Hello!!

I've been doing a little reading up on some information on microprocessor efficiency. I've run into something that is not explained well in the book and I can't figure out what the hell he is talking about. I'll try to explain what I read and saw:

There is a simple 2N3904 NPN BJT with the input going through a 10k resistor into the base. The collector is "pulled" up to 5v through a 10k resistor as well. The emittor is grounded and the output is taken at the collector. This output is the input to an 8051.

Now, as I see it, the 8051's input only requires some 50uA or so to source it. When the transistor is OFF and sourcing a "high", the current produced from Rc is about half a milliamp. This is good because much less power is used...makes sense so far. When the transistor is saturated, or ON, the 8051's input sinks the current however much it may be. Still good so far...

Now here is the problem. The author next says that the 10k collector resistor "could be even larger in value, but the speed would start to slow because of stray capacitance." I'm sure that is true, but what the hell is he saying? As I see it, less the current the better. For me, using a much higher value would decrease the current preferably a slight amount above 50uA needed by the sinking of the TTL input. So how does this capacitance come into play and how does it slow the circuit down?

Thanks for your input!
Phil