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[SOLVED] [Design] Control Slew Rate in Square Wave Form

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TuAtAu

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Hi all, I'm newbie in Analog Design :)

Currently I want to do a circuit that able to control the LINEAR slew rate from a square wave.

I'm using Astable timer555 to provide the square wave. After that, my initial design is :

from the output of the timer555(square wave), I series with a Resistor and Parallel with a Capacitor to control the slew rate. BUT, problem is the slew rate is not linear but exponential!!(because charging capacitor is exponentially)

I only got 1 input source 12V

Anyone got any idea how to design to control the slew rate and the slew rate must linear??(linear = something like ramp function)
:-?
 
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LvW

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In principle, your approach is not false - however, you need an RC element with gain that allows you to use only the very first (nearly linear) part of the charging function. With other words: Use an integrating device (opamp with capacitice feedback).
 
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kak111

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LvW

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As a (perhaps simpler) alternative you can use a classical RC lowpass element in series with a limiting gain stage.
 

TuAtAu

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In principle, your approach is not false - however, you need an RC element with gain that allows you to use only the very first (nearly linear) part of the charging function. With other words: Use an integrating device (opamp with capacitice feedback).

if add an integrator to the output, its only will change the square wave into triangle wave or saw tooth wave right?

forget to add addition detail~ I JUST GOT 1 source which is 12VOLT. That is the limitation :(


Here is some examples , google finds more.............

LINEAR VCO - Circuit Diagram 1 - Forum - SeekIC.com

LINEAR VCO - Circuit Diagram 2 - Forum - SeekIC.com




Regards KAK

---------- Post added at 12:03 ---------- Previous post was at 12:01 ----------

Sorry these are Vco not slew rate controls
Yup, It can't control slew rate :(

As a (perhaps simpler) alternative you can use a classical RC lowpass element in series with a limiting gain stage.

I not understand "with a limiting gain stage" (still newbie ><") can futher explain detailly? THX!!!
 

dick_freebird

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Get a CD4000B series inverter, if you can find one that is a single stage
even better. Drive the input via a resistor and feed back the output to
the input via a capacitor. The dV/dt now opposes the input signal to make
a linear ramp.

Not the greatest scheme, since the inverter will spend a lot of time in
cross-conduction, but it's an approach. Be sure you get the good
CD4000 with a high voltage rating, there are weaker subfamilies.

Since you have all those other inverters for free, you might make
a phase shift oscillator and ditch the 555.
 

kak111

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Dont use 4000B series , those have buffered output stages.

Best choice this kind of circuit is 4000UB series cmos-ic inverter, like 4049UB or UBE

in circuit it acts like inverting op-amp , if more amplification needed add two
inverters.

linear_amp.gif


Linear 10x Amplifier
The above circuit is a simple 10x linear amplifier which the gain determined by R2/R1
 

RCinFLA

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If you want a linear slew rate use the digital control to active a pump up and pump down constant current source.
 

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