I wonder, how can I make edge detecting circuit without using any logic IC's, using only transistors & passive components?
I have digital signal with ~15ns fronts on my digital input (5v TTL), and I want to get 20-50ns long pulses(with ~10-20ns transitions) on the front edge of the input pulse.
What I tried to do is capacitor + resistor and then push-pull BJT cascade - I am getting nice front(~15ns), but trialling edge is just terrible (200ns) - capacitor slooooowly discharges...
Probably push-pull cascase I am using (see first answer Prevent high-side BJT saturation) is not very 'digital' - i.e. it's too linear... If there is a way to make more Schmitt-like , probably that would solve this capacitor discharge problem...
Any ideas?
Answer
I'm not totally clear on what you are asking about, but I think you want a edge to glitch converter? I'll assume that you have a positive going edge and want to have this result in a positive pulse. Two common answers are a one-shot or a flip-flop with a R-C delay from output to reset. However, you want to use discrete analog parts for some reason.
How about a capacitor-coupled emitter follower with a resistor and capacitor in parallel to ground on the emitter? Assuming new edges don't come along until the circuit has had time to reset, the rising edge goes thru the capacitor and makes a rising edge on the output (emitter). A resistor from base to ground makes the input to the transistor go away quickly and reset for the next edge. Meanwhile the length of the output pulse is governed by the R-C time constant of the output capacitor and resistor.
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