Understanding Transistors: How Base Current Controls Collector Current

[jd12345]

I'm trying to understand working of transistor( common emmiter ) and there is one line that i don't understand and i ahve found that line in many books :- " A small change in base current controls the larger collector current" . I don't understand how the base current controls the collector current.
Can someone explain. Also if someone has a link explaining transistors it would be helpful

 

[cabraham]

We typically control the collector current by controlling the emitter current value. Then Ic = alpha*Ie. If we control base current Ib, we get a collector current given by Ic = beta*Ib. This type of method is called "beta-dependent", and is usually avoided. Alpha is very consistent and predictable over speciman, current value, and temperature, having values from 0.98 to 0.998. Hence if we take the value of alpha at 0.99, we are at most off by 1%.'

Beta, OTOH, can be as low as 50 & as high as 500. It varies with temperature, current value, and speciman. The most common application where base current is the contrrolled input is saturated switch. By overdriving the base, we force the collector to saturate. If minimum beta is 50 worst case, but we force base current to be 1/10th of the collector current, the device saturates reliably.

Devices with beta of 50, 100, 200, & 400, all saturate with near identical performance. For most applications, we design a network so that Ie is a specific value, then we know that Ic = alpha*Ie. That is how we control the bjt.

I would recommend university textbooks on electronics and peer-reviewed publications. Under no circumstances do I advise "web surfing". The web is filled with amateur electronics hobbyists who seem to have an insatiable urge to lecture on electronics as if they were a professor. OEM web sites like On Semi, Fairchild, etc. are also a good source of info. Avoid self-proclaimed "experts". Anybody who claims that the uiversities and OEMs got it wrong should be ignored.

I hope I've helped.

Claude

 

[jd12345]

ok thanx

 

[dlgoff]

... Also if someone has a link explaining transistors it would be helpful

... The web is filled with amateur electronics hobbyists who seem to have an insatiable urge to lecture on electronics as if they were a professor.

I concur about the web having lots of trash, but this site is good.

Transistors
Transistor Operation

With interactive images such as this:

 

[alphysicist]

Transistors are a fundamental component in modern electronics and understanding their operation is crucial for any scientist. In the common emitter configuration, the base current controls the larger collector current through a process called amplification.

To understand this, we need to first understand the structure of a transistor. A transistor is made up of three layers of semiconductor material - the emitter, base, and collector. The base layer is very thin, and it acts as a control element for the flow of current between the emitter and collector.

When a small current is applied to the base, it creates an electric field that allows a larger current to flow from the emitter to the collector. This is known as the amplification process. The amount of amplification is determined by the properties of the transistor, such as its size and material composition.

In other words, the base current acts as a switch that controls the flow of a larger current between the emitter and collector. This is why it is often said that a small change in base current can lead to a larger change in collector current.

To better understand the mechanics of transistors, I recommend checking out some online resources that explain the concept in more detail. Some good starting points could be websites such as Electronics Tutorials or SparkFun, which offer step-by-step explanations and visual aids to help you understand the inner workings of transistors. Additionally, you can also refer to textbooks or attend workshops or seminars on transistors to gain a deeper understanding of their operation.