How Do Transistors Function as Switches and Amplifiers?

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Discussion Overview

The discussion revolves around the functioning of transistors as switches and amplifiers, specifically focusing on their circuit arrangements, operational characteristics, and the implications of signal sizes in amplification. Participants explore both theoretical and practical aspects of transistor operation in common emitter configuration.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks clarification on the working of n-p-n transistors in common emitter configuration, indicating a lack of understanding of input and output characteristics.
  • Another participant suggests that a transistor can be viewed as an electronic switch, comparing it to a manual switch and mentioning its applications in sound amplification and voltage regulation.
  • It is noted that transistors operate in different regions: cut-off (off state) and saturation (on state) for switching, and active region for amplification, with the latter allowing for a small collector current.
  • Questions arise regarding the capability of transistors to amplify only small signals, with one participant asserting that the size of the transistor influences the size of signals it can amplify.
  • A participant discusses the limitations of amplification based on supply rails, emphasizing that exceeding these limits results in clipped waveforms.
  • Another participant explains the common emitter mode of operation, correcting a misunderstanding about inputting signals into the emitter versus the base.
  • There is a discussion on the linearity of the relationship between base-emitter voltage and collector current, with references to the Taylor series approximation for small signals.
  • One participant emphasizes that a BJT operates as a current-controlled current source, which is a key concept for designers.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the operation of transistors, particularly in relation to signal sizes and operational modes. There is no consensus on the implications of using larger transistors for small signal analysis, and the discussion remains unresolved on several technical points.

Contextual Notes

Participants mention specific operational regions and the effects of supply rails on amplification, indicating that assumptions about signal sizes and transistor characteristics are critical to the discussion. There are unresolved questions about the applicability of small signal analysis to larger signals.

Rainbow
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Greetings

Can somebody please help me understand the use of transistor as a switch and that as an amplifier?
 
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"Understand" is a pretty vague term. Can you ask a more specific question?

- Warren
 
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
 
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.
 
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
 
Last edited by a moderator:
Thank you. I'll go through the links that you provided and let you know if I have any questions.
 
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?
 
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.
 
  • #10
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.
 
  • #11
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.
 
  • #12
dionysian said:
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.
 
  • #13
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.
 
  • #14
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...
 
  • #15
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.
 
  • #16
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
 

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