EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: w2aew on October 09, 2013, 03:50:52 am
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I was asked to put together a video that discussed the basics of the three BJT transistor amplifier topologies: Common Emitter, Common Collector and Common Base. I figured I'd share it with this forum as well:
Tutorial: Common Emitter, Common Collector, and Common Base Transistor amplifiers (https://www.youtube.com/watch?v=zXh5gMc6kyU#)
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Very nice video.
You should be a teacher! :-+
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Nice video. Definitely helpful for analog n00bs like me. :-+
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thank you Alan for the always great videos, appreciate your time and energy benefiting netizens all over!
a question about the common-emitter-amplifier configuration,
(or a general question about bjt behavior really):
(https://www.eevblog.com/forum/beginners/video-by-request-bjt-amplifier-topologies/?action=dlattach;attach=265881;image)
a bjt is either off, or if it's on, the currents would always go in ONE direction only.
the ac signal at the output is developed across Rc from ic changing from weaker to stronger and vice versa, but always in one and the same direction.
ic's direction, for example, never reverses to get an ac output.
is that correct?
thank you for your time and help!
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The base current in an npn transistor goes from base to emitter only, not the way you've drawn it. There's essentially a diode in the transistor that prevents reverse base current.
So if you feed the ac signal directly to the base, the transistor will conduct only for the positive half of the input waveform.
That's why we use the two biasing resistors. Their purpose is to add a DC bias to the input signal so it becomes entirely positive.
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That's why we use the two biasing resistors. Their purpose is to add a DC bias to the input signal so it becomes entirely positive.
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thank you for the help fubar.gr!
ok, so the arrows are either in the conventional current direction, or the actual direction of electron flow, or a mix of the two, whichever the case may be.
i'm trying to get it positively confirmed that, (when a bjt is biased right for the amplification/operational region, if that's a necessary condition),
currents would only go in one direction, they would never have currents reverse their directions,
and that is the only way bjt's behave, no other way.
does "entirely positive" mean currents in a bjt go only in one direction, and never reverse?
and the so called "ac", either input or output, is merely a current changing between weaker and stronger, but never reverse directions?
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The absolute, instantaneous current can be reverse, but this would be under somewhat unusual circumstances, involving fast signals and junction capacitances, or voltages very different from those indicated.
Under normal operation, the electron current flow is as indicated (or, for a PNP and negative supply and bias, the conventional current flow).
AC and DC are very different things. It's very easy to subtract the average level of current and voltage from a system, and look at small changes from that baseline.
Tim
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But if you take a fluctuating signal like (+10.0 +/- 3.0VDC) which might be across a collector resistor and you capacitively couple that voltage then you can end up with bipolar AC since the capacitor will not pass DC, only the AC compon
Also there is nothing stopping you from biasing the transistor such that the emitter is referred to -10VDC potential and the collector is referred to +10VDC potential and setting the collector current zero signal bias until the collector load resistor is at 0VDC potential. Then if your base signal goes up and down your output voltage will be AC bipolar relative to 0VDC since that is the way you have the DC bias and power supplies of the transistor configured.
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thank you evb149, very helpful, thank you for the links as well!
as quoted above, although a bipolar AC is so generated, but that's just a matter of GND reference. as far as the actual working of the bjt is concerned, currents in it would still all go in one direction only, and they would never reverse directions. that's is to say, all bjt's can do is developing varying unipolar DC current, which in turn is used to develop bipolar AC signals through whatever means, and bjt's own working mechanism would never allow bipolar AC current. hope i got it right this time, what do you think?
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The base current in an npn transistor goes from base to emitter only, not the way you've drawn it. There's essentially a diode in the transistor that prevents reverse base current.
Notice how the arrow showing the collector current points upwards?
(https://www.eevblog.com/forum/beginners/video-by-request-bjt-amplifier-topologies/?action=dlattach;attach=265881;image)
The drawing appears to show electron flow, rather than conventional current flow, in which case it's correct.
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The topologies where you have a single transistor operating in linear mode as an amplifier over the whole signal range are 'Class-A' type amplifiers. Due to the high bias current you must use even at zero input signal they are inefficient.
There is also a way to use two transistors in a push pull Class-AB or Class-B arrangement to get an AC output (if that is the way you set up the circuit power supplies) more efficiently than Class-A.
The following links illustrate more
https://en.wikipedia.org/wiki/Amplifier_class#Power_amplifier_classes (https://en.wikipedia.org/wiki/Amplifier_class#Power_amplifier_classes)
https://en.wikipedia.org/wiki/Push%E2%80%93pull_output (https://en.wikipedia.org/wiki/Push%E2%80%93pull_output)
Here's a video I did on the different classes of operation (Class A, Class B, etc.) for a common emitter BJT amplifier - just in case this helps your understanding...
https://www.youtube.com/watch?v=c6cmkm3UPUI (https://www.youtube.com/watch?v=c6cmkm3UPUI)