The correct answer is (a) Capacitive effect.
The gain of an operational amplifier rolls off at high frequencies primarily due to internal parasitic capacitances within the op-amp. These capacitances, particularly the compensation capacitor used in internally compensated op-amps, create frequency-dependent paths that reduce the open-loop gain at frequencies above the gain-bandwidth product.
Key points:
- Capacitive effects dominate at high frequencies
- The internal compensation capacitor in op-amps intentionally introduces a single-pole frequency response to ensure stability
- As frequency increases, the impedance of capacitors decreases (Z = 1/2πfC), providing an alternate signal path
- This creates the characteristic roll-off at -20dB/decade
While resistive and inductive effects also play roles in circuit behavior, the capacitive effect is the primary factor responsible for the gain roll-off in op-amp frequency response.