How Do Transistors Function as Switches and Amplifiers?

[Rainbow]

Greetings

Can somebody please help me understand the use of transistor as a switch and that as an amplifier?

 

[chroot]

"Understand" is a pretty vague term. Can you ask a more specific question?

- Warren

 

[Rainbow]

I'm sorry for not being specific.
I meant, I couldn't get the working of the circuit arrangement for studying the input and output characteristics of n-p-n transistor in CE configuration. I want a complete description of the whole thing

 

[ranger]

You mean to say that you are trying to build the thing but cannot get it to work?

Here are some references to CE amplifiers. Let us know if you have any specific questions relating to this configuration:
http://en.wikipedia.org/wiki/Common_emitter
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npnce.html

 

[bassplayer142]

Can't you basically call a transistor an electronic switch. A manual switch would take persons finger to push the button allowing flow of current from one side to another. In a transistor, the finger hitting the button would be a third pin which gets a voltage applied to it.
I know that amplifiers for sound are based on transistors which usually get really hot when operating. Also you could use one for regulating voltage.

 

[ranger]

The transistor as a switch is basically having it work in two of the possible three operating regions. The first region is cut off. Cut off is as the names implies; no collector current so the transistor is off. The switch is "OFF". The operating region of interest in the application of a transistor as a switch is the saturation region. In this mode, a large (or useful) current flows. The switch is "ON". Therefore, in using the transistor as a switch, one seeks to have the transistor in either the cut off or saturation regions of operation.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c2
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c3
http://www.allaboutcircuits.com/vol_3/chpt_4/2.html

A point of departure from the transistor switch and transistor amplifier is the operating region the transistor is biased to operate in. For the amplifier, this region is known as the active region. There is some collector current in this mode. Although it does not end here, amplifiers is an extensive topic. You will have to do extra reading for yourself on this one.

http://www.itee.uq.edu.au/~engg1030/lectures/1perpage/lect12.pdf (pdf file)
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c2

 

[Rainbow]

Thank you. I'll go through the links that you provided and let you know if I have any questions.

 

[dionysian]

I have a question along the same lines too. I am studying transistors as amplifier circuits at the moment and it seems to be that they can only be used to amplify small signals is that true?

And if they can only be used to amplifier small signals what is that point of that? Do most of the applications in which we need signal amplifaction pretain only to small signals?

 

[zeitghost]

The size of signals amplified by transistors depends on the transistor...

Small transistors are good for amplifying small signals...

Look up a BC109 for example.

Big transistors are good for amplifying big signals.

Look up 2n3772 for example.


It's horses for courses, much like everything else really.

 

[dionysian]

So does this mean that the small signal analysis holds if we have a big enough diode? Even if we don't have a "small signal" per se.

 

[Number2Pencil]

Well it really depends on what you mean by "small signal". No matter what operation region you're in you can't exceed your supply rails (probably the dc value labeled Vcc in textbook cases..or at least the one i learned in). for instance: if your signal is a 1V signal and your gain is set to 100, you are expecting a 100V output right? well if your Vcc (or whatever your dc component is) is only 15V then your output will be a clipped waveform or a 15V dc voltage that doesn't accurately amplify the signal. on the other hand a 1mv signal times a gain of 100 is 100mv, which could actually produce an accurate amplified signal. this might be what you mean by "small signal". You can't get more out than what you put in. I actually learned this from asking questions on this very forum, so don't feel bad i understand how you feel.

 

[Number2Pencil]

And if they can only be used to amplifier small signals what is that point of that? Do most of the applications in which we need signal amplifaction pretain only to small signals?

In most cases, yes. the purpose of solely amplifying a signal would be to get it in a more "useable" form.

 

[dionysian]

Thank you or your reply.

They way i understand how a transistor amplifier (BJT) works is that you input a small signal into the emitter then based on the change in the base emitter voltage the current Ic changes. Is this not correct?

Then i was under the impression that the only reason why this works is that the change in base emitter voltage is small enough that the change in the ic current is for all practical perposes is linear ( thus introducing no distortion) . Normally its not linear because ic = Ic*e^(vbe/vt) but for small signalls vbe << Vt the taylor series approximation ic = Ic(1 + vbe/Vt) holds.

If there is any error in the above statements please let me know. But i don't see how changeing the size of the transistor would allow us to use the small signal approximation with larger signals. wouldn't Vt have to become large along with the transistors size?

I do understand that we cannot exceed the supply rails but that is not what i am confused on. Thank you for your help.

 

[zeitghost]

What you describe above, putting the signal into the emitter is known as Common Base mode and is not widely used.

The more usual method is common emitter mode where the signal is applied to the base with the emitter effectively grounded so that Vbe is modulated, thus changing the collector current.

No one (that I know of) designs common emitter circuits by contemplating the change in Vbe... they are designed on the base current & collector current, taking account of hFE or Beta, the current gain of the transistor.

As you say, the change in Ic for changes in Vbe is highly non linear.

I suspect this is the difference between the semiconductor physics approach to things & we rude mechanicals who have to get the job done...

 

[dionysian]

O wait i meant the signal is input into the base not emitter.. i mean common emitter mode. I am still just a student so i don't really actively use these things i am just trying to piece this together and figure out where my understaning is wrong.

 

[chroot]

A BJT is a current-controlled current source. That's almost literally all you really need to know when using them from a designer's perspective.

- Warren