A simple pulse generator. ( R , C , op )

[Boki]

Hi All,

Here is a pulse generator.
http://photos1.blogger.com/blogger/1641/2018/1600/comparator.gif

Vcmp is constant.

Vin only 2 level. 0 or 3.3V


My questions as follows:

1) The R , C value will change the pulse width ?

2) The positive point of comparator( V+ of comparator, next to Vcmp)
will have a suddenly high voltage if Vin from low to high (same voltage
as Vin ?) ? am I correct ? or it will only has suddenly 500mV around ?

Vin is DC, a voltage be changed by a manual switch.


Thank you very much for your advice.

Best regards,
Boki.

 

[John Fields]

Hi All,

Here is a pulse generator.
http://photos1.blogger.com/blogger/1641/2018/1600/comparator.gif

Vcmp is constant.

Vin only 2 level. 0 or 3.3V


My questions as follows:

1) The R , C value will change the pulse width ?
---
Yes.
---

2) The positive point of comparator( V+ of comparator, next to Vcmp)
will have a suddenly high voltage if Vin from low to high (same voltage
as Vin ?) ? am I correct ? or it will only has suddenly 500mV around ?

Vin is DC, a voltage be changed by a manual switch.

---
If the time rate of change of Vin causes the voltage on the
comparator side of the capacitor to rise to > VCMP, the output of
the comparator will go high (or open-collector) for as long as Vin >
VCMP.
---

 

[PeteS]

Hi All,

Here is a pulse generator.
http://photos1.blogger.com/blogger/1641/2018/1600/comparator.gif

Vcmp is constant.

Vin only 2 level. 0 or 3.3V


My questions as follows:

1) The R , C value will change the pulse width ?

2) The positive point of comparator( V+ of comparator, next to Vcmp)
will have a suddenly high voltage if Vin from low to high (same voltage
as Vin ?) ? am I correct ? or it will only has suddenly 500mV around ?

Vin is DC, a voltage be changed by a manual switch.


Thank you very much for your advice.

Best regards,
Boki.

This is a pretty basic generator. The pulse width is set by R,c, Vcmp
and Vin. The width of the pulse is set by the differentiator at the
front end (although it's not really that practical as shown due to
slowly rising / falling edges which most comparators have problems
with). The rising edge at the + input will follow Vin very closely,
which starts the pulse. The end of the pulse is determined by the decay
of the differentiator (proportional to e ^^t/RC). When it falls below
Vcmp, the output will go low again.

Don't tell me you've never seen a simple differentiator before

Cheers

PeteS

 

[PeteS]

This is a pretty basic generator. The pulse width is set by R,c, Vcmp
and Vin. The width of the pulse is set by the differentiator at the
front end (although it's not really that practical as shown due to
slowly rising / falling edges which most comparators have problems
with). The rising edge at the + input will follow Vin very closely,
which starts the pulse. The end of the pulse is determined by the decay
of the differentiator (proportional to e ^^t/RC). When it falls below
Vcmp, the output will go low again.

Don't tell me you've never seen a simple differentiator before

Cheers

PeteS

(proportional to e ^^-t/RC)

Cheers

PeteS

 

[John Larkin]

This is a pretty basic generator. The pulse width is set by R,c, Vcmp
and Vin. The width of the pulse is set by the differentiator at the
front end (although it's not really that practical as shown due to
slowly rising / falling edges which most comparators have problems
with). The rising edge at the + input will follow Vin very closely,
which starts the pulse. The end of the pulse is determined by the decay
of the differentiator (proportional to e ^^t/RC). When it falls below
Vcmp, the output will go low again.

Don't tell me you've never seen a simple differentiator before

Cheers

PeteS

What goes up must come down. Some opamps/comparators will go bonkers
when the trailing edge slams the input below ground.

John

 

[PeteS]

What goes up must come down. Some opamps/comparators will go bonkers
when the trailing edge slams the input below ground.

John

That's why I mentioned it wasn't very practical as drawn, although I
didn't go into _all_ the reasons it was impractical

I've seen nasty phase reversal _without_ the input going below ground -
just going below a certain level with loop gain high enough. I'm not a
chip designer, but the reasons are fairly well known by now

Cheers

PeteS

 

[Boki]

PeteS 寫�：

This is a pretty basic generator. The pulse width is set by R,c, Vcmp
and Vin. The width of the pulse is set by the differentiator at the
front end (although it's not really that practical as shown due to
slowly rising / falling edges which most comparators have problems
with). The rising edge at the + input will follow Vin very closely,
which starts the pulse. The end of the pulse is determined by the decay
of the differentiator (proportional to e ^^t/RC). When it falls below
Vcmp, the output will go low again.

Don't tell me you've never seen a simple differentiator before

Cheers

PeteS

One more question, does this circuit possible to happen bounce effect?
( at comparator output side )

The VDD(VCC) for the comparator is keep on ( about 3.7 V) always, Vcmp
is about 0.x (V), divide by two resistor, ( gnd is 0V )



Best regards,
Boki.

 

[Rich Grise]

Hi All,

Here is a pulse generator.
http://photos1.blogger.com/blogger/1641/2018/1600/comparator.gif

Vcmp is constant.

Vin only 2 level. 0 or 3.3V


My questions as follows:

1) The R , C value will change the pulse width ?

2) The positive point of comparator( V+ of comparator, next to Vcmp)
will have a suddenly high voltage if Vin from low to high (same voltage
as Vin ?) ? am I correct ? or it will only has suddenly 500mV around ?

Vin is DC, a voltage be changed by a manual switch.


Thank you very much for your advice.

You've gone and reinvented the Poor Man's One-Shot. I learned about
this in USAF tech school, but they used a transistor back in 1968. ;-)

Cheers!
Rich