LM324 Func. Generator testing

Thread Starter

naitek

Joined May 31, 2008
15
hello everyone. I am building a circuit for the umpteenth time, but dont understand the v+/2 in this circuit. How will i get 1/2 the voltage or is it just stating that 1/2 the voltage will be here. Please see attached
 
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Thread Starter

naitek

Joined May 31, 2008
15
hello everyone. I am building a circuit for the umpteenth time, but dont understand the v+/2 in this circuit. How will i get 1/2 the voltage or is it just stating that 1/2 the voltage will be here. Please see attached

Forgot the attachment..
 

Attachments

AnalogKid

Joined Aug 1, 2013
10,987
The circuit assumes that you will provide V+ (or Vcc) divided by 2 and decoupled for low source impedance. So if the circuit power is +9V, you have to supply +4.5V to the reference pin of the hysteretic comparator that converts the integrator's triangle wave into a square wave.

If you have a spare LM324 section, you can do this with a 2:1 voltage divider driving a LM324 as a voltage follower. If you don't have a spare section, replace with 51K coming from V+/2 with two 100K resistors, one to V+ and one to GND. Do *not* add a decoupling capacitor to the juction of the three 100K resistors and the + input. Just in case you don't know, this is called a Thevenin equivalent (actually a reverse-Thevenin).

ak
 
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Thread Starter

naitek

Joined May 31, 2008
15
I am just starting on this, but the ac coupled invert. amp is assuming ac voltage or no?

if so , then what app would i be using it for since i only want a 9v supply.

thanks for such quick responses..

-i
 

atferrari

Joined Jan 6, 2004
4,764
If you don't have a spare section, replace with 51K coming from V+/2 with two 1000K resistors, one to V+ and one to GND.

Do *not* add a decoupling capacitor to the juction of the three 10K resistors and the + input.

ak
Why not the cap AK? I think I've seen a cap used all the time.

What are the three resistors (10K)?

Confused here.
 

DickCappels

Joined Aug 21, 2008
10,152
OOPs! A Rose Rosanadan moment ("Never mind"). I just simultaneously realized that the second stage gets VCC/2 for bias and clearly understood the original question.

You can use one of Bill's excellent circuits or you can probably do very well by making a voltage divider with a pair of resistors, say 3.3k or so.
 

AnalogKid

Joined Aug 1, 2013
10,987
Why not the cap AK? I think I've seen a cap used all the time.
Because unlike the 2nd schematic in bertus' post, your right opamp is acting as a comparator, not a linear amplifier. The plus input is the trip point for the comparator to change states, and must move "instantly" when it does so to effect the hysteresis effect.

The original schematic is designed for single-supply operation, as long as Vcc/2 comes from somewhere. I've always wondered why they didn't go with the two 100K resistors as in the AC coupled example.

What are the three resistors (10K)?
There were two typos in my post, both corrected now. I meant to say two 100K resistors (not 1000K), and later it was supposed to be three 100K resistors, the original feedback 100K and the two you add to form Vcc/2.

ak
 

AnalogKid

Joined Aug 1, 2013
10,987
You can use one of Bill's excellent circuits or you can probably do very well by making a voltage divider with a pair of resistors, say 3.3k or so.
Don't think so. The thevenin equivalent resistance of the divider (in your case 1.65K) directly affects the amount of hysteresis and therefore the amplitude of the triangle wave.

ak
 
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