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Basic Electronics » Bipolar and MOSFET transistors

June 08, 2010
by CyberGod
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Hi, I was wondering what is the main difference between the BJT and MOSFTET and in what cases one should be preferred than the other. I've read that the BJT is current controlled where the MOSFET is voltage controlled. Can someone try and clarify this please. Thanks!

June 08, 2010
by mrobbins
(NerdKits Staff)

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Hi CyberGod,

There are lots of "gotchas" here in certain situations, but:

You are correct that BJTs require current into the base terminal to turn on, while MOSFETs only require a voltage (except as frequencies increase).

Both BJTs and MOSFETs continue to have a tremendous number of practical uses, so it's certainly not the case that one has "won" over another.

For digital circuits, MOSFETs win because of zero static power dissipation -- the gate consumes no power unless it's switching states. (This wasn't always true -- TTL was BJT-based, and ECL logic families were considered particularly high performance, at the cost of power consumption.) In modern digital circuits like CPUs that have millions and now even billions of transistors, power dissipation is one of the fundamental limiting factors.

For analog circuits, both BJTs and MOSFETs have uses. BJTs might be more common in discrete analog circuits like amplifiers because they can often have higher "transconductance" than a similarly-biased MOSFET. BJTs are also probably easier to bias because there is much less device-to-device variation, whereas for MOSFETs two "identical" parts can be quite a bit different. BJTs also are often utilized for higher-frequency applications because of better performance. However, the use of MOSFETs in high-frequency (like RF) applications is increasing because they are cheaper to integrate with digital logic on the same silicon integrated circuit -- there's a book on my bookshelf called The Design of CMOS Radio-Frequency Integrated Circuits.

An example: if I wanted to build a single-transistor amplifier to amplify an analog signal, then the "bias current" is an important number that largely determines the performance of the transistor, and tells how much current I pass through the transistor just around the "normal" point. The "transconductance" (g_m) of the transistor tells me that if I raise the voltage at the input by a little bit, how much extra current will flow into the collector/drain terminal of the transistor?

If you go look at page 132 of The Art of Electronics, you'll see Figure 3.24, which shows on a logarithmic scale how a bipolar transistor compares with FETs for g_m. There's two things to notice: First is that the g_m of the BJT always beats the FET, making it a better amplifier. Second is that the line for BJTs is perfectly straight, regardless of which BJT -- it has only to do with the underlying physics! The paragraph that follows Figure 3.24 is quite good too:

"The problem of low voltage gain in FET amplifiers can be circumvented by resorting to a current-source (active) load, but once again the bipolar transistor will be better in the same circuit. For this reason you seldom see FETs used as simple amplifiers, unless it's important to take advantage of their unique input properties (extremely high input resistance and low input current)." - The Art of Electronics, Horowitz and Hill page 132

If you already have this book, then browse through some of the earlier parts of this chapter too, which address the unique advantages of FETs.

In any case, if I had to summarize, the simplest rule-of-thumb for electronics hobbyists might be that when you want a switch, use a MOSFET, and when you want an analog amplifier, use a BJT. This rule will probably serve you well 90% of the time. However, there are certainly cases when I've used a BJT as a switch and a MOSFET as an amplifier.

Hope that helps!

Mike

June 09, 2010
by CyberGod
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Thank you very much for the complete answer I really appreciate it!

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