# Why can this circuit generate ramp waves?

Discussion in 'General Electronics Chat' started by zhaojia, Jan 21, 2008.

1. ### zhaojia Thread Starter New Member

Nov 23, 2007
7
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The circuit is shown below.
It can generate ramp waves. Could anybody tell me the principle of this ramp wave generator?
Thanks.

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2. ### rwmoekoe Active Member

Mar 1, 2007
172
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zhaojia, you can read about it in the "The art of electronics" by Horowitz. (Pardon if i'm wrong)

3. ### rwmoekoe Active Member

Mar 1, 2007
172
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zhaojia,

this is what i know about it:

here it goes in the circuit:

the opamp must start in one of the two stable positions, right? (aside from the negative feedback loop w/ the 10k R1

resistor and 1nF cap, it forms a bistable unit).

assume it starts with the opamp output being stable at positive, and the 1nF cap starts empty of charges, i.e. near

negative.
the output is stable at positive, while the cap is being charged through the R1 res from the opamp output.
this is the ramp-up side of the ramp wave, right?

notice that the output also, through the R2 res, pulls up the threshold level of the system (i.e. the non-inverting (+)

input of the opamp) to near the positive voltage.

the opamp is always detecting the difference between its two inputs, right?
as long as the non-inverting (+) input is more positive than the inverting (-) one, it always output positive (or near),

right?

finally, the ramp-up wave at it's inverting (-) input reaches the threshold level at the non-inverting (+) input.
what happened then? that's right, it changes states, stables at negative.

now the output is negative, through the R2 res, shifting the threshold level of the non-inverting (+) input to near

negative, while at the same time discharging the cap at the inverting (-) input through the R1 res.
this is the ramp-down side of the ramp wave, right?

now it's the other way around from above,
as long as the non-inverting (+) input is more negative than the inverting (-) one, it always output negative (or near),

right?

when finally the inverting input reaches down to the near negative threshold level, the system starts it all over again...

that's all i guess,
hope it helps

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Going with the theory that a picture is worth 1e3 words, here's the result of a simulation to go along with rwmoekoe's words.
As you can see, the "ramp" is exponential, not linear as ramp is generally defined.

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5. ### rwmoekoe Active Member

Mar 1, 2007
172
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ron h, u did it great. the pic's more than 1e3 words