how does a transistor amplify?

i wanna know how does the transistor convert the DC power of the amplifier circuit to amplify the ac signal ? in case of BJT for example ?

Re: how does the transistor amplify ?

In BJT transistors is not easy to realize physically what happens inside it when there is amplification (FETs are easier to understand), so I will not try to explain the physics about BJTs (you can search in the web).

In some words, the amplification process is the following:
BJTs are current controlled devices, the base current controls the collector current flow. The base current is very small and a variation in this current will reflect in a large and proportional variation in the collector current (more base current allows more collector current). The collector current is originated by the DC power supply and when the collector current is varied, the voltage between the collector and emitter pins is also changed. Of course in order to do this, you should attach some components to the circuit beside the transistor, this is called biasing the transistor and allows it to amplify a signal that is put into the base pin

how does the transistor amplify ?

well, I will try to explain with the example.
suppose you have a water tank filled and a big valve which lets the water out at a high rate. you have another small valve which is very easy to control and you can vary the amount of water in small valve. but interestingly when you vary the small valve (base current) the large valve (collector current) just follows the same variation pattern as adjusted by the user on small valve.
so if you open and close the small valve in a sinusoidal manner, then the large valve will also follow the sinusoidal variation and thus the water will flow out sinosoidally through the large valve. now although the tank is filled with water and acts like a DC battery, but the the water flowing out of it is sinosoidal. so that's how BJT converts the DC power to AC power.. small power applied by you on the small valve is converted to higher power by the big valve using the water stored in the tank.. thats amplification
i hope you understand it....

how does the transistor amplify ?

thanks for u all .... actually i understand all that ..... i was asking for the physical explanatio for it coz i read it many times in many books but couldnt get it ..... but it seems its not only me ....

how does the transistor amplify ?

Actually what the transistors(BJT, etc, not CMOS) amplified is the current changed in the base, a little change in base current result in a large change in collector current, Am I right?

Re: how does the transistor amplify ?

ahmed_nasr said:

i wanna know how does the transistor converts the DC power of the amplifier circuit to amplify the ac signal ? in case of BJT for example ?

Click to expand...

In reality, there is no amplification. Rather, it's a copy, and, like a photocopier, you don't always get a great copy. Just because I tell the copier to give me copyB=copyA*scaleS it does not mean I'm going to get copyB. Rather, I'll always get copyC, which hopefully is very close to copyB, the ideal. For example, what if the original document is sitting perfectly flat on the glass but the scanning mechanism is skewed even slightly? This is how it is with "amplification." Red electrons are at the input, but blue electrons are the "amplified" signal, not bright red. A truly amplified signal would still be the original signal but larger.

Hence, a transistor does not amplify a signal; rather, it only allows you to try to copy it by a scale factor that you'd like to be linear but never is. With any nonlinearity in the scale factor you do not have an accurate copy. You can only try for the best, assuming that's what you want from your "amplifier."

The signal (AC) never gets bigger; it's just duplicated via another source of energy (DC).

Therefore, to your question: "how does a transistor amplify?" My answer is it doesn't. Rather, the circuit around the transistor hopes to use the transistor to produce the copy of the signal that is desired. We only say "amplify" because it's easier to think of the copy as the orignal signal after multiplcation by a factor.

For a true amplifier, perhaps a resonant cavity may count, as the origninal signal (red) adds to itself (brighter red). But, it's still not a useful amp, because resonance, by definition, is a peaked response = non-linear.

Re: how does the transistor amplify ?

i suggest u go to www.malvino.com for the animation of such topics where u can see actually what happens in the circuit when transistor amplifies and many more topics in electronics circuit

Re: how does the transistor amplify ?

with the help of DC supply,it amplify the small ac signal to large signal.because transistor has low i/p resistance(F.B) and large o/p impedance(R.B).

how does the transistor amplify ?

transisitors are infact transfere resistors. You could change the resistance by the base (or gate ) signals. if you change the resistance of a resistor coonected to a dc voltage supply with the help of gate signal ; an exact replica of the gate signal with higher amplitute will be produced.

Re: how does the transistor amplify ?

