Govt. Exams
Entrance Exams
PSRR (in dB or V/V) measures how much power supply voltage variations appear at output. Higher PSRR (more negative in dB) = better rejection. PSRR varies with frequency and is crucial for low-noise applications.
Coupling capacitors block DC bias from one stage to the next, preventing Q-point shifts. They allow AC signal passage while maintaining proper biasing independently in each stage.
Bridge amplifiers (like instrumentation amp derivative) reject common-mode signals while amplifying differential signals from sensors, ideal for noisy environments.
Current-series (shunt-series) feedback increases both input impedance (series connection) and output impedance (current feedback). This is used in circuits requiring high impedances.
For a summing amplifier, Vout = -Rf(V1/R1 + V2/R2 + ... + Vn/Rn). Each input is weighted by Rf/Rin ratio, not just simple addition.
Hysteresis voltage ΔV = 2Vsat × R1/(R1+R2), where Vsat is op-amp output saturation voltage. It depends on both the feedback resistor ratio and the op-amp's output swing (supply voltage).
Common-gate configuration presents low input impedance (≈ 1/gm) and high output impedance. It provides good isolation and is useful for impedance matching.
In transimpedance configuration, gain = Rf. However, bandwidth is inversely proportional to Rf due to feedback capacitance (Cf ≈ Cp + Cin), creating a gain-bandwidth tradeoff.
Finite output impedance of tail current source reduces the symmetry of the differential pair, degrading Common Mode Rejection Ratio. Higher impedance improves CMRR.
Negative feedback must be applied in opposition to the input signal. For inverting topology, feedback goes to inverting terminal; for non-inverting, to non-inverting terminal. The feedback signal opposes the original input.
