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#### Mao

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##### GAR transistor amplifiers.....
« on: February 11, 2008, 04:42:15 pm »
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i'm doind a bit of reading ahead, and i have a problem...

Can someone please explain to me how transistor amplifiers work? I'm using the nelson textbook, and the explanations about the npn transistor is really just, not making sense (much)

Here's what I'm having trouble with, specifically:
1. The electron flow, where does Ib, Ic and Ie go?? - DONE
2. How the amplification actually works? How is it amplified? - DONE

and most importantly
3. HOW THE HECK DO I READ THESE CIRCUIT BOARDS?? THEY ARE SO DAMN CONFUSING! - DONE
Vcc, all the resistors that seem to be everywhere...... bleh. - DONE
THANKS
« Last Edit: February 11, 2008, 07:35:25 pm by Mao »
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#### abcat

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##### Re: GAR transistor amplifiers.....
« Reply #1 on: February 11, 2008, 05:00:00 pm »
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oh god, transistor amplifiers are horrible. i basically chalked them up as a loss for unit 3. so freakin confusing.

#### cara.mel

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##### Re: GAR transistor amplifiers.....
« Reply #2 on: February 11, 2008, 06:11:12 pm »
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OOH PHYSICS QUESTION =D

1. Electron flow: Basically things start off in the top left (Vcc) or middle left (signal to be amplified), and they end up going right/down. Along the very top line the voltage = Vcc, along the very bottom line V=0. So Ib goes right, Ic and Ie go downwards

2. I havent looked at this since unit 3, I will do so later ie tomorrow unless someone beats me to it, but the supply voltage adds onto the signal to amplify it. You don't need to learn how the actual transistor bit works. I am struggling to remember off the top of my head why the transistor works only when there is a base current, in my head I am only getting diode crap.

3. The first two resistors on the left side act as a voltage divider. This adds onto the input signal as the bias voltage so that none of it gets chopped off/lost when being amplified.
The other two also are a voltage divider, but I can't remember why and I know you don't need Re for it to work.

THIS POST WILL BE EDITED AND 100X BETTER WHEN I READ ITUTES STUFF/FIND MY OWN STUFF

#### Collin Li

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##### Re: GAR transistor amplifiers.....
« Reply #3 on: February 11, 2008, 06:30:43 pm »
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Here's what I'm having trouble with, specifically:
1. The electron flow, where does Ib, Ic and Ie go??
2. How the amplification actually works? How is it amplified?

and most importantly
3. HOW THE HECK DO I READ THESE CIRCUIT BOARDS?? THEY ARE SO DAMN CONFUSING!
Vcc, all the resistors that seem to be everywhere...... bleh.
THANKS

An npn-transistor has 3 streams: 2 streams entering it, and 1 stream exiting it. The two streams entering it are called the collector arm, and the base arm. The stream exiting it is the emitter arm. $I_c$ refers to the current entering through the collector arm, $I_b$ refers to the current entering the transistor through the base arm and $I_e$ refers to the current exiting the transistor through the emitter arm.

The analogy of the transistor to a tap is quite useful. The base current is analogous to the energy required to open a valve (approximately requires 0.30 V if I recall correctly) that allows the collector current to flow through the transistor, and ultimately come out as emitter current. The transistor is supposed to amplify the base current, using the collector current as the feed (still speaking in analogous terms) to do so.

Read the circuit boards like an energy well. Think of it as a ball that is rolling down the circuit. The circuits are always drawn with the wires closer to the top containing more voltage, then eventually dropping down to a voltage of 0 at the ground (bottom line). $V_{cc}$ refers to the input DC voltage (which is related to the collector current) and is ultimately used to amplify the small base AC voltage.

#### bilgia

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##### Re: GAR transistor amplifiers.....
« Reply #4 on: February 11, 2008, 06:38:41 pm »
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ask jack in springvale when he will cover it...he covered them so thoroughly i didnt need to consult the textbook for any of the theory (he said its all confusing in there)
My Subjects:
2006 I.T Systems --> 42
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ENTER: 97.35

#### Mao

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##### Re: GAR transistor amplifiers.....
« Reply #5 on: February 11, 2008, 06:40:56 pm »
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mmm

that's starting to make sense
but i still dnt know how to do the questions, here's a beginner:

i've got Ic = 20mA, as Vc = Vcc - Vout

how do i find out the Ib and Rb?? -DONE
should i be making any typical assumptions such as Vbe=0.7V and stuff like that...? -DONE
« Last Edit: February 11, 2008, 07:35:46 pm by Mao »
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#### Collin Li

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##### Re: GAR transistor amplifiers.....
« Reply #6 on: February 11, 2008, 06:47:36 pm »
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It's got a current gain of 150, and $I_c \approx I_e = 20\mbox{ mA}$. We have to use this assumption because there is not enough information otherwise, and it is safe to make it because $I_b$ is usually extremely small.

$I_b = \frac{20}{150} = 0.13\mbox{ mA}$

Note that you can actually say $I_e = I_b + I_c \implies A = \frac{I_b + I_c}{I_b} = 1 + \frac{I_c}{I_b}$, where $A$ is the current gain, but I just made this up right now, and I believe most calculations use the approximation: $I_c \approx I_e$.

#### Mao

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##### Re: GAR transistor amplifiers.....
« Reply #7 on: February 11, 2008, 06:51:08 pm »
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of course! why didnt i think of it

and, i think this is how the conventional current flow around... right? - FIXED

my(caramel's) arrows in red (my ones are the arrow looking ones, caramel replaced my errors with Arrowretardo)

EDIT: ooops sorry coblin ur reply is all stuffed as i changed the picture after caramel corrected me
« Last Edit: February 11, 2008, 07:35:58 pm by Mao »
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#### Collin Li

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##### Re: GAR transistor amplifiers.....
« Reply #8 on: February 11, 2008, 07:16:11 pm »
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of course! why didnt i think of it

and, i think this is how the conventional current flow around... right?
(Image removed from quote.)
my arrows in red

That's basically right, except for a transistor amplifier to actually have use, there is an AC voltage applied at the node in between $R_1$ and $R_2$. It is referred to as $i_b$ (as opposed to the DC "bias" voltage called $I_b$). Both of these currents are added up and enter the base of the transistor. So basically, you will usually have a 4-way junction at that node in between $R_1$ and $R_2$, with an AC source coming from the left.

#### cara.mel

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##### Re: GAR transistor amplifiers.....
« Reply #9 on: February 11, 2008, 07:31:12 pm »
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my(caramel's) arrows in red (my ones are the arrow looking ones, caramel replaced my errors with Arrowretardo)

my arrows are not retardo they are very fashionable.

#### Mao

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##### Re: GAR transistor amplifiers.....
« Reply #10 on: February 11, 2008, 07:34:39 pm »
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OMG i cant believe this, this was an impossible topic this morning, but now

SOLVED

thank you so much everyone,

PS caramel your arrows can be R(egistered)retardo then xD jk jk

anyways, keep the discussions going, i'm sure later on (in a few months) there will be plenty of physics people going "WTF TRANSISTORS ZOMG I R DEAD" lol

in the meanwhile i shall write notes along with the NMR stuff
« Last Edit: February 11, 2008, 07:37:11 pm by Mao »
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VCE 2008 | Monash BSc (Chem., Appl. Math.) 2009-2011 | UoM BScHon (Chem.) 2012 | UoM PhD (Chem.) 2013-2015