Formula Used
The ideal emitter follower current gain is:
Ai = IE / IB = β + 1
The delivered load current gain is adjusted for emitter and load sharing:
Ai load = (β + 1) × (RE || RL) / RL
The dynamic emitter resistance is:
re = VT / IEQ
The base input resistance is estimated as:
Rin base = (β + 1) × (re + (RE || RL))
The total input resistance also includes the bias network:
Rin total = Rin base || Rbias
How to Use This Calculator
Enter the transistor beta from your datasheet or measurement. Add the base signal current if you want base-to-load current gain. Add source voltage and source resistance if you want source-to-load gain.
Enter the emitter resistor, load resistor, bias resistance, quiescent emitter current, and temperature. Press the calculate button. The result appears above the form. Use CSV or PDF buttons to save the output.
Example Data Table
| Parameter |
Example Value |
Meaning |
| β |
100 |
Small signal transistor current gain |
| Base current |
50 µA RMS |
Input base signal current |
| Emitter resistor |
1000 Ω |
Emitter path resistance |
| Load resistor |
470 Ω |
External load at the emitter |
| Quiescent emitter current |
5 mA |
Bias current for small signal resistance |
| Approximate ideal gain |
101 |
β plus one |
| Approximate load gain |
68.71 |
Current gain after load sharing |
Emitter Follower Current Gain Guide
An emitter follower is a common collector amplifier. It gives high input resistance and low output resistance. Its voltage gain is usually close to one. Its current gain can be much larger. That makes it useful as a buffer stage.
Why Current Gain Matters
The main current gain is often treated as beta plus one. Beta is the transistor small signal current gain. The emitter current equals collector current plus base current. Since collector current is beta times base current, emitter current becomes beta plus one times base current.
Load Effects
Real circuits add loading effects. The emitter resistor and load resistor share the emitter signal current. A small load resistor takes more current. A small emitter resistor can steal part of the signal current. This calculator estimates both ideal emitter current gain and delivered load current gain.
Bias and Resistance
The tool also includes dynamic emitter resistance. This small resistance depends on quiescent emitter current and temperature. Higher bias current lowers dynamic resistance. Lower dynamic resistance improves buffering and voltage transfer. The value also affects input resistance seen at the base.
Source Behavior
Source resistance and bias resistance matter too. The signal source must feed the transistor input and any bias network. A low bias resistance can reduce total input resistance. That lowers source voltage at the base. Then the delivered load current also drops. The calculator reports source based current gain for this reason.
Practical Notes
Use RMS signal values when comparing power. Keep the transistor in its active region. Avoid large input swings that clip the emitter waveform. Also check power rating in the load and emitter resistor. Small signal formulas work best near the chosen bias point.
Design Use
This calculator is helpful during early design. It compares load choices quickly. It shows whether base drive is enough. It also reveals when a bias network is too heavy. You can export results as CSV or PDF for notes, reports, or circuit records.
Accuracy Tips
For best accuracy, enter measured beta when possible. Datasheet beta can vary widely. Temperature also changes the thermal voltage. Use realistic load values. Then compare the estimate with a simulator or bench test before final hardware. Record every assumption. Save the exported file with project data. It helps later troubleshooting and design reviews well.
FAQs
What is an emitter follower?
An emitter follower is a common collector transistor amplifier. The output is taken from the emitter. Its voltage gain is near one, but its current gain is high.
What is current gain in an emitter follower?
The ideal emitter current gain is beta plus one. In practical circuits, load sharing and bias resistance can reduce the current actually delivered to the load.
Why is load current gain lower than beta plus one?
The emitter signal current divides between the emitter resistor and load resistor. Only the part flowing through the load becomes useful load current.
What does beta mean?
Beta is the transistor small signal current gain. It is the ratio of collector current change to base current change under selected bias conditions.
Why is quiescent emitter current needed?
Quiescent emitter current sets the dynamic emitter resistance. This resistance affects input resistance, voltage gain, and small signal accuracy.
Should I use RMS or peak signal values?
Use RMS values when you want power estimates. Use peak values only when you are comparing peak currents or waveform limits.
Can this calculator replace a circuit simulator?
No. It gives fast small signal estimates. Use a simulator or bench test for distortion, clipping, thermal drift, and final design checks.
Why does bias resistance reduce gain?
The bias network loads the signal source. A low bias resistance reduces total input resistance, lowers base signal voltage, and can reduce delivered load current.