How can a transistor amplify the current ??

[tefa921]

Plz, help me guys because i'm really confused .. now i know the circuit very well .. but still i can't understand how the npn transistor amplify the current where Ie = Ib + Ic .. what i mean is that Ic or the output is less than Ie or the input .. so this is not aplification at all .. So please please help me

 

[MikeMl]

Think about the ratio Ic/Ib (dont worry about Ie). For example, if Ic=100mA and Ib=1mA, what is the current gain?

 

[tefa921]

what is the current gain?

I think it will equal 100 .. but this is between 2 results .. we didn't amplify any thing yet .. I think i dont get the meaning of amplification right well .. so what is your definition of AMPLIFICATION ??

 

[MikeMl]

I think it will equal 100 .. but this is between 2 results .. we didn't amplify any thing yet .. I think i dont get the meaning of amplification right well .. so what is your definition of AMPLIFICATION ??

From Wikipedia:

Amplification may refer to:

The operation of an amplifier, a natural or artificial device intended to make a signal stronger.

I think of a Bipolar transistor (NPN or PNP) as amplifying current. For example, in using an NPN to turn on a lamp when a microcontroller pin is high (logic One), we use a resistor in the base of the NPN to limit the current drawn from the pin, but that relatively small current (a few mA) can control a much larger current (a few tens to hundreds of mA) in the collector of the NPN.

 

[tefa921]

Still cant get it

 

[panic mode]

if you have input and output, gain or amplification is simply ratio: A=output/input.
type of gain depends on type of input and output, commonly it is voltage or current.

to measure gain you need pair of input terminals and pair of output terminals (total of 4 terminals).
in case of transistor (such as npn bjt) there are only three terminals so we have a problem.
problem is solved by making one of terminals common (shared by input and output).
there are three possible combinations:
CE or common emitter (here input is at base and output at collector)
CB or common base (here input is at emitter and output at collector)
CC or common collector (here input is at base and output at emitter)

each configuration has different characteristics (voltage/current gain, input and output impedance etc.)
what tefa is mentioning is CB and gain known as α (alpha).
what Mike replies is CE and gain known as β (beta).
both are talking about current gains - although different ones.
it is easily shown that
α=β/(β+1)

this means that α is always less than one.
for example if β=100, we get α=100/101=0.9901

 

[panic mode]

to get current gain greater than 1, one MUST use CE or CC configuration.

 

[panic mode]

to get voltage gain greater than 1, one MUST use CE or CB configuration.

 

[tefa921]

to get voltage gain greater than 1, one MUST use CE or CB configuration.

Finally got it .. but i have one more question .. Does the quote means that when i use CE the current gain is zero or low ??

 

[ronsimpson]

we didn't amplify any thing yet .. I think i dont get the meaning of amplification right well .. so what is your definition of AMPLIFICATION ??

Amplification: If I have one cup of water and I amplify by 100 then I have 100 cups of water. [more water] (not this kind of amplification)

Amplification: I have a valve that turns on/off the flow of water. The valve has a handle. When I place a cup of water on the valve handle then the valve opens up and 100 cups of water flows. When I remove the cup of water from the handle the valve closes and no water flows. (this kind of amplification) [no water is made, water is controlled]
------------------------edited----------------------------
If there is no water in the pipe the valve will not make water to flow when there is no water. Open or closed water can not flow if there is no water.

 

[davenn]

I think it will equal 100 .. but this is between 2 results .. we didn't amplify any thing yet .. I think i dont get the meaning of amplification right well .. so what is your definition of AMPLIFICATION ??

the real confusion comes when you dont realise that the transistor doesnt actually amplify anything !!

a NPN transistor is like a valve ( water/or gas), the base of the transistor is the valve control
by varying the size of the small current to flow between the base and emitter, it controls a larger
current that can flow between the collector and emitter ....
so for example on a power transistor ... you may have 1mA flowing between the base and emitter and this allow 1 Amp (1000mA) to flow between the collector and emitter
increase the base current to 10mA and you will allow 10 Amps to flow between the collector and emitter

The appearance of amplification is given, BUT there is NO actual amplification as such. Just a varying small current flow controlling a larger current flow

hope that helps

Dave

 

[panic mode]

Finally got it .. but i have one more question .. Does the quote means that when i use CE the current gain is zero or low ??

no, see post #7

CE: high current gain, high voltage gain, low input impedance etc.
CB: no current gain (~1), high voltage gain, low input impedance etc.
CC: high current gain, no voltage gain (~1), high input impedance etc.

 

[tefa921]

If there is no water in the pipe the valve will not make water to flow when there is no water. Open or closed water can not flow if there is no water.

that help a lot

 

[tefa921]

the real confusion comes when you dont realise that the transistor doesnt actually amplify anything !!

a NPN transistor is like a valve ( water/or gas), the base of the transistor is the valve control
by varying the size of the small current to flow between the base and emitter, it controls a larger
current that can flow between the collector and emitter ....
so for example on a power transistor ... you may have 1mA flowing between the base and emitter and this allow 1 Amp (1000mA) to flow between the collector and emitter
increase the base current to 10mA and you will allow 10 Amps to flow between the collector and emitter

The appearance of amplification is given, BUT there is NO actual amplification as such. Just a varying small current flow controlling a larger current flow

hope that helps

Dave

A great explanation .. but i've got a tiny question .. does this amplification happens as a result of Accumulation of current ??

 

[tefa921]

no, see post #7

CE: high current gain, high voltage gain, low input impedance etc.
CB: no current gain (~1), high voltage gain, low input impedance etc.
CC: high current gain, no voltage gain (~1), high input impedance etc.

Yep i meant CB
Thaaaaaaaaaaaaaaaaaaaaaaaaanks

 

[ronsimpson]

A great explanation .. but i've got a tiny question .. does this amplification happens as a result of Accumulation of current ??

Accumulation is adding together.

If a transistor worked by accumulation then:
.....1mA in the base for 1 second will cause 100mA flow in the collector.
.....1mA in the base for 2 second will cause 200mA flow in the collector.
.....1mA in the base for 3 second will cause 300mA flow in the collector.
Not like this!


.....1mA in the base for X second will cause 100mA flow in the collector for X seconds.
Like this, with no accumulation

 

[tefa921]

Accumulation is adding together.

If a transistor worked by accumulation then:
.....1mA in the base for 1 second will cause 100mA flow in the collector.
.....1mA in the base for 2 second will cause 200mA flow in the collector.
.....1mA in the base for 3 second will cause 300mA flow in the collector.
Not like this!


.....1mA in the base for X second will cause 100mA flow in the collector for X seconds.
Like this, with no accumulation

So what cause this high ratio ??

 

[davenn]

So what cause this high ratio ??

what hi ratio ?

D

 

[tefa921]

what hi ratio ?

D

I mean where this huge number originated form .. What factor changed 10mA to10 Amps ??

 

[davenn]

there's no changing happening

you have a small current supply on the base and a large current supply on the collector
the small current supply on the base is controlling how much of the large current supply is allowed to flow through the collector to emitter

As much as I hate using water analogies for current flow

EDIT .... Imagine that the + supply is capable of a max of 10 Amps and the design of the transistor is such
that by varying the current flowing through base to emitter can vary the collector - emitter current from 0 to 10 Amps
now we cannot get 10 Amps flowing through the base because the value of the trimpot we chose and the other resistor above it limits the current


does that help ?


Dave