Bipolar Junction Transistor Connections

A bipolar junction transistor has emitter, base and collector terminals. In a two-port circuit, one terminal serves as the reference shared by the input and output. The selected reference gives three connections: common base, common emitter and common collector. Their current gain, voltage gain and impedances differ. The surrounding bias circuit still sets the operating region.

• Common Base Transistor
• Common Emitter Transistor
• Common Collector Transistor.

For linear amplification, the base-emitter junction is forward biased and the collector-base junction is reverse biased. That condition is the forward-active region. A BJT switch also uses cutoff and saturation, so forward-active bias is not required for every transistor application.

Common Base Connection of BJT

In a common-base connection, the base is the shared signal reference. The input is applied between emitter and base, and the output is taken between collector and base. The diagrams show NPN and PNP versions. npn transistor voltage and current polarities are opposite to those of a PNP device. The connection has low input resistance, high output resistance and no phase reversal from emitter current to collector current in the usual small-signal convention.

Current Gain

The input current is emitter current I_E, and the output current is collector current I_C. DC common-base current gain is the collector-current-to-emitter-current ratio at the stated bias point. Small-signal common-base current gain α is the local change in collector current divided by the change in emitter current while collector-base voltage is held constant.
Because emitter current equals collector current plus base current, forward-active α is below one and often close to one. Device and bias conditions determine its value, so 0.9 to 0.99 is not a general specification.

Expression of Collector Current

With the emitter open, emitter current is zero, but reverse leakage can still flow through the collector-base junction. The symbol I_C therefore does not become exactly zero. The collector-base leakage with emitter open is I_CBO. Its magnitude depends on device construction, voltage and temperature; power rating alone does not decide whether it matters. A current model that includes I_CBO is useful when leakage affects the required accuracy, especially at elevated junction temperature. The datasheet limit for I_CBO must use the required temperature. The expression combines the emitter-controlled collector component with leakage. It does not show that collector current depends directly on an independently applied base current in the common-base connection.

Characteristic of Common Base Connection

Input Characteristic

The common-base input characteristic plots emitter current I_E against emitter-base voltage V_EB at a stated collector-base voltage. Because the emitter-base junction is forward biased, I_E changes exponentially with V_EB over much of its useful range rather than switching on at one exact barrier voltage.
Small-signal input resistance is ΔV_EB divided by ΔI_E at constant V_CB, evaluated at the operating point. The ratio is usually low because a small ΔI_E change corresponds to an even smaller ΔV_EB change; its actual value depends on current and temperature, not a comparison between unlike voltage and current units.

Output Characteristic

The common-base output characteristic plots collector current against collector-base voltage for several fixed emitter currents. In the forward-active region, collector current is set mainly by emitter current. The curves still have a finite slope because changing collector-base voltage changes the effective base width, an effect called Early effect. At low or reversed collector-base bias the device leaves forward-active operation, and at excessive voltage it can enter breakdown.
Small-signal output resistance is the local change in collector-base voltage divided by the change in collector current at constant emitter current. Forward-active operation often gives high output resistance, though its value remains finite and bias-dependent.

Common Emitter Connection of BJT

In a common-emitter connection, the emitter is the shared reference. The input is between base and emitter, and the output is between collector and emitter. This connection can provide both voltage and current gain, but it also inverts the small-signal voltage in a resistively loaded amplifier. The diagrams show NPN and PNP versions.

Current Gain

The input current is base current I_B, and the output current is collector current I_C. Small-signal common-emitter current gain β is ΔI_C divided by ΔI_B at a stated collector-emitter voltage and operating point. Emitter current I_E equals base current I_B plus collector current I_C.
The related common-base gain is α = ΔIC/ΔIE under its stated condition.
Current gain varies substantially with collector current, collector-emitter voltage, temperature and individual device. Datasheets give minimum or typical hFE values at named test points; a universal range of 20 to 500 is unsafe for design.

Characteristic of Common Emitter Transistor

The common-emitter input characteristic plots base current against base-emitter voltage at a stated collector-emitter voltage. It resembles a forward-biased PN-junction curve, but base current depends on transistor transport and collector bias as well as junction voltage. Silicon BJTs do not have one exact turn-on voltage; current changes exponentially and the voltage shifts with current and temperature.

Small-signal input resistance is the local change in base-emitter voltage divided by the change in base current at the chosen operating point.

Output Characteristic of Common Emitter Transistor

The common-emitter output characteristic plots collector current against collector-emitter voltage for several fixed base currents. At low VCE the transistor is in saturation and both junctions are forward biased. In the forward-active region, collector current is controlled mainly by base current, while a finite curve slope remains because of Early effect. At high VCE the device approaches breakdown. These regions, not a claim of progressively sufficient reverse bias alone, explain the curve shape.

Output resistance is the local change in collector-emitter voltage divided by collector-current change at fixed base current and the selected operating point.

Common Collector Connection of BJT

In a common-collector connection, the collector is the shared signal reference. The input is applied between base and collector, and the output is taken between emitter and collector. In the usual emitter-follower circuit, the input is at the base and the output follows at the emitter with no voltage inversion.

DC emitter-to-base current gain is approximately β + 1 at the stated operating point.

Small-signal current gain is ΔIE/ΔIB under stated bias conditions and is also approximately β + 1.

Input Characteristic of Common Collector Transistor

For a formal common-collector characteristic, the input variables are base current and base-collector voltage while the output condition is held constant. In practical emitter-follower analysis, designers more often use base-emitter voltage and input current because the collector is tied to the supply reference. The input resistance seen by the source depends on emitter load, transistor gain, bias network and operating point; reverse-bias changes at the collector-base junction alone do not define it.

Output Characteristic of Common Collector Transistor

The common-collector output characteristic uses emitter current as output current and emitter-collector voltage under its chosen sign convention. Its family of curves resembles common-emitter collector curves because emitter current equals collector current plus base current. The low-voltage saturation region, forward-active region, Early-effect slope and breakdown limit still apply. For an emitter follower, the more useful circuit results are voltage gain slightly below one, high input resistance and low output resistance, all dependent on bias and load.