Question 1

What is the difference between a common emitter and emitter follower?

Accepted Answer

Common Emitter (CE) provides large voltage gain Av = -gm(RC||RL) with 180° phase inversion. Emitter Follower (CC) has Av ≈ +1 but provides high input impedance and low output impedance, making it ideal for impedance transformation (buffering). Common Base (CB) has low input impedance (~1/gm) and high output impedance, with excellent high-frequency performance due to no Miller effect.

Question 2

How is the small-signal transconductance gm calculated?

Accepted Answer

gm = IC/VT, where IC is the DC collector current and VT = kT/q ≈ 26 mV at room temperature. For example, at IC = 1 mA, gm = 1 mA / 26 mV ≈ 38.5 mA/V. Higher IC gives higher gm and thus higher voltage gain. The input resistance rπ = β/gm determines input impedance, and output resistance ro = VA/IC depends on the Early voltage VA.

Question 3

What does the emitter bypass capacitor do?

Accepted Answer

An emitter bypass capacitor CE in parallel with RE short-circuits RE for AC signals. Without bypass, gain is Av = -gmRC/(1+gmRE) (reduced). With bypass, Av = -gmRC (maximum). The bypass capacitor introduces a low-frequency pole at fL = 1/(2π×CE×RE). This creates a gain-bandwidth tradeoff: higher midband gain at the cost of a higher lower cutoff frequency.

Question 4

What is the Early voltage VA?

Accepted Answer

The Early voltage VA models the output resistance of a BJT: ro = VA/IC. A larger VA means higher output resistance, making the transistor behave more like an ideal current source. Typical silicon BJT values are 50–200 V. In amplifier design, ro appears in parallel with RC, so a low ro reduces effective load resistance and limits achievable voltage gain in high-gain stages.

BJT Amplifier AC Analysis (CE / CB / CC)

Small-Signal Parameters