# Calculating BJT Base Resistor Value

The circuit below supplies 3.3V to an antenna (3.3VGreen), which then gives GNSS info to a control module (ANT_GNSS). The attached antenna is datasheet and only consumes about 10mA

I'd like to be able to switch the antenna on and off to preserve power, as this application uses a battery. So I thought the circuit below would accomplish this.

But I am wondering what resistor value I need at the BJT's base to bring the transistor to saturation? The transistor's datasheet is linked here datasheet. I am thinking that if h_fe is 100 @10mA (collector current) then base current needs to be at least 10mA/100 or 0.1mA to drive to saturation? Following this BJT Tutorial, but I have no practical experience.

Somewhere around 0.5 to 1mA is fine, forced beta of 10-20. The transistor is specified at a Ic/Ib = 10, but 20 will be fine too, especially at only 1mA Ic.

So the resistor would be about (3.3V - 0.7V)/0.75mA or about 3.3K$$\\Omega\$$.

The hFE is specified at Vce = 1.0V and you would like it to be more like 0.1V to minimize the losses in the transistor.

You could also use an inexpensive logic-level p-channel MOSFET such as AO3401A which will have negligible voltage drop.

• What does it mean that the h_Fe was specified at 1V? Could someone give an example of when this h_Fe would apply Commented Apr 1, 2021 at 0:58
• For example an amplifier with the transistor biased around Vcc/2 rather than used as a switch (your application). Commented Apr 1, 2021 at 2:21