# IC324 OPamp questions?

Discussion in 'Homework Help' started by pfelectronicstech, Nov 8, 2012.

1. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
I've been out of the loop for over a week because of the hurricane here in NJ. Lost power last Monday, finally got it back last night, 9 days without. Anyway I fell behind on my studies because of this so I'm trying to catch back up.
I'm learning about IC324 op-amps. They way its configured in the book is they have a 10K resistor between 1 and 5 but they are asking what happens if you then place a 100K resistor in its place between pins 1 and 5 what happens. Seems easy to me, frequency decreases. Am I on the right track here?
Again I'm just trying to catch up here after not being able to do a lick of studying of my course. Its good to be back though! Thanks for any help. If you need to see the entire circuit I can take a pic, and post it. Just let me know.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes, show as a circuit diagram.

3. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Yeah i knew I would, was just being lazy. I will post now.

4. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Here is the op-amp circuit, thanks.

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5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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This is a "assembler" diagram you don't have a "normal" circuit diagram?

6. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Oh yes I have that too, duh I should have just provided that. Sorry about that, i will post that now.

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7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Are you sure that you want to change this resistor R1 from 10K to 100K ?
Or maybe you simply connect another resistor parallel to R1 ?

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Last edited: Nov 8, 2012
8. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Its not me, its what they want me to do and they want the result. They are asking "what would occur if you did this. Wouldn't it just decrease frequency? I mean that is after all what resistors do right? Thanks again for the help.

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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If we replace 10K resistor with 100K resistor, the circuit stop working.
Why ?
IC2 work as a Schmitt trigger (comparator) with hysteresis defined by R1/R2*Vsup.
And for 100K/22K *10V = 45V the hysteresis is to large for circuit to work properly.

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10. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
OK, that makes sense, thank you.

11. ### Audioguru Expert

Dec 20, 2007
10,606
1,182
Resistors DO NOT change frequencies. But changing the value of a resistor that charges and discharges a capacitor changes the frequency of an oscillator.

Resistor R4 in the circuit charges and discharges the capacitor in the circuit so it changes the frequency when its value is changed.

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12. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Thanks so what would you say the ultimate answer is? Decreases frequency slightly, increases frequency slightly, no voltage present at pin1 or voltage does not vary? I'm just a little confused now. Thanks for the help, I appreciate it.

13. ### Audioguru Expert

Dec 20, 2007
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If the 10k resistor between pin 1 and pin 5 is increased to about 20k or higher then the oscillator will not oscillate because the Schmitt-trigger opamp cannot flip its output. As the resistor is increased to about 20k then the frequency will drop about 1 octave because the capacitor will be charging to double the voltage.

Note that for half of the signal the capacitor has backwards polarity which will also cause the oscillator to not oscillate.

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14. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
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So the ultimate answer is, no voltage will be found at pin1. I think we finally got to the answer. Thank you for the help. I have another circuit I need to post soon, so stay tuned.

15. ### Audioguru Expert

Dec 20, 2007
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Of course pin 1 will have a DC voltage. It can be either high or low.
But pin 1 will not have an AC triangle wave signal.

16. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Ok, I'm not following. I only have 4 answers to choose from, by the way this is just a "knowledge check", nothing being graded. This is for myself, as an end of the chapter non-graded quiz. The four choices I have to choose from is:

1. The frequency increases slightly
2. The frequency decreases slightly
3. The voltage does not vary
4. No voltage found at pin 1

Thanks again for the help again, I truly appreciate it.

17. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
I have another circuit question about a A/D converter.

If i apply 0.8V to pin 6, what is the result?

1. 1111 and 1110 blink alternately
2. 1111 and 0000 blink alternately
3. 1111 is on
4. The circuit counts upward

Thanks for any help or hints.

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18. ### Audioguru Expert

Dec 20, 2007
10,606
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On the previous question, since the oscillator does not oscillate then the output voltage does not vary.

On the new question, I have not used old-fashioned 74xx TTL ICs for about 34 years and I don't want to look at yours.

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19. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Well thank you for the help, I appreciate it very much. I'm almost done with the knowledge check and thus this book. Then its move on time to the next book and set of lessons.

20. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But changing the resistor R1 and R2 values in Schmitt trigger also changes the frequency of an oscillator because the width of the hysteresis also changes.

As for Ad converter, do you know how this simply R/2R converter work?
What is the function of pin 5? What is the function of a second op amp?
I give you a tip
If output voltage from R/2R network is greater than voltage at pin 6 the op amp output voltage (pin 7) in set to high state.