Significance of current gain in transistors

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Discussion Overview

The discussion centers on the significance of current gain in transistors, particularly focusing on the differences between low power and high power transistors. Participants explore the reasons behind varying current gains, the implications of controlling collector current through base current, and the practical applications of these principles in amplifying small signals.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants note that low power transistors typically have a current gain of 100 to 300, while high power transistors have a gain of 20 to 100, prompting questions about the underlying reasons for this difference.
  • One participant suggests that the larger base width in power transistors leads to higher recombination rates, which in turn results in lower current gain.
  • Another participant questions the necessity of higher recombination for achieving higher bandwidth, indicating a potential misunderstanding or disagreement on this point.
  • A detailed explanation is provided regarding the amplification of small signals, such as those from microphones and sensors, emphasizing the role of transistors in controlling larger collector currents through smaller base currents.
  • A later reply clarifies that low power transistors experience less recombination and higher bandwidth, while higher recombination is associated with larger base junction widths needed for heat dissipation.
  • A separate question is raised about the relationship between voltage sources and current sources, specifically regarding the configuration of resistors in series versus parallel.

Areas of Agreement / Disagreement

Participants express varying views on the relationship between recombination, bandwidth, and current gain in transistors. There is no consensus on the necessity of higher recombination for higher bandwidth, indicating an unresolved aspect of the discussion.

Contextual Notes

Some claims depend on specific definitions of terms like "recombination" and "bandwidth," which may not be universally agreed upon. The discussion also touches on practical applications of transistors in amplifying small signals, but the technical details remain complex and not fully resolved.

Who May Find This Useful

This discussion may be of interest to students and professionals in electronics, electrical engineering, and related fields, particularly those focused on transistor applications and signal amplification.

shauns87
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"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.
 
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In power transistors the base width is larger than in signal transistors, hence higher recombination and lower current gain.
 
But, why higher recombination required for higher bandwidth?
 
shauns87 said:
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.
 
Wow, that was great! Absolutely, I got it now.
 
shauns87 said:
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.
 
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?
 

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