Electronics: BJT and FET Transistors

Transistors sit at the center of modern electronics because they can act as controlled switches and as amplifying devices. Two families dominate analog and mixed-signal design: the bipolar junction transistor (BJT) and the field-effect transistor (FET). While both can build amplifiers, their control mechanisms, biasing methods, and small-signal behavior differ in ways that matter in real circuits. This article walks through operating modes, DC biasing design, small-signal models, and the classic amplifier configurations: common-emitter (CE), common-base (CB), and common-collector (CC), with FET counterparts where appropriate.

BJT fundamentals: current control and operating modes

A BJT uses a small base-emitter current to control a larger collector current. In active operation (the region used for linear amplification), the collector current is approximately

$$I_C\approx \beta I_B$$

where $\beta$ (or $h_{FE}$) varies across devices and with temperature. More fundamental is the exponential base-emitter relation:

$$I_C\approx I_S{e}^{V_{BE}/V_T}$$

where $V_T\approx 25.9\text{ mV}$ at room temperature. This exponential behavior is why $V_{BE}$ changes only modestly with current, and why temperature drift is a practical concern.

BJT operating regions

• Cutoff: Base-emitter junction not forward biased; $I_C$ is near zero. Used for switching “off”.
• Forward active: Base-emitter forward biased, base-collector reverse biased; linear amplification region.
• Saturation: Both junctions forward biased; the transistor is “fully on” as a switch, but not linear.
• Reverse active: Rarely used; roles of collector and emitter effectively swapped.

For amplifier design, the goal is to bias the transistor in forward active with enough headroom so the signal does not push it into cutoff or saturation.

FET fundamentals: voltage control and operating modes

A FET controls current through an electric field. In a MOSFET, the gate is insulated, so the DC gate current is ideally near zero. That high input impedance simplifies biasing and is a major reason FETs are popular at sensor inputs and in high-impedance stages.

Common FET operating regions (MOSFET language)

• Cutoff: $V_{GS}$ below threshold; drain current is very small.
• Triode (linear) region: Acts like a voltage-controlled resistor; useful in switches and some analog applications.
• Saturation (active) region: Drain current primarily controlled by $V_{GS}$; used for amplification.

Note that “saturation” in MOSFETs corresponds to the amplifier-friendly region, unlike BJT saturation which is a switching region.

DC biasing design: establishing a stable operating point

DC biasing sets the quiescent point (Q-point): the steady voltages and currents with no input signal. A good bias point provides linear swing, thermal stability, and tolerance to device variation.

Biasing a BJT amplifier

A practical BJT bias uses:

• A resistor divider to set base voltage.
• An emitter resistor to provide negative feedback (stabilization).
• A collector resistor (or active load) to set collector voltage and gain.

The emitter resistor is central. If temperature rises, $V_{BE}$ tends to drop, increasing current. The emitter resistor raises emitter voltage as current increases, reducing $V_{BE}$ and counteracting the change. This feedback greatly improves stability compared to a fixed base current bias.

A typical target in a single-supply CE stage is to set the collector near mid-supply for maximum symmetrical swing, then choose emitter current (sets transconductance and noise/gain tradeoffs), and finally choose resistor values to support that current and voltage.

Biasing a FET amplifier

For MOSFETs, bias often uses:

• A gate voltage derived from a divider or reference.
• A source resistor for local feedback and stability.
• A drain resistor or current source load.

Because MOSFET parameters (like threshold voltage) vary significantly, source degeneration (a source resistor) is commonly used to reduce sensitivity. In small-signal terms, it trades gain for predictable operation.

Small-signal models: turning nonlinear devices into linear amplifiers

Small-signal analysis linearizes device behavior around the Q-point. You replace the transistor with an incremental model and analyze gain, input impedance, and output impedance using standard circuit techniques.

BJT small-signal essentials

A widely used BJT small-signal parameter is transconductance:

$$g_m=\frac{I_C}{V_T}$$

At $I_C=1\text{ mA}$, $g_m\approx 0.038\text{ S}$ (38 mS). This directly links bias current to potential voltage gain.

