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A very basic question about the transistor in Active mode

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scrooch

Sat Mar 20 2010, 07:16AM

Registered Member #908 Joined: Wed Jul 18 2007, 05:53AM
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Posts: 49

One condition is that the Base voltage should be 0.7v above the emitter voltage for an npn silicon transistor.

But I don't understand the second condition where the collector voltage has to be greater by some amount more than the emitter or the base.
Can someone explain this condition is exactly and why it is so? and by what amount it has to be above or below.
I found google to be confusing many people trying to explain in different ways.

thanks a bunch

Steve Conner

Sat Mar 20 2010, 12:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The emitter is called that because it emits electrons, and likewise, the collector collects them.

Electrons have a negative charge, so in order for the collector to do its collecting, it has to be slightly more positive than the emitter, or they'll find it completely unattractive and come out of the base instead. (This is the physical mechanism behind saturation.)

Just how much more positive depends on the particular transistor, and how much current you want to collect. Typically it's somewhere between 0.1V and 1V, though I've seen some high-speed power BJTs that are still "saturated" at 5V Vce.

This applies to a NPN transistor where the majority carriers are electrons. For a PNP transistor, what's being emitted and collected are positively charged "holes", so the polarity is reversed: the collector has to be more negative than the emitter to do its job.

radiotech

Sat Mar 20 2010, 05:48PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Think of a transistor operating with two loops of current flowing in the same direction, one from the emitter to the base, and the other from the emitter to the collector.

The loop from the emitter to the collector can only be controlled by
varying the loop from the emitter to the base. The emitter base current will flow whether or not there is any collector current. Transistors with disconnected collectors are often used as diodes.

If the collector voltage were lower than the base voltage with respect to the emitter, it would imply that current loop was in the reverse direction.

In many transistors, the emitter and collector wires can be flipped over, and the transistor will still work, poorly.

Steve Conner

Sat Mar 20 2010, 07:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

radiotech wrote ...

In many transistors, the emitter and collector wires can be flipped over, and the transistor will still work, poorly.

That's the first I've ever heard of this!

I know that the drain and source on JFETs are interchangeable. You can use either one as the drain and the other as the source, and it performs just the same. But I didn't realise BJTs behaved like that.

Mattski

Sat Mar 20 2010, 11:10PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

It works for MOSFETs, MESFETs, and HEMTs too! Probably all FETs. All of these structures are, at least for first order designs, symmetric.

radiotech

Sun Mar 21 2010, 12:14AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Bjt's with a beta ~<200 or so have a beta of about 10 reversed. Much to the annoyance of Micronta who had to revise the manual for the 23-475 dmm with the automatic transistor tester, that was so clever it showed NPN or PNP, the beta, and then identified the lead connections. Aside from that, the other type of transistor tester, the kind that hooks up the transistor as a blocking oscillator, then uses the output to light a neon lamp, also can be fooled with big heavy
power transistors.

Steve Conner

Sun Mar 21 2010, 10:46AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I learn something new every day!

Mattski wrote ...

It works for MOSFETs, MESFETs, and HEMTs too! Probably all FETs. All of these structures are, at least for first order designs, symmetric.

Almost every MOSFET has the substrate connected to the source, which wrecks the symmetry because of the body-drain diode. It's a shame, if they were symmetrical they would have lots of other handy uses.

scrooch

Mon Mar 22 2010, 07:18AM

Registered Member #908 Joined: Wed Jul 18 2007, 05:53AM
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Posts: 49

thanks guys, now i have a clear picture.
come to think of it i've been reading this forum for like ages since its first version which was a long time back. I can second Steve's comment haha.

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