Discrete-circuit BJT amplifier

A discrete-circuit BJT amplifier is a board-level BJT amplifier — individual transistor and resistors, biased from a single supply, with capacitors that separate the DC bias from the AC signal. The standard form is the four-resistor common-emitter stage.

Four-resistor voltage-divider bias

The biasing scheme is the Voltage-divider bias four-resistor topology:

• $R_{B1}$ (from $V_{CC}$ to base) and $R_{B2}$ (from base to ground) form a divider that pins the base voltage $V_B$.
• $R_C$ from collector to $V_{CC}$.
• $R_E$ from emitter to ground.

Then $V_E=V_B-V_{BE}$ and $I_E=V_E/R_E$ set the operating point — independent of $\beta$, provided the divider current is much larger than the base current (rule of thumb, 5–10×). If the divider were starved, $I_B$ would load it and a $\beta$-dependent error would creep back in; sizing it stiff is what makes the bias robust. The DC procedure is BJT DC analysis; the other common bias schemes (single-resistor, divider+$R_E$, dual-supply, collector-to-base feedback) are summarised below.

BJT bias schemes left to right: four-resistor voltage divider, single-resistor, voltage-divider with emitter resistor, dual-supply $(+V_{CC}/-V_{EE})$, and collector-to-base feedback. The slide title “Basing” is a typo for “Biasing.” The voltage-divider and dual-supply schemes give the best stability against β variation.

Coupling and bypass capacitors

Three capacitors keep the DC bias and the AC signal from interfering:

• Input coupling capacitor $C_{C1}$ — between the source and the base. Passes the AC signal but blocks DC, so the source cannot disturb the carefully-set $V_B$.
• Output coupling capacitor $C_{C2}$ — between the collector and the load. Passes the AC output but blocks the collector’s DC level from the load.
• Emitter bypass capacitor $C_E$ — across $R_E$. At DC it is an open circuit, so $R_E$ is fully present and stabilises the bias. At signal frequencies it is a short, grounding the emitter for AC and restoring the full CE gain $-g_m{R}_C$ that the un-bypassed $R_E$ would otherwise reduce (the Emitter degeneration gain penalty). This is how you get DC stability and high AC gain at once.

Discrete-circuit common-emitter amplifier: voltage-divider bias for stability, input/output coupling capacitors $C_{C1}$ and $C_{C2}$, emitter bypass capacitor $C_E$ to short $R_E$ for AC while leaving it for DC.

Small-signal input and output resistance

With the bypass and coupling capacitors treated as AC shorts, the bias divider appears in parallel with the transistor’s [[BJT input resistance|$r_{\pi }$]], and the output sees $R_C$ in parallel with the device $r_o$:

$R_{in}=R_{B1}\parallel R_{B2}\parallel r_{\pi },R_o=R_C\parallel r_o$

Coupling caps pass the signal and block DC; $C_E$ shorts $R_E$ for AC. The small-signal model gives $R_{in}=R_{B1}\parallel R_{B2}\parallel r_{\pi }$ and $R_o=R_C\parallel r_o$.

Common-base variant

The discrete Common-base amplifier uses the same four-resistor bias but the input is coupled into the emitter and the base is held at AC ground by a bypass capacitor. There is no emitter bypass capacitor — the emitter is the signal node, so it must not be shorted to AC ground.

Discrete-circuit common-base amplifier: signal coupled into the emitter, base held at AC ground via a bypass capacitor.

Frequency response

These three capacitors also create the low-frequency rolloff. Each capacitor, with the resistance it sees, sets a low-frequency corner ($f\approx 1/(2\pi RC)$); below that corner it stops being a good short/pass and the gain falls. The dominant low-frequency corner is usually the emitter bypass $C_E$, because the resistance it sees ($\approx R_E\parallel r_e$, often only tens of ohms) is the smallest of the three, which makes its corner the highest — so it is the last to “kick in” as frequency rises and therefore dominates the low end. At the high end the response is limited by the BJT’s internal $C_{\pi }$ and $C_{\mu }$, the latter magnified by the Miller effect — which is why a CE stage has lower bandwidth than a CB stage built around the same transistor. Full treatment in Amplifier frequency response.