# Using a Capacitance Multiplier in a RC differentiator

Discussion in 'General Electronics Chat' started by Aisha.Raj, Feb 15, 2013.

1. ### Aisha.Raj Thread Starter New Member

Nov 22, 2012
14
0
I am building a trigger circuit for my monostable multivibrator ( op-amp design) where the pulse width has to be varied from 0.1ms to 10s and also provision needs to be given for varying the duty cycle from 25% to 75 % .

The trigger circuit essentially takes care of my duty cycle path and i am using a RC passive differentiator ( to which a square wave i/p is given) to give triggers at required time period. Since i have to vary my pulse width up 10s , my R , C values of the trigger ckt is enormous . So i fixed my R at 470k and tried to vary my capacitance . I tried doing this using a capacitance multiplier ( since my design specifications says i cant use capacitors greater than 1u) .

The problem i am facing now is how i can connect the capacitance multiplier in place of my capactior of the trigger ??? ( Suppose the +ve of capacitor is connected to i/p square wave and -ve side connected to R , then how do i do the same using a capacitance multiplier, which terminals do i connect to the i/p and which to the resistor ???)

Another question is will the frequency of the input or any other frequency component affect my capacitance multipliers' problem ?? Will it cease to work as the one i originally designed for ??? If so what can be a solution to this problem ??? I ask this because , in order to satisfy my pulse width and duty cycle conditions, i may have to give square pulses of frequency from hz - Mhz( upto approx. 5Mhz , if am not mistaken) range .

Mar 14, 2008
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3. ### Ron H AAC Fanatic!

Apr 14, 2005
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Why are you using an op amp for a one-shot?

4. ### Aisha.Raj Thread Starter New Member

Nov 22, 2012
14
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PROBLEM STATEMENT
Without using a microcontroller or a dedicated timer IC, implement a circuit which, when a trigger is applied , will provide a +ve pulse of amplitude 3V, whose width can be varied from 0.1ms to 10s. You may not use any capacitor greater than 1uF or resistor larger than 470kohm. You are provided with op-amps which operate off + or – 10V. Provide a provision to continuously vary the duty cycle from 25% to 75%.

MY SOLUTION
Monostable multivibrator to provide positive one-shot pulses ( stable state is –Vcc) of amplitude +Vcc.

Pulse Width varied by varying R1C1 time constant . So I fixed R1 = 470kohm and varied C1 . But C1 cant take values greater than 1uF . So I want to use a capacitance multiplier to increase the value of capacitance and also this capacitance has to be varied so I will vary the gain(R7/R6) of the capacitance multiplier.

Duty cycle can be varied by varying the R2C2 time constant of the trigger circuit (which is basically a passive RC differentiator-high pass filter to which square waves of time period T>> R2C2/10 is fed by a function generator) . Here too I fixed R2 = 470kohm and varied C2 . Since very large and variable C2 values were required for this , I again want to use a variable gain capacitance multiplier as above.

So essentially I am using 2 different – variable gain capacitance multiplier whose capacitance value b/w i/p and o/p node is 1uF , and the amplified version of which is seen from the input side ( due to Miller’s Effect) given by the equation, Cin =C(1+R7/R6) where either R7 or R6 is a variable resistance.

Then at the o/p stage of the multivibrator , I put back to back zeners to clip the voltages to (Vz + Vf) and –(Vz to Vf) which roughly estimates to be +(3.3+1.1) and –(3.3+1.1) (ie) 4.4V to -4.4V. Zeners used have a forward drop of 1.1 V max and Vz = 3.3 V (1N5226) . This is further clipped by using a +ve clipper circuit of 2 pn- diodes (1n4001 of Vd =0.7V ) and a load resistor Rl = 10kohm .
The o/p of this entire design is taken from Rl (10kohm) .

MY PROBLEMS
1. I do not know how to connect the capacitance multipliers to my circuit such that , for one capacitance multiplier Cin appears as C1 and the other capacitance multiplier Cin appears as C2.

2. Will there be any effects due to frequency which’ll impede the working of my circuit as it should behave?? Because I remember that Miller effect has something to do with frequency. I ask this as to provide the required pulse width and duty cycle I may have to give the input to the trigger from few Hz to few MHz . So will the circuit function good for this ??

3. Also, at the clipper end I used a random value of Rl as 10kohm. Is there a specific way to choose this Rl value???

4. Any other problems I will face while simulating or implementing this circuit that you can see. Any errors in logic or design that you can detect???

I’d be really grateful if you could advice me suitably on the issues I raised . I have been given a deadline – 20th Feb 2013, gotta present the design to my panel on 21st . So I’d be extremely happy if we can figure out something by then ……Sorry for the extreme short notice and tks for your help in advance.

I am attaching the circuit with this post.

5. ### crutschow Expert

Mar 14, 2008
20,825
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Your post didn't work. Select "Go Advanced" in the Quick Reply window and then select the "Attachments" (paper clip) icon to attach a file to your post.

6. ### timescope Member

Dec 14, 2011
298
44
If you use a variable current source to charge the capacitor, a wide range of time constants can be obtained easily.

