Gain stage vs. Booster stage - What's really happening to that signal?

[MohiZ]

Ok, say a signal enters a BJT base. The base-emitter junction is much like a diode. So the signal goes from base to emitter through a diode. If the emitter is connected to ground, then the signal goes to ground.

But, the transistor is a device that "monitors" the amount of current flowing in the base-emitter junction, and "adjusts" the collector-emitter current to be a multiple of this. As the signal current flows from base to emitter to ground, a multiple of this current flows through collector to emitter to ground. If there is a resistor between the power supply and the collector, the resistor drops a voltage across it, that is proportional to the collector current (Ohm's law). And, thus, the signal is "formed" again at the collector, and its magnitude depends on the value of the collector resistor.

This is called a common emitter stage.

In a FET the situation is a little different. The gate "monitors" the voltage instead of current. The drain-source current is proportional to the gate-source voltage. Again, the signal is then "formed" again at the drain resistor.

[evanj1969]

the diodes should be placed from the signal path to ground, as simple as that, I can't really clarify it any more than that

maybe I should have said, "does it matter where in the signal CHAIN it's placed". placed from the signal path to ground I understand, I'm just not getting any results. One side of the diode is in the signal path, the other connects to ground. Positive in path, negative to ground / negative in path, positive to ground. Been there, tried it, no results.

could this be a biasing/voltage issue. not getting enough power to the opamp for it to function the way it is intended?
If there is an area I REALLY have trouble with, that would be it (understanding the powering issue)

[Earthscum]

evan:

I'm not saying I'm right, but I had the best luck with dropping the diode to ground right before the coupling cap, but I think this changes the bias when it clips? Anyways, I was getting an LED to light up and it wouldn't after the coupling cap (obviously, there's no DC running through it at that point).


So, I figured out where my big hangup was...

I had the Rb confused with the feedback loop of an OP Amp. Basically I just went back and (RE)read about transistor biasing and found that the same document I've been going back to again and again... well, I missed one sentence somehow every time I've read it. Now all the parts' interactions make sense.

So, now I have a more (seemingly) intelligent question. I notice on some single transistor amps (either alone, or part of a larger circuit) have just a resistor between base-emitter for the bias. (I really gotta quit mixing up FET terms with BPJ terms). Others have the resistor from emitter-base, and another from base-ground. I couldn't find an explanation for this, just that some articles I've read have it, and some don't, and none that I've seen have both and explain it.

When you do a combination bias (self+fixed), how does that resistor (ground-collector) affect the other biasing resistors? Basically, I assume you can't just figure out your bias using a single resistor (B-E) and then assume that anything else is not going to change how that value is affecting the bias, right? Maybe just a quick explanation on  the process of biasing a self-biased+fixed bias and just fixed bias? (don't have to go into detail, unless it's needed. More of "If you have 9V battery, and you want the swing right in the center, you want your base to be 4.5V, so you calculate (ResistorA) first." Like, say if I wanted my swing to be, like said, 4.5, and I want mondo gain, and I use a 100K from emitter to V+, I could use a 10K from B-E, and another from Grd-B, right? It seems like it would be a bit easier to calculate that one than the single B-E biasing. Anyways, maybe a quick small rundown of "Start at point-A, then figure this, then you calculate this one". Also, I didn't see this, fixed bias there's a resistor to ground. What other resistors are needed for fixed bias alone? All I've read didn't really mention this, either, so I've kinda assumed that the "fixed" biasing is something that is not really stand-alone, but rather an addition for added stability.

Another quick thing: The gain vs. boost question... Would I be correct in saying that one of the biggest differences is that a gain stage's biasing will change more with signal while boost is designed to be more stable biased? I've noticed that boost stages have the full 4-resistor biasing, while most Gain (unclean, clipping) stages are made to allow the signal to effectively re-bias the transistor.

[R.G.]

Well, now I'm utterly confused then. And, btw, I understand voltage biasing, I'm just not understanding where the signal is going in the path and what it's doing from input to output (in this case, a single transistor amplifier). I understand how the signal travels along the DC path, but I am not getting where these paths lay, which direction things are going, etc.
...

You're confused and having problems because you're asking questions that properly are answered by about 2-3 semesters of college level courses or the equivalent in experimentation or independent study. In a forum like this, it is only possible to get the most condensed, terse answers no matter how much the person answering wants to explain it to you. Worse, you don't know how to separate the questions to get the answers you need. I mean this as no belittling of you or your efforts to learn. On the contrary, it's really good that you want to dig in and really learn instead of just subbing in parts to make a mean sounding distortion.

To get the level of understanding you are asking for, to even separate the topics so you can ask single-point questions, you're going to have to spend some time preparing the background knowledge that will support the answers. This is the reason you're getting many long answers that don't answer your questions. They're really huge topics if you have to explain all the way back to the beginning.

