from the main power supply rail, while the capacitor shunts any AC voltage variations on that local supply line directly to ground. This effectively prevents the unwanted AC signals from propagating between stages via the common power supply, breaking the feedback loop, and thereby preventing parasitic oscillations. Therefore, decoupling networks ensure stable amplifier operation by preventing these oscillations.