Video by request: BJT amplifier topologies - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: October 09, 2013, 03:50:52 am
content: 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:

author: FoXX
author_rank: Contributor
timestamp: October 09, 2013, 07:42:28 am
content: Very nice video.You should be a teacher!

author: mrflibble
author_rank: Super Contributor
timestamp: October 09, 2013, 12:31:38 pm
content: Nice video. Definitely helpful for analog n00bs like me.

author: questron
author_rank: Regular Contributor
timestamp: October 29, 2016, 03:51:12 pm
content: 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):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!

author: fubar.gr
author_rank: Unknown
timestamp: October 29, 2016, 04:17:57 pm
content: 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.

author: questron
author_rank: Regular Contributor
timestamp: October 29, 2016, 04:43:26 pm
content: Quote from: fubar.gr on October 29, 2016, 04:17:57 pmthank 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2016, 05:01:47 pm
content: 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

author: questron
author_rank: Regular Contributor
timestamp: October 29, 2016, 05:10:40 pm
content: Quote from: evb149 on October 29, 2016, 04:30:09 pmthank 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?

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2016, 07:15:44 pm
content: Quote from: fubar.gr on October 29, 2016, 04:17:57 pmNotice how the arrow showing the collector current points upwards?Quote from: questron on October 29, 2016, 03:51:12 pmThe drawing appears to show electron flow, rather than conventional current flow, in which case it's correct.

author: w2aew
author_rank: Super Contributor
timestamp: October 30, 2016, 01:48:00 pm
content: Quote from: evb149 on October 29, 2016, 04:37:56 pmHere'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...