Calculator Inputs
Choose the most appropriate engineering method, enter known values, and submit to estimate output resistance and related small-signal metrics.
Example Data Table
| Method | Sample Inputs | Output Resistance | Engineering Meaning |
|---|---|---|---|
| Voltage-current slope | 5.0 V at 2.0 mA, 4.2 V at 3.2 mA | 666.67 Ω | Small-signal resistance from measured operating-point slope. |
| Open-circuit and load test | 12 V open circuit, 9 V with 1 kΩ load | 333.33 Ω | Source resistance inferred from voltage sag under load. |
| BJT Early-effect model | Early voltage 80 V, collector current 2 mA | 40 kΩ | Transistor output resistance in small-signal analysis. |
| MOSFET channel modulation | λ = 0.02 1/V, drain current 1.5 mA | 33.33 kΩ | Saturation-region drain resistance estimate from device model. |
Formula Used
1. Voltage-current slope: R_out = |ΔV / ΔI|. Use two nearby operating points from a measured I-V curve. The magnitude is reported because output resistance is normally expressed as a positive value.
2. Open-circuit and load test: R_out = R_L × (V_oc / V_load − 1). This comes from the voltage divider relationship between internal resistance and the known load.
3. BJT Early-effect model: r_o ≈ V_A / I_C. This approximation is widely used for small-signal transistor analysis around the chosen bias point.
4. MOSFET channel modulation: r_o ≈ 1 / (λ × I_D). It estimates drain output resistance in saturation when the channel-length modulation parameter is known.
Output conductance: g_out = 1 / R_out. Conductance is useful when comparing devices, bias points, or gain equations.
How to Use This Calculator
- Select the method that matches your available engineering data.
- Enter voltages, currents, load resistance, or device parameters.
- Choose the proper units for each numeric input.
- Click Calculate Output Resistance to generate the result block above the form.
- Review resistance, conductance, and method-specific supporting metrics.
- Use the CSV or PDF buttons to export the calculated summary.
- Compare results with the example table to check reasonableness.
Frequently Asked Questions
What does output resistance represent?
Output resistance describes how strongly a source or active device resists delivering additional current. Lower values usually hold voltage better under changing loads, while higher values show greater sensitivity to load variation.
When should I use the slope method?
Use the slope method when you have two measured points from an output characteristic. It works well for laboratory data, simulations, or datasheet curves near a chosen operating point.
Why does the loaded-source method need open-circuit voltage?
Open-circuit voltage acts as the unloaded reference. Comparing it with the loaded voltage reveals how much voltage is lost inside the source, which makes the internal resistance calculable.
Is transistor output resistance constant?
No. It changes with bias current, device geometry, temperature, and modeling assumptions. The value reported here is a small-signal estimate around the operating point you entered.
Why is conductance shown with resistance?
Conductance is the reciprocal of resistance. Many small-signal equations, gain models, and parallel-network calculations become easier when expressed as conductances instead of resistances.
What if my calculated resistance is extremely large?
A very large value often means the source behaves close to an ideal voltage source or the device has weak output-current dependence on voltage. Always verify units and bias assumptions before drawing conclusions.
Can I use this for amplifier stages?
Yes. The calculator is useful for emitter followers, collectors, drains, regulators, signal sources, and modeled outputs, provided your chosen method matches the stage behavior and available measurements.
Why do some methods return different values?
Different methods describe different conditions. Measured slope values reflect actual data, while device-model methods estimate small-signal behavior from parameters. Bias point changes and nonlinearity can create valid differences.