Are you allowed to use transistors ? You can use an op amp and a transistor to make a precision voltage controlled current source

Timescope

Last edited: Feb 16, 2013
7. ### Aisha.Raj Thread Starter New Member

Nov 22, 2012
14
0
The circuit is attached, i dnt think transistors are allowed.....

• ###### Circuit.png
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Last edited: Feb 17, 2013
8. ### Ron H AAC Fanatic!

Apr 14, 2005
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I don't understand your problem definition. A monostable multivibrator doesn't have a duty cycle. It has a pulse width. Oscillators have duty cycles. If a monostable multivibrator is triggered at a constant frequency, then the resulting repetitive pulse waveform will have a duty cycle. This is NOT related to the pulse width of the trigger pulse.
I think that you, or your instructor, or possibly both, have confused monostable multivibrators with astable multivibrators.

9. ### timescope Member

Dec 14, 2011
298
44
Thanks. I thought I was the only one confused by this.

Timescope

10. ### Aisha.Raj Thread Starter New Member

Nov 22, 2012
14
0
Very Very sorry, just gotit clarrified from my professor again, there was a typographical error , which occured......the circuit has to produce a TRAIN OF PULSES whose width and duty cycle can be varied ..... so yes, i would be requiring a astable multivibrator ckt, but i would also need a capacitance multiplier circuit that works using op amps!!!!....Any ideas or suggestions!!

11. ### Aisha.Raj Thread Starter New Member

Nov 22, 2012
14
0
My apologies for creating such a big confusion.......
I now have to design a circuit which provides a train of pulses of width ranging from 0.1ms to 1s of duty cycle that can be varied from 25% to 75%

As suggested i have decided to go with the astable multivibrator configuration. My trigger is actually giving supply to the op amps , ie when i give a signal to an electronic switch , it has to close a circuit which provides the Vcc to my astable multi-vibrator. I was given an idea to use components like CD4051,CD4052 , CD4053 . Can someone explain what they are???? I tried googling it but i couldnt understand much from the datasheets!!!!

But yet again i gotta use a capacitance multiplier to amplify and vary my capacitance value ........
How do i vary dutycycle now ???

I finally have got hold of the capacitance multiplier circuit.......Could someone please find the transfer function of it ( Relation connecting input and output ) ??? I would be grateful if someone can give me the derivation of the transfer function, as i am not able to zero in on it even after several attempts!!!!!

I am attaching the circuit along

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18.6 KB
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43
12. ### Aisha.Raj Thread Starter New Member

Nov 22, 2012
14
0
My apologies for creating such a big confusion....... my design question has been modified

I now have to design a circuit which provides a train of pulses of width ranging from 0.1ms to 1s of duty cycle that can be varied from 25% to 75%

As suggested i have decided to go with the astable multivibrator configuration. My trigger is actually giving supply to the op amps , ie when i give a signal to an electronic switch , it has to close a circuit which provides the Vcc to my astable multi-vibrator. I was given an idea to use components like CD4051,CD4052 , CD4053 . Can someone explain what they are???? I tried googling it but i couldnt understand much from the datasheets!!!!

But yet again i gotta use a capacitance multiplier to amplify and vary my capacitance value ........
How do i vary dutycycle now ???

I finally have got hold of the capacitance multiplier circuit.......Could someone please find the transfer function of it ( Relation connecting input and output ) ??? I would be grateful if someone can give me the derivation of the transfer function, as i am not able to zero in on it even after several attempts!!!!!

I am attaching the circuit along

File size:
18.6 KB
Views:
34
13. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
You need to post your revised problem definition.
Also, can you switch in different capacitor values to get the required frequency range?

I don't see why you need a capacitance multiplier.

14. ### timescope Member

Dec 14, 2011
298
44
Probably because of the restrictions in component values capacitance < 1uF, resistance < 470k for a time constant of 10 seconds.

Timescope

15. ### Ron H AAC Fanatic!

Apr 14, 2005
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With the "standard" function generator circuit comprised of an integrator and a Schmitt trigger, 470k and 1uF should work OK for a 10 second ramp. it might get a little tough if he needs 30 seconds or more.

We need to see the revised problem definition.

16. ### timescope Member

Dec 14, 2011
298
44
How ? 470k x 1 uf = 470ms

Timescope

17. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
Well, I can't tell you without giving away the circuit. I believe this is homework, so our OP needs to do some of the work.
I have already given a big hint. Hopefully, you and he are both familiar with Google.
Here's another hint: With a small amount of current, e.g., 1uA, I can make a very slow ramp if I charge my 1uF cap with it.

18. ### timescope Member

Dec 14, 2011
298
44
I've got it. I posted that idea earlier.

Have you seen the circuit ? 1n4001 is a rectifier diode.

"He" may be a "She"

Timescope

19. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
In the circuit I've been referring to, the current source is the input resistor in an operational integrator.

I can't find any circuit on this thread that was posted by you.

I know. It doesn't worry me.

20. ### timescope Member

Dec 14, 2011
298
44
The circuit posted by the op.

Timescope