Hi friends,

I want to understand in depth knowledge abt transisitor as we keep on doing lots of stuff with this device.

i too feel we need to dig deeper because the explanations were like patches,here and there lot of things were already given the standard books but yet i could not understand why it amplifies.

everyone here understoods that base control the collector current,also we read in the books that 'amplification' (so called increase in size of ac signal applied at base,where as dc supply is only for the transistor remain with life other wise it goes to dead state) takes place due to transfer of resistance from low resistance circuit to high resistance circuit.i.e(low resistance due to F.B and high resistance due to R.B)

Here is one explanation i got from yahoo i really don't understand as that of explanation with copier given above. "A transistor does not amplify
What it does is it controls the amount of power that is delivered to a load by means of the input signal .The whole kit is called an amplifier but it really is the energy from the power supply that is conveyed to the load.
If your question is “can an AC voltage be augmented) then a transformer would do that.There would be a reduction in current; therefore the output power would be the same as the input power."

My real doubt is if signal is flowing through low to high resistance then it should get attenuated at high resistance at output we should not get any output at all.but surprisingly we get signal with increase in amplitude .

also we never spoke abt the Q point, we knew that transistor operates as 'amplifier' only in active region whats the resistance factor to do with Q point of the transistor.

please go though all the threads and pls some one links all the patches and enlighten us.

thanks for your patience...

Re: how does the transistor amplify ?

according to practical base ,
when comes the input signal (having to amplify) at the base of then the transistor passes the dc signal (connected through emittor and collector ) may vary (reversly ,means 180 degree) in accordance with the input signal

Re: how does the transistor amplify ?

i agree with all of you...
but my answer is...
if you look at bjts construction the emitter is highly
doped to shot charge carriers and collector
doping level is less than emitter but its size is bigger
than emitter so to have large power handling
capacity when an input signal (time varying) is applied
to base emitter-to-junction charge carriers
start tarnsfering energy of the input signal there they have
to ways to transfer energy to base
charges or to transfer energy to collector charges but as
you all know base resistance is quite
high so they will choose to strike with collector charges
at the same time you know that base-to
-collector junction is biased reversely with strong direct current
voltage which withdraws the incoming
energy very forcely it means that energy supplied by biasing
is used which reinforce the input energy due to which
its amplitude increases.
and this is all about how the input signal is amplified.

---------- Post added at 18:40 ---------- Previous post was at 18:40 ----------

i agree with all of you...
but my answer is...
if you look at bjts construction the emitter is highly
doped to shot charge carriers and collector
doping level is less than emitter but its size is bigger
than emitter so to have large power handling
capacity when an input signal (time varying) is applied
to base emitter-to-junction charge carriers
start tarnsfering energy of the input signal there they have
to ways to transfer energy to base
charges or to transfer energy to collector charges but as
you all know base resistance is quite
high so they will choose to strike with collector charges
at the same time you know that base-to
-collector junction is biased reversely with strong direct current
voltage which withdraws the incoming
energy very forcely it means that energy supplied by biasing
is used which reinforce the input energy due to which
its amplitude increases.
and this is all about how the input signal is amplified.

Re: how does the transistor amplify ?

ahmed_nasr,

i wanna know how does the transistor converts the DC power of the amplifier circuit to amplify the ac signal ? in case of BJT for example ?

Click to expand...

Then read this thread below, especially posts #43 and #47. Come back if you have any questions about it.

https://www.electro-tech-online.com/general-electronics-chat/126200-understanding-transistor-5.html

Ratch

---------- Post added at 10:52 ---------- Previous post was at 10:33 ----------

rkodaira,

BJTs are current controlled devices, the base current controls the collector current flow.

Click to expand...

Vbe controls both the collector current and the base current. That means that the collector current and base current have a relationship with each other called "beta". That does not mean that the base current is controlling the collector current. It is the Vbe that is controlling everything, so a BJT is a voltage controlled device in a causal manner, and functionally mimics a current controlled device.

Ratch

---------- Post added at 11:01 ---------- Previous post was at 10:52 ----------

wahabmalik,

well, I will try to explain with the example.

.... small power applied by you on the small valve is converted to higher power by the big valve using the water stored in the tank.. thats amplification
i hope you understand it....

Click to expand...

