9.1 Bipolar Junction Transistors

9.1 Bipolar Junction Transistors

Quick Answer: A BJT has base, collector, and emitter (NPN or PNP). Beta (β / hFE) = change in collector current with respect to base current; alpha (α) = change in collector current with respect to emitter current. Class A: BE forward, CB reverse. Saturation: both junctions forward, collector current maximum. Cutoff: no emitter–collector current. Common-emitter is the workhorse RF/IF amp; common-base has high voltage gain; emitter-follower (CC) has high current gain.

Topic 3-C (circuit components) moves from passive parts and diodes into active devices that amplify and switch. Key topics 025 (Transistors-1) and 026 (Transistors-2) are pure BJT literacy for the GROL tech who adjusts, repairs, and maintains radiotelephone equipment.

NPN and PNP structure — three terminals

What are the three terminals of a bipolar transistor? Base, collector, and emitter.

Do not mix vacuum-tube labels (cathode, plate, grid) or FET labels (gate, source, drain) into a BJT question. Element 3 also distinguishes the unijunction transistor (UJT), whose elements are base 1, base 2, and emitter—a different three-terminal device used in timing and trigger circuits, not a standard NPN/PNP amp.

Roles in a working amplifier:

Two junctions exist: the base-emitter (BE) junction and the collector-base (CB) junction. How those junctions are biased decides whether the transistor is amplifying linearly, fully on, or fully off.

Current gain: alpha, beta, and hFE

Alpha (α)

Meaning of “alpha” with regard to bipolar transistors: the change of collector current with respect to emitter current.

[ \alpha = \frac{\Delta I_C}{\Delta I_E} ]

Alpha is slightly less than 1 (typical 0.95–0.99) because a small fraction of emitter current leaves as base current. Alpha is the natural gain figure for the common-base configuration.

Beta (β) / hFE

Meaning of “beta” with regard to bipolar transistors: the change of collector current with respect to base current.

[ \beta = h_{FE} = \frac{\Delta I_C}{\Delta I_B} ]

Beta is the common-emitter current gain. Datasheets list DC current gain as hFE (static) and small-signal gain as hfe. Values from tens to a few hundred are common for small-signal RF and audio devices; power transistors may be lower.

Relationship (exam-useful):

[ \beta = \frac{\alpha}{1 - \alpha}, \quad \alpha = \frac{\beta}{\beta + 1} ]

If β = 99, then α ≈ 0.99. A tiny base current can control nearly 100× as much collector current—the entire point of the BJT as a current amplifier.

Beta cutoff frequency

Beta cutoff frequency of a bipolar transistor is the frequency at which collector current gain has decreased to 0.707 of its low-frequency (maximum) value. That is the −3 dB point for β versus frequency. Above that frequency the device still works, but current gain rolls off—critical when selecting transistors for VHF/UHF RF amplifiers versus audio stages.

Operating regions: cutoff, active (linear), saturation

Cutoff

What it means for a transistor to be cut off: there is no current between emitter and collector. The device is an open switch for practical purposes. Both junctions are reverse-biased (or BE is not forward enough to inject carriers). Switching stages (keying, mute, logic-level drivers) spend half their life in cutoff.

Active / linear (Class A amplifier bias)

When an NPN transistor operates as a Class A amplifier: the base-emitter junction is forward biased and the collector-base junction is reverse biased. That is the classic forward-active region: carriers injected at the emitter are swept into the collector, and a small change in base current produces a proportional change in collector current. Class A bias keeps the Q-point mid-range so the RF or audio waveform stays linear—exactly what SSB linear amps and many receiver IF stages need.

Saturation

What it means for a transistor to be fully saturated: the collector current is at its maximum value (limited by the external circuit, not by β × I_B).

Bias condition in saturation: the base-emitter junction and collector-base junction are both forward biased. The collector-emitter voltage collapses to a small V_CE(sat) (often a few tenths of a volt). Saturation is the “closed switch” state for digital and keying circuits. Do not confuse saturation bias with Class A: in Class A the CB junction stays reverse biased.

Silicon NPN turn-on voltage

For current to flow in an NPN silicon transistor’s emitter-collector path, the base must be at least about 0.7 V positive with respect to the emitter. That is the silicon PN junction forward drop applied to the BE diode. Germanium devices are lower (~0.3 V), but Element 3 emphasizes the 0.7 V silicon figure. If V_BE is well below that threshold, the transistor stays cut off.

Worked bias sketch — Class A NPN stage

A small-signal NPN IF amp runs from +12 V with emitter grounded through a bypassed resistor and collector loaded by a tank or resistor.

1. Bias divider sets base ≈ 0.7 V + I_E R_E so BE is forward.
2. Collector sits several volts below the supply so CB remains reverse-biased over the full signal swing.
3. Small RF at the base modulates I_B → larger swing at the collector (common-emitter voltage gain).

If a fault pulls the collector nearly to the emitter potential while base drive is huge, the stage may be saturated (both junctions forward)—distortion, no linear gain, possible excess dissipation if the load is wrong.

The three amplifier configurations

Element 3 compares common-base, common-emitter, and common-collector (emitter-follower) by which terminal is AC-grounded (common to input and output).

Pool hooks:

• A common-base amplifier has more voltage gain than common-emitter or common-collector.
• An emitter-follower has more current gain than common-emitter or common-base.
• A common-emitter amplifier has more voltage gain than a common-collector (from the semiconductor intro pool)—the CE stage is the default voltage amp in radios.

RF amplifier use for GROL techs

• Common-emitter: general-purpose RF, IF, and audio voltage amplification; many discrete preamps and driver stages.
• Common-base: low input impedance suits some RF front ends and grounded-base VHF stages; good isolation and high voltage gain.
• Emitter-follower: buffer between high-Z stages and low-Z loads (cables, next stage inputs); power gain without voltage step-up.

When you replace a transistor in a marine SSB IF strip or an aircraft transceiver audio chain, match configuration intent: a follower substituted for a CE voltage amp will not restore gain; a CE part in a CB RF front end may mis-match impedance and oscillate or go deaf.

Linear bias vs switching bias — shop summary

Exam-day checklist for topics 025–026

1. Terminals → base, collector, emitter (UJT → base 1, base 2, emitter).
2. Alpha → ΔI_C/ΔI_E; beta → ΔI_C/ΔI_B.
3. Saturated → I_C maximum; both junctions forward.
4. Cutoff → no emitter–collector current.
5. Class A → BE forward, CB reverse.
6. NPN silicon → base ≈ +0.7 V vs emitter to conduct.
7. Beta cutoff → β down to 0.707 of max.
8. CB → highest voltage gain; emitter-follower → highest current gain; CE → balanced workhorse voltage amp.

Master these hooks and Element 3 transistor questions become pattern matching instead of memorizing random phrases. Next section extends the solid-state toolkit to FETs, LEDs, optoisolators, and related specialty devices.