Why output comes before the collector on single transistor amplifiers? - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: January 31, 2019, 04:31:11 am
content: I thought the:- clean current would get into the transistor through the collector- the signal inserted through the base- the current from the collector would combine itself with the signal- that would emit from the emitter an amplified signalbut by looking at most circuits that amplify with one transistor they all place the output before the collector. Why?

author: oPossum
author_rank: Super Contributor
timestamp: January 31, 2019, 04:37:38 am
content: A common emitter amplifier like that has voltage gain. A common collector (emitter flower) only has current gain.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 31, 2019, 04:38:21 am
content: https://www.quora.com/What-is-a-transistor-how-does-it-work-and-how-can-it-be-used-as-an-amplifier-or-switch

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: January 31, 2019, 04:43:38 am
content: If you have the money to spare Sedra Smith is a wonderful book to read.Though you could surely find it in some "dark" places on the internet, for me it´s really worth having in your library if you are serious about electronics.The issue is a bit more complicated than you might think to just reply in a few sentences. It´s more like pages upon pages in books.Cheers!

author: gkmaia
author_rank: Frequent Contributor
timestamp: January 31, 2019, 05:00:32 am
content: this one?http://global.oup.com/us/companion.websites/umbrella/sedrasmith/

author: bd139
author_rank: Super Contributor
timestamp: January 31, 2019, 07:41:09 am
content: In this configuration the transistor is a voltage controlled current source. The resistor is a current to voltage converter. Try and think of it like that as two separate devices.Voltage -> current -> voltageIt’s a bit more complicated than that really but the basic idea is a valid mental model.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 31, 2019, 08:03:48 am
content: Quote from: gkmaia on January 31, 2019, 04:31:11 amBecause that's not how transistors, or FETs, or vacuum tubes (valves) work!Imagine, if you will, the transistor in your Schematic being replaced by a variable resistor with a knob you can turn to increase or decrease the resistance.This device, which we can call VR1, along with Rc, forms a voltage divider between your "power source" & ground.The "output signal" is the voltage between the junction of Rc & VR1, with respect to ground.If you move the knob controlling VR1 fast enough, you get a varying voltage at the output which looks like a very low frequency audio signal .,'Imagine if you had superpowers, you could listen to some music & twiddle the knob in time with it.If you listened to the signal with an amplifier & speaker, you would hear music!Unfortunately, nobody has super powers, but early experimenters looked for ways to vary the resistance of part of a voltage divider to do something along those lines.The first success was the carbon microphone.The sound of someone speaking into the microphone varied the resistance of the resistive element, & hence, varied the voltage available at the output of the voltage divider made up of that element & its load resistor--- in many cases, the earphone of a telephone somewhere remote from the mic.This gave them a very sensitive microphone, but it still didn't solve the problem of amplifying a signal which was already an electrical one.One idea was to use a very sensitive earphone, mechanically coupled to a carbon mic.This worked, but not too well.Ultimately, the first workable vacuum tubes were made, revolutionising the field of Electronics.The "tube" was still a "variable resistor", but one where the input signal controlled the amount of current flow through it, & hence, its effective resistance value in the "voltage divider".Most amplifying devices, although the mechanism varies significantly, still are "part of a voltage divider".Some circuit configurations are a bit hard to visualise as a simple "voltage divider", but it works well with the "Common Emitter" & " Common Collector"(Emitter follower) circuits, & their equivalents with other devices.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: January 31, 2019, 12:20:13 pm
content: Quote from: gkmaia on January 31, 2019, 05:00:32 amYup, that one. I got the fourth edition, so I do not know what they added or removed, still, I very good read!

author: hli
author_rank: Frequent Contributor
timestamp: January 31, 2019, 01:46:18 pm
content: RSD academy has a nice youtube video explaining common-emitter amplifiers:

author: rstofer
author_rank: Super Contributor
timestamp: January 31, 2019, 02:36:24 pm
content: W2AEW explains amplifier configurations quite well:

author: JungleTits69
author_rank: Newbie
timestamp: January 04, 2025, 02:18:42 am
content: Thank you so much for that. I am new to the world of electronics engineering, and I never had any schooling for it so I'm just finding online resources and teaching myself that way. And man, idk if it's just me, but...SO many people explaining how different components or circuits work, seem to be so terrible at it, so many confusing or seemingly contradictory things. But you explained this beautifully!

author: MrAl
author_rank: Super Contributor
timestamp: January 04, 2025, 05:48:04 am
content: Quote from: gkmaia on January 31, 2019, 04:31:11 amWhat do you mean that they "place the output before the collector" that does not make sense. Did you mean they take the output from the collector?Yes it's a sort of misnomer that they call it the collector in an amplifier too, but that's because the transistor operation as an isolated device has a different description than when we look at the entire circuit. The collector in a transistor works in a certain way when we look at how the transistor works, but in a circuit the collector works a little differently when we look at how the amplifier works. Once the transistor is placed into a circuit, we become more concerned with how the amplifier works not how the transistor works. The transistor still works the same, but we no longer care about that as much as how the entire circuit works. We begin to care ONLY about how the current flows out of the output terminal of the amplifier not how it flows with regard to the collector itself. So although the transistor works in a certain way, we can use it any way we want to use it. For a sarcastic example, if we used it for a paper weight it would not have any current in any lead but it's still a transistor.Also, this happens to be a common emitter amplifier but with a common collector amplifier you would find that the collector viewed on the transistor itself would work the same as when viewed from the point of the entire amplifier circuit. That's also called an emitter follower and the output comes from the emitter not the collector.