BJT & FET AC Analysis

Short circuits (AC ground)

During AC analysis of a transistor amplifier, DC voltage sources are treated as

To pass AC signals while blocking DC

Why are coupling capacitors used in transistor amplifiers?

Increase AC voltage gain by bypassing the emitter resistor for AC

The primary purpose of an emitter bypass capacitor in a BJT amplifier is to

A short circuit

At midband frequencies, a coupling capacitor ideally behaves as

Coupling and bypass capacitors

The lower cutoff frequency of an amplifier is mainly determined by

Decrease

If the emitter bypass capacitor becomes open-circuited, the AC voltage gain will generally

It introduces negative feedback (emitter degeneration).

Why does an unbypassed emitter resistor reduce AC voltage gain?

Determines the small-signal parameters.

In AC analysis, the transistor’s DC operating point is important because it

Increases

If the collector resistor is increased while keeping the Q-point in the active region, the voltage gain generally

Source Follower

Which amplifier configuration generally has the highest input impedance?

Decreases

As frequency decreases below the lower cutoff frequency, amplifier gain

Coupling capacitor and Bypass capacitor

Which capacitor usually has the greatest influence on the low-frequency response if its reactance becomes significant?

70.7% of midband gain (−3 dB)

The lower cutoff frequency is commonly defined as the frequency where the voltage gain falls to approximately

A defective coupling or bypass capacitor

A BJT amplifier suddenly exhibits much lower gain at low frequencies, while its midband gain remains normal. The MOST likely cause is

The capacitor behaves as a short circuit only for AC signals.

Why does bypassing the emitter resistor increase voltage gain without significantly changing the DC bias?

It provides excellent impedance matching due to its high input and low output impedance

A source follower has a voltage gain close to one. Why is it still widely used?

Their reactance is negligible compared with the surrounding resistances.

In AC small-signal analysis, why are coupling capacitors assumed to be short circuits in the midband region?