Common-Emitter Amplifier

Core Idea

The common-emitter (CE) amplifier is the most widely used BJT amplifier topology, providing substantial voltage gain with 180-degree phase inversion between input and output. The transistor's emitter terminal is common to both the input and output circuits (grounded through a bypass capacitor at signal frequencies). Voltage gain is determined by the ratio of collector resistance to the small-signal emitter resistance: A_v = -g_m * R_C (or equivalently -R_C / r_e for the simplified T-model), where the negative sign reflects phase inversion. A voltage divider network at the base establishes a stable DC operating point (Q-point), while coupling capacitors isolate the amplifier's DC bias from signal source and load. An emitter resistor R_E provides DC stability against beta variation, but an emitter bypass capacitor must short R_E at signal frequencies to preserve full voltage gain.

How It's Best Learned

Start by establishing the DC bias point using Thevenin equivalent analysis at the base, then replace the BJT with the hybrid-pi small-signal model. Systematically short all DC sources and large capacitors, apply KCL at the collector node, and derive gain and impedance expressions. Compare measured gain with and without the emitter bypass capacitor to see how the un-bypassed emitter resistor trades gain for linearity and bandwidth.

Common Misconceptions

• Assuming the bypass capacitor is optional — without it, the emitter resistance R_E appears in the gain expression and reduces voltage gain dramatically (A_v = -R_C / (r_e + R_E)).
• Confusing the DC bias resistors with the small-signal input impedance — the bias network loads the input in parallel with the transistor's r_pi, so the amplifier's input impedance is lower than r_pi alone.
• Expecting the CE amplifier to have constant gain across all frequencies — coupling and bypass capacitors create low-frequency roll-off, while parasitic capacitances and the transistor's f_T limit high-frequency response.

Explainer

From your study of BJT fundamentals, you know that a small base current controls a much larger collector current — that is the transistor's essential action. The common-emitter amplifier exploits this by routing a small AC signal into the base and extracting an amplified version at the collector. The name "common emitter" means the emitter is the terminal shared by both the input loop (base–emitter) and the output loop (collector–emitter) — it is the reference node at signal frequencies, held to AC ground by a bypass capacitor across the emitter resistor.

Before you can analyze the amplifier's gain, you must set its DC operating point (Q-point). Two resistors, R₁ and R₂, form a voltage divider that fixes the base voltage. This divider is designed so that its Thevenin resistance is small compared to the transistor's input resistance, making the base voltage largely independent of transistor beta — a form of negative feedback that stabilizes the Q-point against part-to-part variation. The emitter resistor R_E further stabilizes the collector current: if temperature causes I_C to drift upward, the voltage across R_E increases, reducing the base-emitter voltage and pulling I_C back down. This DC stability is why the emitter resistor exists, even though it fights against AC gain.

To find the voltage gain, you replace the transistor with its hybrid-pi small-signal model — a controlled current source g_m·v_be in parallel with the output resistance r_o, and an input resistance r_π from base to emitter. At signal frequencies, the bypass capacitor shorts R_E (so it no longer appears in the gain), and the coupling capacitors are also short circuits. The output voltage develops across R_C (or R_C || R_L if a load is connected). Applying KVL around the input loop: v_be = v_in. Applying KCL at the collector node: v_out = −g_m·v_be·R_C. Dividing gives A_v = −g_m·R_C — the negative sign is crucial and reflects phase inversion: the output swings opposite to the input. When the input rises (forward biasing the junction more), collector current increases, the voltage drop across R_C increases, and the collector voltage falls.

The bypass capacitor is the key to understanding the gain-stability tradeoff. With the bypass capacitor in place, R_E is shorted at signal frequencies and the full gain −g_m·R_C is achieved. Without it, R_E appears in series with r_e = 1/g_m, and gain drops to −R_C/(r_e + R_E) — often much smaller. This un-bypassed configuration is actually useful: the emitter resistor provides series-series feedback that linearizes the amplifier, increases input impedance, and extends bandwidth. The designer's choice — bypass or not — depends on whether gain or linearity matters more. This tradeoff between gain and stability through feedback is a theme you will encounter repeatedly as you study multi-stage and differential amplifiers.