BJT Input Output Characteristics

The input and output characteristics of a BJT in the common-emitter configuration are the two curves every ECE student must be able to read and draw. These curves define everything about how the device behaves in a circuit.

Core Concept

The input characteristic (IB vs VBE) for a BJT in common-emitter configuration looks like a diode I-V curve. No base current flows until VBE crosses about 0.5V. Above 0.7V, IB rises sharply. This curve is taken at a fixed VCE, usually 5V or 10V.

The output characteristic (IC vs VCE) is a family of curves, one for each value of IB. In the active region, IC is nearly flat, meaning IC does not change much with VCE. The spacing between curves is proportional to β: a fixed increase in IB produces a proportional increase in IC.

The output characteristics show three regions clearly. Below VCE(sat) ≈ 0.2V is saturation, where curves cluster together and IC depends on VCE. Above this is the active (linear) region. The nearly flat line at IC ≈ 0 for IB = 0 is the cutoff region. The slight upward slope of active-region curves is due to the Early effect, caused by base-width modulation at higher VCE.

Key Equations

Input characteristic follows the diode equation: IB = IBS × (e^(VBE/VT) - 1). VT = kT/q = 26 mV at 300K. IBS is the base-emitter saturation current, very small (nanoamperes).

From the output characteristic, β can be read as: β = ΔIC / ΔIB at constant VCE. Pick two curves in the active region and divide the change in IC by the change in IB.

The Early effect: output curves slope upward with a slope of 1 / ro = IC / VA. VA is the Early voltage, typically 50V to 100V for BC547. This gives the small-signal output resistance ro = VA / IC.

Given:
  From output characteristic graph:
  At IB = 30µA: IC = 3.0 mA (at VCE = 5V)
  At IB = 10µA: IC = 1.0 mA (at VCE = 5V)
  VCE = 5V (constant)

Why this formula:
  β = ΔIC / ΔIB at constant VCE (from the characteristic curves)

Formula:
  β = (IC2 - IC1) / (IB2 - IB1)

Substitution:
  β = (3.0 mA - 1.0 mA) / (30 µA - 10 µA)

Calculation:
  β = 2.0 × 10^-3 / (20 × 10^-6)
    = 2.0 × 10^-3 / 2.0 × 10^-5
    = 100

Final Answer:
  β (hFE) = 100 at VCE = 5V
  This is the DC current gain read directly from the output characteristic.

Exam Tip: GATE graph-reading questions ask you to extract β from the output characteristic. Always pick points in the flat (active) region at a fixed VCE. Do not pick points near the saturation knee (VCE < 0.5V) because curves merge there and you cannot get a clean ΔIC. Also, the Early voltage VA is found by extrapolating the active-region curves backward to the VCE axis; they all meet at -VA.

Key Properties

• Input characteristic looks like a PN junction diode curve. The knee voltage is about 0.5V to 0.7V for silicon BJTs.
• Output characteristics are plotted with IB as parameter, one curve per IB value. Equal IB steps give equal IC spacing in the active region.
• In saturation (VCE < 0.2V), all the IC-VCE curves merge and rise steeply. IB has little control here.
• The active region curves are nearly horizontal but have a slight upward slope due to the Early effect. The Early voltage VA for BC547 is about 100V.
• At cutoff (IB = 0), IC equals the leakage current ICEO = (β+1) × ICBO, which is a few nanoamperes at room temperature.
• The maximum power hyperbola PD = VCE × IC = constant is superimposed on the output curves to define the safe operating area.

Quick Revision

• Input characteristic: IB vs VBE. Looks like a diode curve. Knee at 0.7V.
• Output characteristic: IC vs VCE, family of curves for different IB.
• β = ΔIC / ΔIB at constant VCE in the active region.
• Early effect: active region curves slope up. ro = VA / IC.
• Saturation knee at VCE ≈ 0.2V. Curves merge below this.
• ICEO is the leakage current at IB = 0. ICEO = (β+1) × ICBO.
• Exam trap: Reading β from the saturation region of the graph. Always pick points in the flat active region (VCE > 1V) for an accurate β extraction.