transistors

[f4cepl4nt]

I have a few questions about transistors, so here we go:

How can you tell how much current must flow between base-emitter to allow current to flow collector-emitter??

How can you tell which connector on a transistor is the emitter?

When a transistor gets a small current running through the base-emitter, which then allows current to flow collector-emitter, does the current through the base-emitter get cut off? Or does it keep flowing in tiny amounts?

One last one: If a transistor has a current flowing base-emitter which then allows current to flow collector-emitter, if the base-emitter current is turned off (by a switch or something) does the collector-emitter current turn off too? Or does it just need a kick start to get going then nothing can stop it except a switch or broken circuit?


I'm very sorry if my questions are unclear, I'm not very good at describing things  ;D

Again, thanks for all the help, you guys are life savers (no not the candy  8) )

 

[prateeksikka]

using a multimeter but with some jiggling u can come to know about the terminals base,emitter ,collector.Emitter conducts most heavily.(Q2)

It is the base-emitter voltage which drives the transistor into saturation and makes the heavy current to flow from collector to emitter.(Q3)

It is the beta and internal mechanism of conduction in a transistor the answer to the
first question.

 

[audioguru2]

How can you tell how much current must flow between base-emitter to allow current to flow collector-emitter??

A transistor has a range of DC current gain. It is different between two transistors with the same part number and is different at different collector currents. The transistor's datasheet explains it all.

How can you tell which connector on a transistor is the emitter?

By looking at the labelled picure of it on its datasheet.

When a transistor gets a small current running through the base-emitter, which then allows current to flow collector-emitter, does the current through the base-emitter get cut off? Or does it keep flowing in tiny amounts?

Only the input circuit to your transistor can cutoff the base-emitter current.

If a transistor has a current flowing base-emitter which then allows current to flow collector-emitter, if the base-emitter current is turned off (by a switch or something) does the collector-emitter current turn off too? Or does it just need a kick start to get going then nothing can stop it except a switch or broken circuit?

If you disconnect the base current, collector to base leakage current might cause the transistor to conduct. Usually the base of a transistor is shorted to its emitter with a resistor or another conducting transistor to make certain it is turned off.
The collector current follows changes in the base current, but is amplified. ;D

 

[f4cepl4nt]

whooof talk about quick reply, thanks guys

 

[prateeksikka]

hi audioguru!
i feel that it is the value of beta which is responsible for determining the gain of the transistor.
It is its high value which makes it a good amplifier.
;D

 

[audioguru2]

Hi Prateek,
I don't think a high beta (current gain) makes a transistor amplifier have a high voltage gain. A darlington transistor has an extremely high beta, and its gain is about the same as an ordinary transistor. A high beta transistor in a common-emitter stage has a higher input impedance.

A good amplifying transistor has a linear or flat beta, and a very low Re. The voltage gain is the collector resistor's value (and load in parallel) divided by the total of its Re plus emitter resistor's value. If you buffer its output so it isn't loaded down, and use bootsrapping or a constant current source as its collector resistor, its gain will be very high and linear. Here is a good explanation with measured results: http://www.dself.dsl.pipex.com/ampins/discrete/twoq.htm

 

[prateeksikka]

hi audioguru!
here we reach a conflict ,for a good amplifier we need a low Re (emitter res) as u said and at the same time low Re causes the transistor to limit the i/p signal below a certain value. higher value of Re enables us to feed a gignal more than say 1 V.
;D

 

[audioguru2]

Hi Prateek,
No conflict. With a very high gain, an amplifier can use more negative feedback to reduce the distortion and allow increased input.
Negative feedback can be from an emitter resistor or from feedback directly to the input. ;D

 

[Kevin Weddle]

I think that high beta transistors are considered high gain because of the current multiplication. Generally, to get gain, you look to the collector and emitter resistors. It is always safer to play the voltage game. What you end up doing in the current gain scenario is to lower the change in base current rather than affect the change in collector current and thus the voltage. So with a constant change in input voltage, you have not gained anything with a high beta transistor.

Now let's consider the current game for a moment. If we had a current source as the signal, you would apply that to the base of a high beta transistor only to realize that yes you have the change in collector current, but oh no, you also have a greater input voltage, so your gain is not as high as you thought.

 

[audioguru2]

Hi Kevin,
Have you seen the distortion of a transistor with a pure voltage input on a 'scope?
It's really awful. Sounds worse than it looks. You must use negative feedback to improve it or reduce the input level.

View attachment 37317

 

[Kevin Weddle]

Hi Audioguru. I think that feedback, zero time difference, is only there as a necessity. It let's current flow around the opamp. But of course why do you need that. It just so happens that when you put a resistor from the output to the  inverting input, your gain looks like it mimicks the resistors. I have done several things with opamps. I hook them up seemingly backwards with fairly reasonable results. But what negative feedback appears to be is just an adjustment to the signal at the input of the amplifier. And it's there because you don't want too high a gain. It is my opinion that the only reason they use opamps, which are simple circuits, is because they need high gain. How high? Well, let the designers be the judge of that.

 

[audioguru2]

I have done several things with opamps. I hook them up seemingly backwards

Why?

what negative feedback appears to be is just an adjustment to the signal at the input of the amplifier. And it's there because you don't want too high a gain. It is my opinion that the only reason they use opamps, which are simple circuits, is because they need high gain.

An opamp circuit doesn't have to have high gain. Its negative feedback reduces its extremely high open-loop gain with two resistors to however much gain you want. The extra gain makes the opamp an error amplifier so that any non-linearity in its output is reduced to very, very close to zero.

 

[prateeksikka]

hi kevin!
we use negative feedback through an emitter resistor just to stabilize the gain and make the amplifier much more linear.
and to increase the range of i/p signal. ;D
hi audioguru!
do we have any disadvantage of a transistor with 2000 value of beta as amplifier or is it an excellent amplifier?what do u think? :

 

[Alun]

The higher the gain the better, I've never seen a single transistor with such a high gain - the highest - I've seen is 800,

 

[audioguru2]

Transistors with a beta as high as 900 is the highest I've seen and they work about the same as transistors with lower beta, except the input bias current is very low and the input impedance is high. BC549C for example.
Some opamps use "super-beta" transistors on their inputs for a very low input bias current and they have a very low breakdown voltage, requiring the use of clamp diodes between the inputs.

 

[prateeksikka]

Hi audioguru!
do we require a high or low beta for a high gain amplifier?
How do we chose beta for a given application?
;D

 

[audioguru2]

Hi Prateek,
I don't think beta affects a transistor's voltage gain. It affects a transistor's input impedance and input bias current.
A common-emitter transistor's voltage gain is simply Rc/Re, where Rc is its collector resistor in parallel with its load. The voltage gain is higher at higher current (if the collector resistor remains the same then the supply voltage must be increased) because Re is decreased. ;D

 

[Kevin Weddle]

Beta is determined by the current capability of the transistor. A high current transistor has a lower beta. If you want to know the truth, I like low beta because a beta like 50 is more likely to remain near 50 while a high beta of 500 will not hesitate to go to 450 or 550. Am I following any logic here?

 

[audioguru2]

It doesn't matter if a transistor has a high current rating and a low beta, or a low current rating and a high beta. Beta changes with temperature and amount of current.

The beta of a 200mA 2N3904 drops to about 70% of its highest value at half of its current rating.
The beta of a 15A 2N3055 drops to about 14% of its highest value at half of its current rating.
You think the high current, low beta transistor keeps its beta more constant?
I don't think so.

View attachment 37339