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#### chid

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##### Transistor Amplifiers...a few questions
« on: April 30, 2008, 09:20:18 pm »
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Hi guys,

I've read a lot about transistor amplifiers (we haven't actually covered them at school yet) but I still have a few difficulties:

Why does an increase in the base current cause a large change in the collector current? (something to do with npn junctions but I'm not sure exactly what!)

Is Vout just an AC voltage or a combination of AC and DC? (I'm getting confused with them using peak to peak sometimes... I think)

Why do they often talk about the device being 'disconnected' and then a 20 mV input signal is used?

And finally is the currrent gain similar to the voltage gain... or not?

I think I understand the quantitative side to these questions (Mao's notes are really helpful) but I feel like I don't really understand what is actually happening.

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#### Mao

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##### Re: Transistor Amplifiers...a few questions
« Reply #1 on: May 01, 2008, 08:51:28 pm »
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i'm not exactly sure about the first question, but I do know that we dont need to know it.

for your second question, Vout can be a combination of both (usually is), unless a capacitor is placed somewhere there (there's two positions to put it) which decouples it to only AC
peak-to-peak refers to twice the amplitude of the AC component. its basically $\Delta V_{out}$ or can be written as $v_{out}$

"disconnected" just refer to 0V from Vin, or basically just 0.7V going into Vb

and current gain is the current amplification, it depends on the transistor itself, it is $\frac{I_{CE}}{I_{BE}}$. Since ICE is practically IC, and IBE is practically IB, a current gain of 100 means an Ib of $20 /mu A$ will be amplified to $2mA$ at RC
Voltage gain depends on the circuit configuration and the current gain, where $A_V=\frac{\Delta V_{out}}{\Delta V_{in}}$
they are kind of related.... based on similar principles, but very different things.

« Last Edit: May 03, 2008, 02:14:34 pm by Mao »
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#### chid

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##### Re: Transistor Amplifiers...a few questions
« Reply #2 on: May 01, 2008, 10:44:16 pm »
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Thanks Mao. Much appreciated.
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#### Collin Li

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##### Re: Transistor Amplifiers...a few questions
« Reply #3 on: May 03, 2008, 01:26:51 am »
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Why does an increase in the base current cause a large change in the collector current? (something to do with npn junctions but I'm not sure exactly what!)

You do not need to know about the operation of npn-junctions, or p-n junctions. However, to not really answer your question technically, it is helpful just to think of a base current as an input energy to open a tap. If you have more base current, you can turn the tap more, and allow more collector current to flow through. This is why a variation in the base current causes a variation in the collector current (an amplified one)