Transistor Switching: Am I Doing it Right?

[Raven Luni]

Greetings,

I just wanted some input from the experts on the best practices when it comes to using transistors for switching in digital logic circuits.

What I do: I use BJTs pretty much exclusively both because I have an endless supply of 2N3904s and 06s (and I still cant get my head round some of the stranger aspects of mosfets like having to charge pump them because the gate voltage rises and stuff).

Anyway - what I do with the BJTs is make sure they are always in saturation by having Rb much lower than Rc (usually 1K and 10K respectively). The reason I'm asking is because I want to know if this would work at high frequencies or would this need more precision and/or better components to ensure faster switching times?

 

[(*steve*)]

The speed of switching is really determined by how fast you can charge and discharge various capacitances associated with your circuit.

In general, reducing the size of the resistors (allowing higher current) will increase the speed at which you charge and discharge parasitic capacitances (and for this purpose I will include junction capacitances).

If you look at the history of TTL and the various variants, you will note that the major difference between standard, low power, and high speed variants was the current at which the transistors operated.

 

[Raven Luni]

cool thanks

 

[BobK]

To get the highest switching speeds on a 2N3904 you are going to need a collector resistor in the hundreds of Ohms, not 10K.

Bob