Help with transistors

[yxqjzz]

is there a different method that can give a gain? because at the level of the ttl signal the led was not bright enough since the typical forward voltage of the led is 3.8V and the ttl is only 3V

thanks

 

[Roff]

is there a different method that can give a gain? because at the level of the ttl signal the led was not bright enough

thanks

You're going to have to use two transistors. What supply voltages do you have available?

 

[yxqjzz]

You're going to have to use two transistors. What supply voltages do you have available?

I have a dual channel supply that can go up to 30V, and another one that is fixed at 5V

 

[joeh100]

You'll need a pnp transistor to source the current.

 

[Roff]

This should work, but it won't handle a 100ns pulse.

 

[unclejed613]

is there a different method that can give a gain? because at the level of the ttl signal the led was not bright enough since the typical forward voltage of the led is 3.8V and the ttl is only 3V

thanks

your problem here is the source current available from the TTL output. the transistor should provide the required current gain.

 

[Roff]

your problem here is the source current available from the TTL output. the transistor should provide the required current gain.

He says the TTL signal is 3V, and the LED fwd voltage is 3.8V. An emitter follower won't work.

 

[yxqjzz]

This should work, but it won't handle a 100ns pulse.

Thanks, this is very helpful. I'll get the parts and try this out today.

 

[yxqjzz]

Hi, sorry to bring this up again. The circuit finally worked, it managed to send an amplified square pulse to the LED. I did it using the original design with the 2N2369A transistor. But now the pulse has this strange teeth-like shape as shown in the picture. Is it due to the transistor or the reflections of the BNC cable or something else? I'm wondering if there is a way to fix this.

 

[Roff]

Those are classical transmission line reflections due to the load and/or source impedance not matching the Z0 of the line (coax).