The small-signal base-emitter resistance is:

$$r_{\pi }=\frac{\beta }{g_m}$$

You may also include output resistance $r_o$ due to the Early effect, which reduces gain and increases output impedance compared to an ideal current source collector.

FET small-signal essentials

For a MOSFET in saturation, the small-signal model centers on:

• Transconductance $g_m$ (how much drain current changes with gate voltage).
• Output resistance $r_o$ from channel-length modulation.
• Very high gate input resistance (ignoring leakage).

Conceptually, MOSFET gain stages behave like “voltage-in, current-out” devices similar to BJTs, but with a different bias sensitivity and typically lower $g_m$ for the same current in many practical conditions.

Classic amplifier configurations

Common-Emitter (CE) amplifier (BJT)

The CE stage is the workhorse voltage amplifier:

• Voltage gain: High (often negative sign, meaning inversion).
• Input impedance: Moderate (set by $r_{\pi }$ and bias network).
• Output impedance: Moderate to high (set by collector resistor and $r_o$).

With an unbypassed emitter resistor $R_E$, gain is reduced but linearity and bias stability improve. With a bypass capacitor across $R_E$, AC gain increases while DC stability remains, a common practical compromise.

A rule-of-thumb small-signal gain (ignoring $r_o$) is approximately:

$$A_v\approx -\frac{g_m{R}_C}{1+g_m{R}_E}$$

This captures the core design trade: larger $R_C$ and higher $g_m$ increase gain, while emitter degeneration $R_E$ reduces gain but improves predictability.

Common-Base (CB) amplifier (BJT)

CB is less common in low-frequency discrete designs but important in RF and wideband work:

• Voltage gain: Can be high.
• Current gain: Approximately 1 (no current amplification in the usual sense).
• Input impedance: Low (useful for matching low-impedance sources).
• Bandwidth: Often excellent because it reduces the Miller effect seen in CE stages.

CB is frequently chosen when you need wide bandwidth and can tolerate a low input impedance.

Common-Collector (CC) amplifier (BJT emitter follower)

CC, also called the emitter follower, is a buffer:

• Voltage gain: Near +1 (no inversion).
• Input impedance: High (especially with large $\beta$ and emitter resistance).
• Output impedance: Low (good for driving loads).

It is used for impedance matching, level shifting, and isolating a high-gain stage from a heavy load.

FET counterparts: common-source, common-gate, source follower

FET amplifiers mirror BJT topologies:

• Common-source is analogous to CE: high voltage gain, inverting, moderate output impedance.
• Common-gate is analogous to CB: low input impedance, good high-frequency behavior.
• Source follower is analogous to CC: near-unity gain buffer with high input impedance and low output impedance.

A practical distinction is input biasing. A MOSFET gate draws negligible DC current, so the bias network can use large resistors without loading the source, which is valuable in sensor front ends.

Practical design insight: choosing between BJT and FET

• Input impedance: FET stages win when you must avoid loading a source.
• Transconductance per milliamp: BJTs typically provide higher $g_m$ at a given current because $g_m=I_C/V_T$ is strong and predictable, making BJTs attractive for low-noise, high-gain analog stages.
• Bias stability and variation: MOSFET thresholds vary widely; BJTs vary too (notably $\beta$), but emitter resistors and proper bias networks make BJT operating points quite stable.
• Switching vs linear: Both can switch and amplify, but be careful with terminology: BJT saturation is a switching condition, MOSFET saturation is an amplifying condition.

Closing perspective

Understanding operating modes, DC biasing, and small-signal models turns transistors from mysterious three-terminal parts into predictable building blocks. The CE/CB/CC family and their FET equivalents cover most single-transistor amplifier needs: gain, buffering, impedance transformation, and bandwidth control. Once you can set a clean Q-point and translate it into $g_m$, input/output resistance, and gain, you can design amplifier stages that behave well on the bench