Fast switching PNP transistor request

They don't mention it because you don't need to know - you're looking at totally meaningless figures, ANY transistor will do.

Standard small signal transistors (like the BC107) are rated at 300MHz, and that's an audio transistor.
 
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Man, I'm trying to tell you. Use an Emitter follower NPN. You can use a 2N3904. Transistors switch much faster in common collector configuration. I have done this to drive an LED and it worked so well I swore I would never go back to common emitter LED driver.

It will have a linear response so I don't know if your input signal is already switching or not???? If it is great! If not you will have to configure a switch. There are plenty of devices that switch very quickly.
 

I was under the impression that emitter followers were a SLOWER configuration? - and why would you want high speed to drive an LED?.

Emitter followers also don't 'switch' it's a purely linear configuration.
 
No they are faster in common collector from what I remember.

Anyway, that's good question. Maybe he wants to use it like a TV remote control.
 
Alright,

Thanks again for everyone's assistance so far. I have another question, same topic...

I have tested a PNP powerFET with an LED @ +5v 350mA...

Theory:

0v turns on the LED while +5v turns off the LED.

The issue I face is that the LED is not turning completely on at 0v. I notice slightly more light output when I connect the LED directly to the 4.7Ohm resistor I'm using to limit current, rather than going through the MOSFET.

Attached is the datasheet for the IRF9540n I have.

https://www.electro-tech-online.com/custompdfs/2009/07/irf9540n.pdf

Is this because the mosfet is listed as having a Vgs threshold of -2v to -4v?
 

Depending on the FET type, they are often more complicated to drive than bipolar transistors - any reason for switching from bipolar?.
 
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Yes. ANd be careful not using the driver. With only 4.7 ohms as a current limiter, if the LED is on for prolonged periods of time it will probably get dimmer as it slowly burns out.
 
Correction...

I meant 5.6 Ohms, not 4.7 Ohms.

LED: Vf = 3.5v, If = 350 mA, driven with +5v = 5.6 Ohm resistor @ 2 Watts

My reasons for selecting a MOSFET were purely due to the 350 mA to 700 mA load. I thought it would be safer when connecting to the logic of an IC.

So, if anyone disagrees, please give example of an alternate design capable of driving a 350 to 700 mA load from a 0v active IC of 3.3v to 5v.
 
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