# Problem with Ramp signal Generator using IC 555

#### Pr1y4n5hu

May 9, 2022
2
According to theory the time period for ramp signal generated using the following circuit should be:
T= [2.Vcc.C1.RE . (R1+R2)] / [(3Vcc .R2 ) - (3VBE.(R1+R2))]
Now... putting the value of Vcc, R1, R2, VBE, RE... the time period comes out to be around 0.9ms.
But from the simulation it appears to be around 3ms.
Whats wrong here?

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#### Pr1y4n5hu

May 9, 2022
2
Ok...but i just want to know why the difference in theory and practical value of time period of the ramp?

Feb 19, 2021
331
Did you measure the accuracy of your R & C used in timing loop ?

Datasheet says -

What accuracy are you seeking ?

Fixed frequency or variable needed ?

Regards, Dana.

#### kellys_eye

Jun 25, 2010
5,480
More to the point, what are the defined values for the components used in the simulation software?

#### Alec_t

Jul 7, 2015
3,330
i just want to know why the difference in theory and practical value of time period of the ramp?
Check your maths. The voltage at the R1/R2 junction is not 4.33V if Vcc=5V..
What value have you assumed for Vbe?

#### AnalogKid

Jun 10, 2015
2,698
The Thevenin equivalent resistance of R1 and R2 is 31 K. That is a pretty high base resistance, so the voltage at the Q1 base will not be very "stiff". That is, it will change as the collector voltage ramps up. As the collector-emitter voltage changes, the transistor gain changes, so its base current changes. The base current goes through R2, which changes the voltage at the R1-R2 node. Because R2 is so large, a very small change in base current is noticeable. This affects the linearity of the ramp waveform.

ak

Last edited:

#### AnalogKid

Jun 10, 2015
2,698
Alec: The problem is not Vbe; it is hfe.

If you run the numbers assuming a transistor gain of 50, it almost perfectly explains why the sim voltage at the R1-R2 node differs from the Thevenin equivalent by around 1 V. That is 10 uA through 100 K.

TS: Where did that equation come from?

Also, as a test, divide R1 and R2 by 1000, leave everything else the same, and see how this affects the base voltage and the sim.

ak

Last edited:

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