Transistor Base Resistor Calculator

This transistor biasing calculator finds the base resistor needed to drive a bipolar transistor (BJT) as a switch. Enter the control voltage, the base-emitter voltage drop, the collector current you want to switch and the transistor\u2019s current gain, and the calculator returns the base resistor and the required base current. The base current is the collector current divided by the gain, and the resistor drops the control voltage minus the base-emitter drop at that current. In practice you over-drive the base a little to ensure the transistor saturates fully. The formula, a worked example and the assumptions are below.

Control voltage Vcc (V)

Base-emitter Vbe (V)

Collector current Ic (A)

Gain (hFE / \u03B2)

Base current Ib

Gain used

The formula

The base current needed is Ib = Ic / gain. The base resistor is Rb = (Vcc minus Vbe) / Ib, where Vbe is about 0.7 V for silicon. For a hard switch, divide Rb by a factor of 2 to 10 to over-drive the base.

Worked example

To switch 2 mA of collector current with a gain of 100 from a 9 V control line: Ib = 0.002/100 = 20 microamps, and Rb = (9 minus 0.7) / 0.00002 = 415 k ohms. Enter 9, 0.7, 0.002, 100 to confirm.

Frequently asked questions

How do I choose a base resistor?

Work out the base current (collector current over gain), then Rb = (control voltage minus 0.7) divided by that base current.

Why over-drive the base?

To make sure the transistor fully saturates as a switch, designers use a base current several times the minimum, so use a smaller Rb.

What is Vbe?

The base-emitter voltage drop, about 0.7 volts for a silicon BJT when conducting.

Who this calculator is for

This calculator is for electronics students, hobbyists and engineers.

What this calculator assumes

You enter values in the units shown.
Ideal components are assumed.
Results are rounded for display.

Formula and sources

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