- #1
Transistor 2N4401: DC Current Gain (hFE) for 5V/60mA
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Discussion Overview
The discussion revolves around the DC current gain (hFE) of the 2N4401 transistor, specifically in the context of using it in a circuit with 5V and 60mA. Participants explore the implications of different gain specifications, bias conditions, and alternatives to the 2N4401 transistor.
Discussion Character
- Technical explanation
- Debate/contested
- Experimental/applied
Main Points Raised
- One participant notes that the datasheet for the 2N4401 provides multiple DC current gains (hFE) and questions which value is most useful for their application.
- Another participant mentions that while the datasheet shows minimum and maximum hFE values, one can expect at least around 100, but emphasizes the need to measure the actual transistor for precise values.
- A participant requests clarification on the specific bias conditions (Vbe, Vce, and Ib) for the 5V and 60mA scenario, highlighting that current gain can vary significantly between individual transistors.
- There is a clarification that hFE is not the same as the DC current gain "B," with hFE being a small-signal parameter derived from a specific model of the BJT.
- Another participant explains the distinction between hFE (DC current gain) and hfe (small-signal current gain), suggesting that a better datasheet with graphs could aid in understanding the gain values.
- One participant shifts the focus away from the 2N4401, proposing a simple circuit example with a 30mW lamp and asking for recommendations on suitable transistors and how to determine their specifications.
- There is a reiteration of the distinction between hFE and hfe, with a participant noting that in Germany, both terms are often used interchangeably.
- Another participant advises that for switching applications, the Vce should be minimized by running the transistor in saturation, and mentions that specifications are typically quoted at a forced beta of 10.
- It is suggested that if low base current is desired, a PMOS power FET could be a better alternative due to its characteristics.
Areas of Agreement / Disagreement
Participants express differing views on the interpretation of hFE and hfe, as well as the best practices for using the 2N4401 transistor. There is no consensus on a single approach or solution, and multiple competing perspectives are presented throughout the discussion.
Contextual Notes
Participants highlight the variability of current gain across different transistors and the importance of specific bias conditions, indicating that assumptions and definitions may influence the discussion.
- #2
BvU
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Table only shows minimum hFE and one maximum. there may be some non-linearity at low current but you can expect at least around 100. If you need the precise value you'll have to measure it on your actual transistor.
- #3
phyzguy
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You need to be more specific. 5V and 60mA where? What are the bias conditions, specifically Vbe, Vce, and Ib? Even after you have these, you need to realize that current gain varies a lot from transistor to transistor. Bipolar transistor amplifier circuits are usually designed so that the gain is determined by the passive components (usually resistors) and not the transistor itself.
- #4
LvW
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Just for your understanding: The parameter hfe is NOT identical to the DC current gain "B".
The parameter hfe is a small-signal parameter resulting from the small-signal 4-pole model of the BJT.
The parameter hfe is a small-signal parameter resulting from the small-signal 4-pole model of the BJT.
- #5
meBigGuy
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This sheet has two different gain specifications
hFE (DC current gain) which is beta and has many specified conditions.
hfe (Small-Signal Current gain) at one specified condition, which the the small signal gain.
You can probably find a better 4401 data sheet that has some graphs you can interpolate.
In order to help we need a little more information about your circuit.
If you don't like the gain values, look at the 2N2222
hFE (DC current gain) which is beta and has many specified conditions.
hfe (Small-Signal Current gain) at one specified condition, which the the small signal gain.
You can probably find a better 4401 data sheet that has some graphs you can interpolate.
In order to help we need a little more information about your circuit.
If you don't like the gain values, look at the 2N2222
- #6
- #7
LvW
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Yes - agreed. Here in germany, we do not discriminate between hFE and hfe.meBigGuy said:
We are using the small-signal values hfe (or h21) and the DC current gain B.
- #8
meBigGuy
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In a circuit like that you want Vce to be as low as possible. That means you should run in saturation. Notice that in the high hFE specs there is significant Vce.
In the spec for Vce(sat), notice two things. Vce is very low, and Beta is assumed to be 10. Whenever you want to use a transistor as a switch, you assume a Beta of 10. That is, you overdrive the base to get Vce as low as possible. That reduces dissipation in the transistor, and maximizes the voltage across the load.
All transistor specifications quote Vcs(sat) at a forced beta of 10.
If you need low base current, you should change to a PMOS power fet. It will have 0 gate current (except when switching) and lower Vds(on).
In the spec for Vce(sat), notice two things. Vce is very low, and Beta is assumed to be 10. Whenever you want to use a transistor as a switch, you assume a Beta of 10. That is, you overdrive the base to get Vce as low as possible. That reduces dissipation in the transistor, and maximizes the voltage across the load.
All transistor specifications quote Vcs(sat) at a forced beta of 10.
If you need low base current, you should change to a PMOS power fet. It will have 0 gate current (except when switching) and lower Vds(on).
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