How does input voltage for common emitter amplifiers affecting biasing?

Thread Starter

abu1985

Joined Oct 18, 2015
22
Hello

I've been playing around a lot with common emitter amplifiers. I've just been making general signal amplification circuits. I use calculator tools to find the right biasing for any particular setup. At least, I think I'm biasing correctly (I'm probably not). But even if I decide to throw together know working circuits, I don't see much on how the input signal affects the Q point of a transistor, or what the best input voltage should be?

Understanding this will help me understand why different amplification stages are needed. Or what the true definition of a preamp is. What about an amplifier circuit makes it a preamp, or is it just the application it's used in?

Thank you!
 

Zeeus

Joined Apr 17, 2019
411
Hello

I've been playing around a lot with common emitter amplifiers. I've just been making general signal amplification circuits. I use calculator tools to find the right biasing for any particular setup. At least, I think I'm biasing correctly (I'm probably not). But even if I decide to throw together know working circuits, I don't see much on how the input signal affects the Q point of a transistor, or what the best input voltage should be?

Understanding this will help me understand why different amplification stages are needed. Or what the true definition of a preamp is. What about an amplifier circuit makes it a preamp, or is it just the application it's used in?

Thank you!
Hi..dunno much but don't think the input signal affects the Q point really
and the input voltage : this sets the emitter voltage which in turn sets collector current (emitter current) so we can set collector voltage at half the supply

but also, the base-collector diode should be reversed bias for NPN

Hello

I've been playing around a lot with common emitter amplifiers
hmm, that's nice :) sounds like you're having fun

Can you post schematic of some you've been playing with? don't play alone:rolleyes:

Thank you too!
 

Bordodynov

Joined May 20, 2015
2,386
As long as the non-linear distortion of the amplifier is small, it can be assumed that the influence of the input signal is small, but when the distortion is large, changes in the mode are noticeable. I took the simplest circuit and got the consumption current and output voltage for four different amplitudes. I drew up the results in the diagram. The higher the signal, the lower the consumption current and the higher the average voltage at the collector.
See
simpl.png
 

Thread Starter

abu1985

Joined Oct 18, 2015
22
The Q point
Hi..dunno much but don't think the input signal affects the Q point really
and the input voltage : this sets the emitter voltage which in turn sets collector current (emitter current) so we can set collector voltage at half the supply

but also, the base-collector diode should be reversed bias for NPN



hmm, that's nice :) sounds like you're having fun

Can you post schematic of some you've been playing with? don't play alone:rolleyes:

Thank you too!
Thanks for the reply!

You are correct, the collector current will set the Q point. I don't think I asked the question correctly, though. The comment after yours gave a link, and that link is what I'm asking about. I just didn't know how to ask lol.

As far as schematics, I just use an iOS app called EE Toolkit Pro from Tom Gruber. So I don't really have anything written down. I just swap out the resistors and caps as I go.

Also, and completely off topic, I love your avatar. That quote is the best.

Thanks!
 

Thread Starter

abu1985

Joined Oct 18, 2015
22
As long as the non-linear distortion of the amplifier is small, it can be assumed that the influence of the input signal is small, but when the distortion is large, changes in the mode are noticeable. I took the simplest circuit and got the consumption current and output voltage for four different amplitudes. I drew up the results in the diagram. The higher the signal, the lower the consumption current and the higher the average voltage at the collector.
See
View attachment 179313
What software is this? Free by chance?

And thanks for the time putting that together!
 

Zeeus

Joined Apr 17, 2019
411
The Q point

The comment after yours gave a link, and that link is what I'm asking about. I just didn't know how to ask lol.


Thanks!
lol...Jony and Bordodynov : reserve comments

Post a circuit and question though and guaranteed, you will learn more
 

shteii01

Joined Feb 19, 2010
4,667
Ok. This is homework and should be moved.
The big clue is this statement: "I don't see much on how the input signal affects the Q point of a transistor"

The whole point of an amplifier is to amplify input signal WITHOUT effecting the Q-point.
OP needs to go and read his textbook. Not waste our time on the internetz.
 

Thread Starter

abu1985

Joined Oct 18, 2015
22
Ok. This is homework and should be moved.
The big clue is this statement: "I don't see much on how the input signal affects the Q point of a transistor"

The whole point of an amplifier is to amplify input signal WITHOUT effecting the Q-point.
OP needs to go and read his textbook. Not waste our time on the internetz.
Wow, thanks for the insult. Much appreciated.

Actually I graduated in 2012 from a crappy school with a Bachelor of Science degree in Industrial Automation. I've been a controls systems engineer ever since. I've ground up designed electrical systems for GM's cooling tower systems (one of the cooler things I'm done). I program industrial controllers on the daily. I make robots dance with other robots. I'm a rare person in the industrial automation field and I'm very good at what I do, and I make a good living at it.

But you know what I'm not good at, my hobby; electronics. I'm fine with passive components, but I lack experience with analog and active components. So I decided to ask here on this great website, only to find your rude comment.

Please except my apology for wasting your time...
 

MisterBill2

Joined Jan 23, 2018
3,862
The other question was about the "true definition of a pre-amplifier", and I am going to add the qualifier of "stage". The pre-amplifier stage is usually the first stage with an active device,that follows an input. In audio equipment it is usually before any tone controls and switching circuits. In general purpose applications it is usually considered the first stages of amplification, and the section, including phase inverters, before the part that drives the output stages.
 
I can recommend a good book that will cover biasing of bipolar amplifiers and a lot more. It is an old book but still very useful. It is called Handbook of Simplified Solid State Circuit Design by John D. Lenk.
You can probably purchase it for less than $10. Make sure you get the second edition.
 

MisterBill2

Joined Jan 23, 2018
3,862
Certainly that is one of the very useful books that provide some insight into the operation of bi-polar transistor circuits. Now I am wondering if you are asking about AC coupled amplifiers or DC coupled amplifiers, because those are quite a bit more complex. My experience has been that what I anticipate the input signal to be affects my decisions as to setting the "Q" point, which is where the circuit sits without any input signal. I should dig through my small library of textbooks and find a great title to suggest.
There are also some great discussions and articles by Bob Pease, unfortunately deceased, who had a fantastic grasp of how things worked. His articles are all great collections of insight, mostly into transistor circuits. So there is another source of understanding.
 
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