What is Early effect and how does ro influence amplifier output resistance and gain?

• A. The Early effect causes ro to be infinite.
• B. The Early effect has no impact on output resistance.
• C. The Early effect causes finite ro due to base-width modulation; ro reduces gain accuracy and makes gain depend on Vce.
• D. The Early effect only affects input impedance.

Answer: (C)

Early effect is the base-width modulation that happens when the collector-base voltage changes. As Vcb grows, the depletion region at the base-collector junction widens into the base, effectively narrowing the base. This makes the collector current creep slightly with changes in collector-emitter voltage, so the transistor doesn’t behave as an ideal current source.

In small-signal terms, this introduces a finite output resistance ro, roughly ro ≈ Va/Ic, where Va is the Early voltage. Because ro is finite, the amplifier’s output sees RC in parallel with ro (RC || ro). That lowers the usable gain from gm·RC to gm·(RC || ro), and the gain becomes sensitive to Vce since ro itself depends on Ic and thus on Vce through the Early effect. In short, the Early effect creates finite ro, reduces gain accuracy, and makes the gain depend on the collector-emitter voltage.

Other statements aren’t accurate because ro is not infinite (the Early effect does affect output resistance), and it does influence both output impedance and gain, not just input impedance.