Calculator Inputs
Example Data Table
These sample points illustrate a nearly linear active-region output characteristic for one base-current setting.
| Point | VCE (V) | IC (mA) | Comment |
|---|---|---|---|
| A | 4.0 | 4.35 | Measured operating point in the active region. |
| B | 10.0 | 4.95 | Second point on the same output curve. |
| Reference | 12.0 | 5.15 | Predicted current at a higher voltage using the fitted line. |
Formula Used
For a BJT in the active region, the collector-current curves are not perfectly flat. Their backward extrapolations often meet near a negative voltage on the VCE axis. The magnitude of that negative intercept is the Early voltage. Larger values generally indicate less modulation of collector current with collector-emitter voltage.
How to Use This Calculator
- Measure two VCE and IC pairs from the same transistor output curve at one fixed base current.
- Enter both operating points in volts and milliamps.
- Provide a reference collector current so the calculator can estimate output resistance.
- Enter gm if you also want intrinsic gain.
- Set a target VCE to predict collector current on the extrapolated line.
- Press the calculate button. The result appears above the form under the header.
- Review the Plotly chart to confirm the intercept and slope look physically reasonable.
- Use the CSV or PDF button to export the results.
Frequently Asked Questions
1. What is Early voltage?
Early voltage is the magnitude of the negative VCE-axis intercept found by extrapolating a transistor’s active-region collector-current curves backward. It indicates how strongly collector current changes with collector-emitter voltage.
2. Why do both measured points need the same base current?
The calculator assumes one output characteristic line. If the two points come from different base-current settings, the slope and intercept will mix different curves and the Early voltage estimate becomes unreliable.
3. Can I use this for MOSFET channel-length modulation?
The math looks similar, especially through λ and output resistance, but the interpretation changes. This page is framed for BJT Early effect, so use transistor-appropriate measurement data and terminology.
4. What does a larger Early voltage mean?
A larger Early voltage usually means flatter output curves, weaker current modulation, and higher small-signal output resistance. That often improves gain in amplifier stages when other device parameters stay favorable.
5. Why is my extrapolated intercept positive?
A positive intercept usually means the chosen points are not in the expected active region, measurement noise is significant, or the selected points do not lie on the same nearly linear segment.
6. How is output resistance estimated here?
The calculator uses ro = VA / IC at the reference collector current you enter. This is a common small-signal approximation around that operating point, not a full nonlinear device model.
7. Do I need transconductance gm?
You only need gm if you want intrinsic gain gm·ro. The Early voltage and output resistance calculations still work without it, as long as your measured current and voltage data are valid.
8. Is two-point fitting always accurate?
Two-point fitting is fast, but real output curves may bend slightly. For better accuracy, choose points from the most linear active-region section and avoid saturation, breakdown, or noisy data.