NPN transistor with lowest Vbe?

In summary, a transistor with a lower Vbe will allow for a more efficient switch operation. However, a standard transistor will not work because it is not low enough voltage. In order to use a transistor, you may need to find one with a Vbe of around 0.2 volts.
  • #1
fizz_it
29
0
Hi - I am looking for an NPN transistor with the lowest Vbe I can get.

Any suggestions for specific transistors? (rather than a material class i.e. GaAs)

I only have 3.1 volts on the collector and the emitter voltage has to be above 2.5 volts to get a circuit to trip. So 3.1v - 0.7v = 2.4 (too low) So a standard BJT won't work. I don't have much PCB space so I can't add a boost circuit of any sort

This will operate only as a switch. And it is a low power application.


Help! Thanks
 
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  • #2
A germanium transistor has Vbe of about 0.2 volts. I'm not sure if they are used anymore.

you could also just put an op-amp with variable gain to amplify voltage to trip the transistor.
 
  • #3
  • #4
skeptic2 - Thanks a 1x10^6

I am ordering the BFU725F from NXP
 
  • #5
fizz_it said:
So 3.1v - 0.7v = 2.4 (too low) So a standard BJT won't work. I don't have much PCB space so I can't add a boost circuit of any sort

This will operate only as a switch. And it is a low power application.
Any suggestions for specific transistors?

I see you already ordered the transistor but another way to go was with a PNP. So the transistor would be normally on then shut off in the 2.5V situation.

Of course this would invert the logic of the switch. If that was unacceptable an inverter could bring it back.
 
  • #6
fizz_it said:
Hi - I am looking for an NPN transistor with the lowest Vbe I can get.

chances are, you need to rethink your design.
 
  • #7
Phrak said:
chances are, you need to rethink your design.

I'd suggest going with a basic BJT inverter:
http://courseware.ee.calpoly.edu/~dbraun/courses/ee307/F03/13/02_13_PhilippeGonzaga.html

With the output current known, choose the base and collector resistances such that the BJT operates in saturation or cut-off, and you'll have almost the full operating voltage as the 'high' logic level, and only the BJT saturation voltage (usually 0.2V or lower) as the 'low' logic level.

If you can't have an inverted signal, you can cascade two of these stages together (though you'll have to run through the design calculations again for the output stage). This is something you wouldn't be able to do with simple base followers, as you're losing that 0.7V every time!

If you were attempting to actually power something (instead of just sending an output to another piece of logic) you can often use a FET as the output stage (and usually achieve a lower "0")
 
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1. What is the purpose of a NPN transistor with lowest Vbe?

The purpose of a NPN transistor with lowest Vbe is to act as an amplifier or switch in electronic circuits. The low Vbe (base-emitter voltage) makes it more efficient and allows for better control of the transistor's behavior.

2. How does the Vbe affect the performance of a NPN transistor?

The Vbe affects the performance of a NPN transistor by controlling the amount of current flowing through the base-emitter junction. A lower Vbe means less current is needed to turn on the transistor, making it more efficient and precise in its operation.

3. What factors influence the Vbe of a NPN transistor?

The Vbe of a NPN transistor is influenced by factors such as temperature, current, and the doping levels of the semiconductor materials used. Higher temperatures and currents can increase the Vbe, while different doping levels can alter the Vbe and other properties of the transistor.

4. How can I choose a NPN transistor with the lowest Vbe for my circuit?

The Vbe value for a NPN transistor can be found in its datasheet. Look for transistors with a low Vbe value, typically around 0.7V, for maximum efficiency. You can also compare different models and choose the one with the lowest Vbe for your specific circuit needs.

5. Are there any disadvantages to using a NPN transistor with lowest Vbe?

One potential disadvantage of using a NPN transistor with lowest Vbe is that it may have a lower gain (amplification factor) compared to other transistors. This could lead to decreased performance in certain circuits. Additionally, these transistors may be more expensive or harder to find compared to other models.

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