Isn't that a generic explanation that applies to every amplifier including tubes, FETs, magnitic amplifiers, etc? Does that really explain in particular how a BJT works?

Ratch

---------- Post added at 11:06 ---------- Previous post was at 11:01 ----------

BigDogBear,

Actually what the transistors(BJT, etc, not CMOS) amplified is the current changed in the base, a little change in base current result in a large change in collector current, Am I right?

Click to expand...

No, you are not. The current change in the collector is due to the change in Vbe, The collector current tracks the base current, but the base current does not control the collector current.

Ratch

---------- Post added at 11:14 ---------- Previous post was at 11:06 ----------

Euler's Identity,

Hence, a transistor does not amplify a signal; rather, it only allows you to try to copy it by a scale factor that you'd like to be linear but never is.

Click to expand...

That sounds like Jabberwocky if I ever heard it. Copy or not, if something comes out scaled differently than it went in, that's amplification.

From the dictionary:am·pli·fi·ca·tion n. 1. The act or result of amplifying, enlarging, or extending. 2.a. An addition to or expansion of a statement or idea. b. A statement with such an addition. 3. Physics. a. The process of increasing the magnitude of a variable quantity, especially the magnitude of voltage, power, or current, without altering any other quality. b. The result of such a process. 4. Electronics. See gain.

Ratch

---------- Post added at 11:17 ---------- Previous post was at 11:14 ----------

suvendu,

with the help of DC supply,it amplify the small ac signal to large signal.because transistor has low i/p resistance(F.B) and large o/p impedance(R.B). .

Click to expand...

Does that really explain how a BJT works?

Ratch

---------- Post added at 11:21 ---------- Previous post was at 11:17 ----------

farooq afghan,

....
...and this is all about how the input signal is amplified.

Click to expand...

Do you think the OP can understand your explanation?

Ratch

---------- Post added at 11:25 ---------- Previous post was at 11:21 ----------

thannara123,

according to practical base ,
when comes the input signal (having to amplify) at the base of then the transistor passes the dc signal (connected through emittor and collector ) may vary (reversly ,means 180 degree) in accordance with the input signal .

Click to expand...

What does the above explanation mean?

Ratch

---------- Post added at 11:28 ---------- Previous post was at 11:25 ----------

rajesh.rj,

please go though all the threads and pls some one links all the patches and enlighten us.

Click to expand...

First, we have to separate the wheat from the chaff.

Ratch

---------- Post added at 11:32 ---------- Previous post was at 11:28 ----------

mdanaie,

transisitors are infact transfere resistors. You could change the resistance by the base (or gate ) signals. if you change the resistance of a resistor coonected to a dc voltage supply with the help of gate signal ; an exact replica of the gate signal with higher amplitute will be produced.

Click to expand...

How does that differ from a tube or FET? Don't they also change their effective resistance so as to modulate the signal? That still does not explain how a BJT works.

Ratch

---------- Post added at 12:10 ---------- Previous post was at 11:32 ----------

ahmed_nasr,

thanks for u all .... actually i understand all that ..... i was asking for the physical explanatio for it coz i read it many times in many books but couldnt get it ..... but it seems its not only me ....

Click to expand...

Do you really understand all the above?

Assume a NPN transistor. The excess electrons in the N-type material of the emitter always diffuse into the P-type material of the base, which is deficient in electrons. Likewise, the excess holes in the P-type material of the base always diffuse into the N-type material of the emitter, which is difficient in holes. This causes an emitter current to exist just from diffusion. Since the base is extremely thin, the electrons pass through the base and continue on to the collector, which is biased positive to attract them. So the mechanism for charge movement in a BJT is diffusion. That is not the case for a tube or FET. When the electrons diffuse from the emitter into the base, they leave behind a positive ion. And when a hole diffuses from the base into the emitter, it is leaves behind a negative ion. The positive and negative ions cause a barrier voltage to exist across the emitter-base junction which stops the diffusion process. Applying a counter voltage Vbe across the emitter-base junction lowers the barrier voltage and allows the current to continue. Some of the current is siphoned off into the base circuit as waste current, as it does not contribute to the collector current. This base current is proportional to the collector current, but it does not control the collector current. Vbe controls both the base current and the collector current according to the above explained mechanism. A PNP transistor works the same way in reverse. The diffusion mechanism also explains why increasing the collector voltage does not change the collector current all that much unless secondary effects are taken into consideration. The current of a BJT is mostly governed by diffusion of the emitter-base, not by the collector voltage. So a large change in collector voltage with little change in current means a high resistance, which implies a current source. A BJT is a voltage controlled current source.

