Bipolar vs FET: Def. and when to use one or the other?

In school (if memory serves) I learned that a bipolar transistor is a current-controlled current "amplifier", and that a FET is a voltage-controlled current "amplifier". Replace "amplifier" with "switch" to complete the 2x2 matrix I learned about most uses for active devices.

Is this -- on the basic level -- true?

Without going into biasing configurations, when would I want to use a bipolar and when, for example, a MOSFET? I know FETs have high input impedance, so drive current is miniscule. Other reasons for using one or the other?

Just trying to refresh my understanding of semi's.

Thanks, Doc

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Bipolars are cheaper than FETs !

Graham

Good answer!

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Re FET's, they are useful when you want a simple means to convert logic circuit voltage levels, and unlike BJT's (bipolar junction transistors), they can be used directly as voltage controlled resistors with extremely low minimum resistances, making them useful for shunt modulation of laser diodes, or as analog signal switches for AC or DC signals, or for power switching. They might cost more than BJT's but I think it's always worth considering them in new situations, on the offchance that they might do a job with a reduced part count. Sometimes they do, and so dramatically that the saving in parts, time and effort makes a FET extremely useful.

I don't know if this is a serious thread . . .

On a more fundamental level, FET's will generally have higher (typically much higher) input resistance than BJT's, but in similar circuits, will generally provide lower voltage gains. BJT's are also vastly more robust in ESD space.

If your interest is genuine, do a google search on something like "bjt vs fet" and see a lot more detail.

Chuck

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Hi, Doc. Here are a couple of differences between Bipolar Junction Transistors (BJTs) and MOSFETS when used as switches:

• MOSFETS are voltage controlled whereas BJTs are current controlled. This can mean the difference when your output logic can't source any real current (such as with 4000-series logic gates -- they can typically only source 1/2mA at a Vdd of 5V). But even if you're driving the BJT with an HC output (4mA, 20mA, or 64mA depending), there's a limit on the current you can switch directly. It's best with saturated switching to supply 10 times the current the transistor current gain would suggest in order to achieve minimum Vce (saturation). So, let's say you've got a TIP power transistor with a DC current gain of 100 (typical). In order to switch 1A, the beta spec suggests you could supply 10mA of base current. But in order to saturate the transistor, you'd have to supply ten times that, or

100mA. No logic gate can do that, so you'll have to add a second transistor to boost the base current. Needless complexity. Of course, you could use a darlington BJT transistor (with a DC current gain of 1000), but then Vce(sat) will be at least 1V, which might result in too much power disspation.

• MOSFETs may require a higher gate voltage in order to turn on than your logic gate can supply. You should be careful with this -- even many "logic level" MOSFETs which allow use with a 5V gate drive may require something like 8V to 10V to achieve the miraculous Rds(on) in the data sheets.

• Vds when the MOSFET is on is a simple product of Rds(on) times current. This can be a big advantage or a disadvantage, depending on the device and the current. Pd is the product of Vds and switching current. Ideally you want the switch power dissipation to be as small as possible.

• MOSFETs tend to be more static sensitive than BJTs, and have a tendency to spit up and die unless you're a lot more careful with such things as circuit transients and momentary reverse voltages. In addition to being less expensive, BJTs have a reputation for being somewhat more rugged.

I hope this has been of help with your homework.

Cheers Chris

Fets generally require higher voltages to bias on the gate. also FETS tend to act differently when it comes to current because a FET can actually act as a current regulator if you want it too. There are different types of FETs, JFET, MOSFET with enhanced mode and non enhanced mode (not common). JFets are on by default, you need -V (N-channel) bias voltage to turn them off. basically if you read the voltage specs. you might find a reference to pinch off voltage, this is the voltage that is required to turn it off.

Mos in Enhanced mode works the other way around for gate control and is more favorable in many applications.

Bipolar types are current control via the Base and Emitter current etc. the advantage there is it only takes an average of .6 volts between the base and emitter to get current flowing which governs the current flow from the Collector and emitter etc. There are other factors to consider but that is the basics. Fets require much higher voltages on the get but offer very high impedance with the cost of lots of capacitance verses the bipolar types.

In R.F. circuits, the use of Fet's with their high gate capacitance plays a heavy roll in the resonating input circuit, that is why, when doing replacements you must get a fet that has gate capacitance close to what came out of there among other things.

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MOSFETs are easy to use in microcontroller based applications since they can be driven directly off the pins of many microcontrollers. The biggest single advantage that I have found for bipolars in my applications is their ability to block current in both directions. When using mosfets, the design gets more complicated when you have to protect against reverse battery connection.

chuck wrote in news:1173533808 snipped-for-privacy@sp6iad.superfeed.net:

Conversely, FET's are better at withstanding ionising radiation. Weird.

Gary Reichlinger wrote in news: snipped-for-privacy@4ax.com:

Forward based diode in series with power input? If the losses that causes are not wanted, wouldn't it be enough to put the diode across the supply terminals, reverse biased, after a fast fuse? If you used a Schottky diode, that could limit the FET's experience of reversal to a short surge of -0.4V or less. I don't know if any are vulnerable to that, but I suspect not.

Well, at 50-plus, I'm doing "homework" only assigned by myself.

Thanks for your educational comments. (c:

Doc

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Dr. Leonard. H. McCoy
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