BJT Configurations

A BJT can be connected in a circuit in three different ways depending on which terminal is made common between input and output. Each configuration, Common Emitter (CE), Common Base (CB), and Common Collector (CC), produces a different set of input-output characteristics and is suited for different applications. Understanding the I-V characteristics of each configuration is a core requirement for both GATE and analog circuit design.

Core Concept Explanation

The output characteristics of any BJT configuration plot output current versus output voltage for a family of fixed input current or voltage values. For the Common Emitter configuration, the output characteristic plots IC versus VCE for different values of IB. These curves show three distinct regions: the cutoff region (IB = 0, IC approximately 0), the saturation region (both junctions forward biased, small VCE), and the active region (where IC is nearly constant for a given IB and increases slightly with VCE due to the Early effect).

In the CE active region, each curve for a fixed IB shows a nearly flat IC, confirming that IC = β × IB is independent of VCE in an ideal transistor. The slight upward slope of these curves is the Early effect. Extending the active region lines backward, they intersect the negative VCE axis at a point equal to -VA (the Early voltage). A larger VA means flatter characteristics and better transistor quality as an amplifier.

The Common Base output characteristic plots IC versus VCB for different values of IE. The current gain is α = IC/IE, which is less than unity but very close to it. The CB configuration provides very high output resistance because the reverse-biased collector junction sees almost no change in current with VCB variations. This makes CB useful in RF and microwave amplifiers where matching to high-impedance loads is needed.

The Common Collector configuration (also called the emitter follower) produces output at the emitter terminal. The voltage gain is just below unity, meaning the emitter output follows the base input voltage closely. However, the current gain is (β + 1) = IE/IB, which is high. The input resistance is very high (β × RE) and the output resistance is very low. This makes CC ideal as a buffer stage that transfers a signal from a high-impedance source to a low-impedance load without voltage loss.

Mathematical Expression

For the CE configuration, the input characteristics plot IB versus VBE and resemble the forward I-V curve of a diode. The output characteristics plot IC versus VCE. The slope of the active region curves in the output plot equals IC / VA, and the incremental output resistance is ro = VA / IC. The voltage gain of a CE amplifier is AV = -gm × RC || ro, which is high and negative (phase-inverted).

For the CC emitter follower, the voltage gain is AV = RE / (RE + 1/gm) which approaches unity as RE increases or gm increases. The input resistance looking into the base is Rin = β × (RE + 1/gm), which is large because the emitter resistor is seen multiplied by β from the base terminal. The output resistance looking into the emitter is Rout = (RS/β) + 1/gm, which is small because the source resistance is divided by β.

Practical Understanding

In real amplifier design, CE is the workhorse configuration for voltage amplification. Its high voltage gain and moderate input/output impedance make it suitable for signal amplification in audio and instrumentation circuits. The phase inversion (180 degrees) must be accounted for when cascading stages. Adding an emitter degeneration resistor in CE improves stability and linearity at the cost of gain.

CC (emitter follower) is placed at the output of a CE stage to drive low-impedance loads such as speakers, cables, or ADC inputs. CB is used at high frequencies because it eliminates the Miller effect, which in CE causes the base-collector capacitance to appear multiplied by (1 + AV) at the input, severely limiting bandwidth.

Given:
β = 120, IB = 25 μA = 25e-6 A
VCE = 6 V, VA = 100 V
Collector resistance RC = 2 kΩ

Why this formula applies:
CE configuration, active mode; need IC and small signal output resistance.

Formula:
IC = β × IB
gm = IC / VT
ro = VA / IC
AV ≈ -gm × (RC || ro)

Substitution:
IC = 120 × 25e-6 = 3 mA
gm = 3e-3 / 26e-3 = 115.4 mS
ro = 100 / 3e-3 = 33.33 kΩ
RC || ro = (2k × 33.33k)/(2k + 33.33k) = 66660/35330 ≈ 1886 Ω

Calculation:
AV = -gm × (RC || ro) = -0.1154 × 1886

Final Answer:
IC = 3 mA,  ro = 33.33 kΩ,  AV ≈ -217.6 (voltage gain, CE with Early effect)

Exam Tip: CE inverts phase (negative AV), CC does not invert (AV near +1), CB does not invert (AV positive, large). For GATE, remember CB has the lowest input impedance and highest output impedance of the three configurations.

• CE characteristics show three regions: saturation, active (amplifying), and cutoff.
• In active region, IC is nearly constant for fixed IB; slight slope is the Early effect due to base width modulation.
• CB has the highest output resistance and lowest input resistance; suited for RF.
• CC (emitter follower) has unity voltage gain, very high input resistance, very low output resistance; suited for buffering.
• CE provides high voltage gain with 180 degree phase inversion; most used configuration in amplifiers.

Quick Revision

• CE: Input at base, output at collector; high AV (negative), medium Rin, high Rout; used in amplifiers.
• CB: Input at emitter, output at collector; AV positive, Rin very low (~20 Ω), Rout very high; used in RF.
• CC: Input at base, output at emitter; AV near +1, Rin high (β × RE), Rout very low; used as buffer.
• In CE output characteristics, slope in active region = 1/ro; ro = VA/IC.
• GATE trap: CC has current gain of (β+1), not β; Rin = β × (1/gm + RE), not just β.
• CB eliminates Miller effect, making it bandwidth-superior at high frequencies despite current gain less than 1.
• Saturation voltage VCE(sat) for silicon BJT is typically 0.1 to 0.3 V.