Ratch

Re: how does the transistor amplify ?

this is not the answer for that question

Re: how does the transistor amplify ?

for physical significance you can take analogy of transistor as a tube of water flowing in it, and and the input signal is like a you pressing the tube and as you vary the pressure on the tube the pressure of water coming out varies, the larger the pressure on tube larger is pressure of water,
power supply is analogous to incoming pressure and input resistance is analogous to the diameter of the tube larger the diameter less is opposition to water.
note in a BJT beam electron is flowing from Emitter to Collector and a pressure(voltage) is applied at base that controls the current in the emitter.
note if you press the tube very hard then a situation comes when no more water is allowed that is analogous to saturation of transistor

Re: how does the transistor amplify ?

pratomer,

or physical significance you can take analogy of transistor as a tube of water flowing in it, and and the input signal is like a you pressing the tube and as you vary the pressure on the tube the pressure of water coming out varies, the larger the pressure on tube larger is pressure of water,
power supply is analogous to incoming pressure and input resistance is analogous to the diameter of the tube larger the diameter less is opposition to water.

Click to expand...

Does the circuit leak? Do you get wet when you do that?

power supply is analogous to incoming pressure and input resistance is analogous to the diameter of the tube larger the diameter less is opposition to water.

Click to expand...

Tell me, do hydraulic engineers use electrical analogs to learn their craft?

note in a BJT beam electron is flowing from Emitter to Collector and a pressure(voltage) is applied at base that controls the current in the emitter.
note if you press the tube very hard then a situation comes when no more water is allowed that is analogous to saturation of transistor

Click to expand...

A BJT is not a CRT. It does not have a beam of current. In a PNP transistor, the electrons go from collector to emitter.

Ratch

Re: how does the transistor amplify ?

I fully support/confirm the explanations from Ratch as given with his post#13.

To all fellows who still believe that the BJT would be a current-controlled device:
If this would be true, we have to face the following contradiction:
1.) We apply a B-E voltage of app. 0.65 volts in order to allow a certain quiescent collector current Ic (with the consequence of a corresponding base current Ib); I think, nobody will deny that this B-E voltage "opens" the transistor.
2.) But the periodical variation of the collector current around this quiescent value suddenly should be caused by variation of the the base current (and not by variation of Vbe) ?

Re: how does the transistor amplify ?

A low impedance voltage input swing to the base of a common-emitter transistor causes SEVERE distortion because the base-emitter voltage change causes logarithmic output voltage and current changes.
If the input to the base is a high impedance current change then the output voltage and current changes are very linear.

Re: how does the transistor amplify ?

I think, the shown characteristics are well-known.
The distortions you call "severe" are kept within limits by using an appropriate bias point - together with signal feedback.

"If the input to the base is a high impedance current change then the output voltage and current changes are very linear."

So - what do you propose? Not to use anymore all the classical voltage-in transistor amplifier stages in C-E configuration?
Should we convert all signal voltages into signal currents before feeding the amplifier?

Re: how does the transistor amplify ?

LvW said:

I think, the shown characteristics are well-known.
The distortions you call "severe" are kept within limits by using an appropriate bias point - together with signal feedback.

"If the input to the base is a high impedance current change then the output voltage and current changes are very linear."

So - what do you propose? Not to use anymore all the classical voltage-in transistor amplifier stages in C-E configuration?
Should we convert all signal voltages into signal currents before feeding the amplifier?

Click to expand...

Almost every signal source is a fairly high resistance current source. The collector of a previous transistor stage has the resistance of its collector resistor.
The output of a signal generator or opamp has a very low resistance voltage source that causes severe distortion in a following common-emitter transistor.

Here is a common-emitter transistor with a sinewave voltage source. The input is a sinewave but the output isn't: