BJT Collector Current Calculator
Early effect model: IC = IS · eVBE/VT · (1 + VCE/VA).
Inputs
Drag the handle (or focus it and press ↑/↓) to reorder. Scientific notation such as 1e-14 is accepted. Pick any unit; conversion is automatic.
Results
Enter values and press Calculate. Your answer will appear here.
Collector current vs. collector-emitter voltage
Output characteristic at your VBE. The slight upward slope is the Early effect. Dashed lines show VBE changes of ±VT·ln 2, which double or halve IC. The dot is your input.
What is being calculated
Common examples
| Case | VBE | VCE | IC |
|---|
Illustrative inputs, not manufacturer data.
Thermal voltage VT = kT/q
| Temperature | VT |
|---|
Change in IC for a change in VBE (VT = 25.85 mV)
| Current change | Increase in VBE |
|---|
The Early effect does not change these figures, because the factor (1 + VCE/VA) does not depend on VBE.
Unit conversions
| Quantity | Conversion |
|---|---|
| Current | 1 mA = 10⁻³ A · 1 µA = 10⁻⁶ A · 1 pA = 10⁻¹² A · 1 fA = 10⁻¹⁵ A |
| Voltage | 1 V = 1,000 mV |
| Temperature | K = °C + 273.15 · K = (°F + 459.67) × 5/9 |
| Constants | k = 1.380649×10⁻²³ J/K · q = 1.602176634×10⁻¹⁹ C · k/q = 8.617333×10⁻⁵ V/K |
Formula
- IC: collector current (A)
- IS: saturation current (A)
- VBE: base-emitter voltage (V)
- VT = kT/q: thermal voltage (V), about 25.85 mV at 300 K
- VCE: collector-emitter voltage (V)
- VA: Early voltage (V)
Example: IS = 10 fA, VBE = 0.65 V, VT = 25.85 mV, VCE = 5 V, VA = 100 V. Without the Early effect, IC0 = 10⁻¹⁴ × e25.145 ≈ 0.8325 mA. The Early factor is 1 + 5/100 = 1.05, so IC ≈ 0.8741 mA.
The model applies in the forward-active region (VBE forward biased, VCE above about 0.2 V). Advanced mode also gives gm = IC/VT, ro = (VA + VCE)/IC, the intrinsic gain (VA + VCE)/VT, IB = IC/β and rπ = β/gm. IC is extremely sensitive to VBE and to temperature (through both IS and VT), so datasheet IS values should be used with care.
References
- Wikipedia, Early effect: the (1 + VCE/VA) term and ro
- Wikipedia, Bipolar junction transistor: exponential IC–VBE model
- NIST, Fundamental Physical Constants: k and q for VT = kT/q
How to Use This Tool
- Pick a preset, or enter the saturation current IS and its unit.
- Enter VBE, VCE, the Early voltage VA and the thermal voltage VT.
- In Advanced, calculate VT from temperature, add β for IB, or choose the IC unit.
- Click Calculate.
- Read IC in the highlighted result and check the output-characteristic chart.
What This Tool Offers
- Basic and Advanced modes on one calculation engine
- Current, voltage and temperature unit conversion
- Early effect on or off (leave VA blank for the ideal model)
- gm, ro, rπ, IB and intrinsic gain (Advanced)
- Output-characteristic chart and labelled NPN diagram
- Reorderable inputs, presets and a mobile-friendly layout
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| I_C | Collector current | A |
| I_S | Saturation current (scale current) | A |
| V_BE | Base-emitter voltage | V |
| V_T | Thermal voltage (kT/q, ≈26mV at 300K) | V |
| V_CE | Collector-emitter voltage | V |
| V_A | Early voltage (finite output impedance parameter) | V |
What it means
The Early effect is the increase in collector current with increasing V_CE due to the base‑width modulation. The collector current is I_C = I_S · e^(V_BE/V_T) · (1 + V_CE/V_A), where V_A is the Early voltage (a parameter of the transistor). The factor (1 + V_CE/V_A) accounts for the increase. The Early effect is a non‑ideality that affects the output resistance of the transistor. It reduces the amplifier gain and must be considered in high‑precision analog designs. Example: If V_A = 50V, at V_CE = 10V, the factor is (1 + 10/50) = 1.2. So the current is 20% higher than the ideal value. This must be accounted for in a current mirror or amplifier design.
Worked example
BJT Collector Current with Early Effect – Practical Example
Real‑World| Parameter | Value |
|---|---|
| IS | 1×10⁻¹⁵ A |
| VBE | 0.65 V |
| VT | 0.026 V |
| VCE | 5 V |
| VA | 50 V |
| Formula | IC = IS·eVBE/VT·(1 + VCE/VA) |
Common mistakes
- I_S: Saturation current – depends on the transistor.
- Thermal voltage V_T: About 25 mV at room temperature.
- Early voltage V_A: Accounts for base‑width modulation – higher V_A means better.
- V_CE: Collector‑emitter voltage – must be sufficient for forward‑active region.
- Exponential: The current depends exponentially on V_BE.
Applications
BJT collector current including Early effect: I_C = I_S·e^(V_BE/V_T)·(1 + V_CE/V_A). This accounts for base‑width modulation, which causes the collector current to increase with V_CE. Engineers use this model to predict output resistance, to design amplifiers with high gain, and to minimise distortion. The Early voltage V_A is a parameter that characterises the transistor's output impedance. By understanding the Early effect, professionals can design more linear and stable circuits. This formula is important for high‑precision analog design and for SPICE simulation modelling.
- High‑gain amplifier design with improved linearity
- Output resistance and load driving capability analysis
- SPICE model parameter extraction and simulation
- Current source and cascode circuit design
- Educational understanding of transistor output characteristics
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