Calculating bias resistor for transistor switch

This may be a question that reveals my ignorance but I am trying to calculate the proper base bias resistor to bias a transistor to saturation. I have several applications and I would like to know the proper method of calculating the resistance value. The transistors are types ranging from 2N2222 to TIP-31. I can get the transistors to work but I am guessing at the resistance values. I would rather know what I am doing.

Jim

Divide the circuit's collector current by the gain (Hfe), this will be the max value base resistor (generally choose a lower value). The minimum value will be how much drive current can be adequately supplied from the source. If you had a TTL driver that could supply 4V, and you used a 3.3k resistor, and an Hfe of 100, then:

Ib = 1mA = (4V-.7V) / 3.3K
Ic = 100mA = 1mA * 100

Choose a value that is lower than max, but is nominal for the source current.

hFE is used to bias a linear amplifier transistor, not a saturated transistor. It has plenty of collector to emitter voltage when it is a linear amplifier. It has a very low collector to emitter voltage when it is saturated.

The max saturation voltage is spec'd with a base current equal to 1/10th the collector current, even if the hFE is very high when it is a linear amplifier.

I agree with Audioguru. Most general transistors for switches that I had characterised specify force gain of 10, meaning that the base current is 1/10th of the maximum collector current. There are high gain transistor switches that are specified with FG of 25 or more, but they were pretty rare. The highest FG for a transistor switch I've seen was around 50.

Thanks for your replies! This points me in the right direction but there are still questions. I am looking at the spec. sheet for the 2N2222 from Fairchild and mention of max. collector current is 1 amp! This would come out to a base current of 100ma which seems awful high! I have seen this transistor used with a 1K ohm resistor in the base circuit to bias it on. This is with 12 volt supply. I have used this example with good results thought the base current of about 11ma may have been minimalistic (12 volts - .7 base to emitter voltage drop for 11.3 volts / 1k) or am I figuring wrong? I also see nothing mentioned about a Force Gain.

I assume I am reading the transistor specs. wrong so can someone help dispel my ignorance

Jim

The Fairchild datasheet for their 2N2222 has a max allowed collector current of only 800mA.
Its minimum current gain at only 500mA is 30 when it has 10V across it (it is not saturated).
Its max saturation voltage is 1.6V when its collector current is only 500mA and its base current is 50mA.

With a base current of only 11.3mA then many of them won't saturate with more than 250ma.

Pick the maximum current value for your load, not the transistor. Make sure that it is below the maximum the transistor can handle.

Calculate the base current you need, FG of 10 is a safe bet. If you want smaller Ib, look at the Vce against Ic graphs, pay attention to the Ib, some manufacturers include several curves with different Ib, thus different FG. Just make sure that the transistor is saturated at your chosen Ic and Ib.

Calculate the power dissipation at maximum load current (Vce*Ic + Vbe*Ib), make sure the transistor can dissipate that much power within your safe operating temperature, if not, use a heatsink or get another transistor.

For low load current, saturation or FG is not as important compared to when the load current is high. This is because the power dissipated by the Vce*Ic would be small and usually falls within the capability of the device. This would not be the case for a high current load.