Fast switching PNP transistor request

[Nigel Goodwin]

Yes, as suspected, the internal clock frequency is 4MHz, the output frequency looks to be that divided by 4096, nothing special required for the transistors.

 

[James77]

Thank you both for your help. I have been looking at several transistors, they don't mention switching times on the datasheet. Is it assumed that most transistors switch at higher speeds than 1,000Hz?

For example, https://www.electro-tech-online.com/custompdfs/2009/07/21559.pdf

Can I purchase NXP BC859C's to do the job? (I know they're SMD - they're rated to 35V, I'm only driving 5v)

 

[Nigel Goodwin]

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.

 

[Hero999]

Standard small signal transistors (like the BC107) are rated at 300MHz, and that's an audio transistor.

This doesn't defeate tyour point but the BC107 has a GBWP of only 150MHz.

http://www.datasheetcatalog.org/datasheet/SEME-LAB/BC107.pdf

 

[Nigel Goodwin]

This doesn't defeate tyour point but the BC107 has a GBWP of only 150MHz.

http://www.datasheetcatalog.org/datasheet/SEME-LAB/BC107.pdf

Well I'm only going from memory (and probably 30 year old memory at that), but I'll check the Mullard databook at work tomorrow

In any case, they work quite happily as an RF oscillator in a 'bug' at 100MHz+.

 

[Space Varmint]

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.

 

[Nigel Goodwin]

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.

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.

 

[Space Varmint]

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.

 

[James77]

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?

 

[Nigel Goodwin]

This doesn't defeate tyour point but the BC107 has a GBWP of only 150MHz.

http://www.datasheetcatalog.org/datasheet/SEME-LAB/BC107.pdf

Just checked the 1973 Mullard Databook - ft of a BC107 is 300MHz - thought so

 

[Nigel Goodwin]

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-1.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?.

 

[Space Varmint]

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-2.pdf

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

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.

 

[James77]

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.

 

[Space Varmint]

I don't disagree. Go for it! It sounds like to get your hands dirty.