Govt. Exams
Entrance Exams
For sustained oscillation: (1) Barkhausen condition: |Aβ| = 1 (unity loop gain), (2) Phase condition: ∠Aβ = 0° or 360°. In Wien bridge, gain needed ≈ 3 to compensate losses.
Bootstrapping increases input impedance by reducing effective base current drawn from source through capacitive feedback. This minimizes loading effects on high-impedance sources.
Log amplifier uses diode in feedback. Temperature changes cause Is (reverse saturation current) to vary exponentially, directly affecting the logarithmic transfer function. This is the dominant non-ideal effect.
Series-shunt (voltage-series) feedback reduces output impedance by factor (1+Aβ). Other configurations either increase impedance or have minimal effect on output impedance.
Class AB combines Class A and Class B characteristics. Maximum efficiency approaches π/2(√2) ≈ 78.5% for Class B, but with Class A bias, typical maximum is around 88.5% under ideal conditions.
Gain is inversely proportional to load transconductance. To maximize |Av|, we need to minimize gm,load, which is achieved by reducing W/L ratio of the load transistor.
The input stage of a 3-op-amp instrumentation amplifier provides very high input impedance (prevents loading) and adjustable gain through a single external resistor, which is then amplified by a differential stage.
BJT current mirror output impedance = 1/gm (Early effect) × (1+λ), where λ is Early effect parameter. This is typically in the range of MΩ, providing high impedance.
Increasing Rf increases gain proportionally (V_out = I_in × Rf). However, thermal noise of Rf (4kTRf/Δf) increases, degrading noise figure.
For negative feedback, Acl = A/(1+L) where L = Aβ is the loop gain. This formula shows how feedback reduces gain but improves stability.