[petemoore]

  Start with a schematic.
  Read at GEO and all links, feel free to google any ''terminologies''.
  Simple is very good for beginning this study.
  LPB or other single transistor boost=gain stage, anything that outputs a larger signal, nearly the same shape as the input signal.
  Notice everything about the bias support:
  First of all it is of the more common NPN/Neg. Ground variety.
  Emitter has small resistor to ground.
  Collector has a larger resistor to V+.
  The relationship between the E/C resistor values can be used to set gain and influence bias.
  The base is held at a voltage somewhere between V+ and Gnd. by the base bias resistors [2] which form the 'resistive divider' [a two resistor, resistive voltage divider].
  http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=72&Itemid=26
  Diddling with a simple circuit such as this, or breadboarding it...provides yourself with a nice boost [very handy item] and a chance to gain a better understanding of what happens with ''electron-flow...when...under 'x' scenario.''
  Starting with a schematic puts questions in a scenario, and allows better understandings to work out related answers.

[Earthscum]

You're confused and having problems because you're asking questions that properly are answered by about 2-3 semesters of college level courses or the equivalent in experimentation or independent study. In a forum like this, it is only possible to get the most condensed, terse answers no matter how much the person answering wants to explain it to you. Worse, you don't know how to separate the questions to get the answers you need. I mean this as no belittling of you or your efforts to learn. On the contrary, it's really good that you want to dig in and really learn instead of just subbing in parts to make a mean sounding distortion.

To get the level of understanding you are asking for, to even separate the topics so you can ask single-point questions, you're going to have to spend some time preparing the background knowledge that will support the answers. This is the reason you're getting many long answers that don't answer your questions. They're really huge topics if you have to explain all the way back to the beginning.

I'm ADHD, OCD, and Bipolar... it's extremely hard for me to get things from my head to my mouth or fingers. Basically, I'm technically not a noob in the sense of the world of electricity all together, but admittedly a noob in the area of electronics. The first thing I started studying concerning schematics was the Real McTube. I already had a basic understanding of how valves worked because of other projects in the past concerning electron flows through other substances, including electron knock-off from the sun, and how it flows through vacuum, etc.... I know ALOT of stuff, but it's all in snippets because I haven't been able to use the info in application... like having a puzzle that you've only put parts of together, then put it in a box to finish some other day when your attention span allows for it. 

If I have to learn 3 new things to understand a single word, I'm a happy camper. That's really the only reason I pick up hobbies like this, to learn... then I get bored and move on to something else. I've spent who knows how many hours reading datasheets, explanations of different circuits... and right now it's all a jumble in my head. Like the biasing thing, which I apparently understood the entire time, it was just the variations of biasing that were throwing me off... that and not paying attention to things like the "feedback loop", which looks like the bias resistor network on a single transistor amplifier. I should have made the separation a long time ago between the two, and that the feedback loop was completely different, and actually an external function of the internal amplification network. So far through this thread I've actually learned a whole bunch, and  found that I already knew the answer to my questions, I just needed to actually put the darn puzzle together.

So, all in all, this thread can probably be left to rest... I'm good for now, I believe. Thanks for putting up with me, lol... and see ya on the boards (and I'll TRY to structure questions and writing in general a bit better).

[frank_p]

Hey, it's pretty normal to want instant knowledge to everything.  Only, it does not seem to work like that. You seem to have motivation to learn more, that is the base.  Then if you have already a technical inclined mind, that is a plus.  Perhaps what could help you is some methodology.  When you york on a project, try to keep track of (in a notebook):

- What are your specific goals in a particular project
- Be aware of projects that might be more suited to what you want to learn.
- Write down your hypothesis about how you think the things you are studing is working.
- Try to confirm your hypothesis with readings, breadboarding and taking measures.
- If you still don't have a clue, ask more specific questions (you will already had seen the battlefield so you will know on what front you are fighting)
- When you feel down, take a break instead of getting discouraged and mixed-up and be tempted to leave the hobby.
- Instead of collecting everything in your computer, fill your notebook by hand, you will have no choice to think of one thing at a time.
- Use outlining markers of different color to point out: questions, things to do, ideas, completed things, etc.
- Etc.

What I am trying to say is: give yourself the means to organise your learning journey, work on one thing at a time and be aware if you should not go back to study a point you had leaved behind, ie: a point that is an obstacle to your progression.

[Ben N]

FWIW, I think your pesky insistance on trying to understand is worlds better than people who just want someone else to do a paint-by-numbers layout so they can save a few bucks off the cost of a commercial pedal. Good on you, and take your time.

[frank_p]

FWIW, I think your pesky insistance on trying to understand is worlds better than people who just want someone else to do a paint-by-numbers layout so they can save a few bucks off the cost of a commercial pedal. Good on you, and take your time.

Yep, and on a forum, everything is not structured like in school or in books.
If you have time and like a classroom style learning at your own pace (and tired of texts and books), be sure to have a look at these "open source" treasures (the videos are on youtube and all the material is online and well organised):

http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/index.htm