Significance of current gain in transistors

[shauns87]

"For low power transistors (under 1 W), the current gain is typically 100 to 300. High power transistors (over 1 W) usually have current gain of 20 to 100." — Electronic principles by Malvino

I want to understand why the current gain is high for low power transistors and why it is low for high power transistors.

Besides, I want to know that why we are trying to control the collector current by the base current. What do we get out of controlling these currents.

 

[CEL]

In power transistors the base width is larger than in signal transistors, hence higher recombination and lower current gain.

 

[shauns87]

But, why higher recombination required for higher bandwidth?

 

[vk6kro]

Besides, I want to know that why we are trying to control the collector current by the base current. What do we get out of controlling these currents.

Many useful devices we use produce quite small voltages and currents.

A microphone may produce only 10 mV AC output which would be inaudible if it was connected directly to a speaker.
Using transistors, we can get this small current to vary a base current which controls a much bigger collector current.
This can then be used to control the base current of another transisitor and so on until the signal becomes large enough.

Without going much deeper into amplifier design, you can probably see that this process could produce a very loud signal from a very small one.

There are other sensors that produce very small outputs which need to be amplified before they are of any use.
There are thermisters for monitoring temperatures, photodiodes and light dependent resistors for measuring light and even ECG sensors that doctors attach to your body to measure the extremely small voltages produced when your heart beats.

All of these need amplifying and using them to control the base current of transistors is one way of doing this.

 

[shauns87]

Wow, that was great! Absolutely, I got it now.

 

[CEL]

But, why higher recombination required for higher bandwidth?

It is the inverse. Low power transistors have less recombination and higher bandwith.
A higher recombination is a consequence of the larger base junctions width. The larger width is necessary for higher heat dissipation.

 

[shauns87]

Voltage source with resistor in series = current source with the same resistor in parallel?
Could you please help me understand the explanation behind connecting the resistor in parallel and not in series for transforming the voltage source into the current source?