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non-inverting amplifier with high pass filter input

earckens

Member
A title to knock you over just to find a simple two-transistor (or mosfet) circuit that receives a one-time down transition from 5V to GND, and a single pulse at the output is needed.

When going from 0V to 5V at the input, a simple 100nF capacitor in series with the base of a NPN transistor, emitter to ground and a 10k resistor between the collector and 5V, with the output connected to the collector will give me a pulse at the output (collector).

But I need, as stated in my first line, something that gives a short output pulse (duration not important as long as it goes shortly to ground) when supplying a downward transition on the input. Output load is high impedance.
 

rjenkinsgb

Active Member
Mirror that circuit top to bottom using a PNP transistor, so a low-going pulse in gives a high pulse out.

Then add the NPN transistor, resistor coupled (instead of capacitor coupled) to the output of that.
eg. another 10K from collector on the PNP to base on the NPN, plus a 10K base-emitter on the NPN.

You also need a reasonable value resistor base to emitter on the first transistor to discharge the cap after a pulse and define the DC state; 10K - 100K depending how sensitive it needs to be.

I'd also add eg. a 1K resistor in series with the capacitor, to limit the current in to the transistor base when an edge occurs.
 

MikeMl

Well-Known Member
Most Helpful Member
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rjenkinsgb

Active Member
Not a lot in it, either should work pretty much the same, with Mike's being a bit simpler.
I was trying to work it around a circuit you were familiar with.

I would add the 1K in series with the input if you use Mike's version though, to avoid any possibility of excess base current.
 

earckens

Member
Not a lot in it, either should work pretty much the same, with Mike's being a bit simpler.
I was trying to work it around a circuit you were familiar with.

I would add the 1K in series with the input if you use Mike's version though, to avoid any possibility of excess base current.
:) Thank you!
 

MikeMl

Well-Known Member
Most Helpful Member
\[ \Omega \]
...I would add the 1K in series with the input if you use Mike's version though, to avoid any possibility of excess base current.
You notice that I modeled the V2 source with an internal resistance of 100 Ohms, which intrinsically limits the base current on the positive going transition.
 
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