saturated operational amplifier
Hi all!
This is my first post so, sorry if I would not be proficient in discussing electronics or if maybe this is to demanding for the forum 😒
I am stuck in a practical experiment, I studied how amplifier for obtaining gain work. And I created a small circuit with an astable multivibrator, and operational, 2 resistors, two batteries of 9v:
Something like this:
![[IMG]](proxy.php?image=http%3A%2F%2Fimg253.imageshack.us%2Fimg253%2F5039%2Fcircuitgain.jpg&hash=a8b4374ef42d501409a2771a56d29af8)
Where the settings are :
non inverting input.
Rf = 300 K, Rg=150K
And the expected gain should be : Av= 1 + Rf/Rg = 3.
Doing a simulation I obtain reasonable results.
Where the output in volts of the operational is effectively augmented.
But when I build the real circuit it doesn't happen:
The oscillator generates a square wave that I can detect with
oscilloscope and even with a tester (between -4.80v +4.80v ), the output of the amplifier is instead stuck to 7.8v.
I have done this with a LM741, and also using a LM386 (one with
single supply) and nothing.
The crazy thing is that I obtain constant 7.8 v independently from the value of the feedback resistors (like they don't matter) and even if I completely disconnect the input.
So, it is like if whatever I do with this configuration
![[IMG]](proxy.php?image=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F6%2F66%2FOperational_amplifier_noninverting.svg%2F146px-Operational_amplifier_noninverting.svg.png&hash=f41b05135e55bd9b84b38046ac01c06e)
the amplifier saturates to 7.8v and end of the story.
I also tried the inverting input configuration and nothing.
Someone told me I need to insert a capacitor between the two part of the system and one in the output., but what is the logic in this ?
1- Why do I need a cap between the 555 and the op? and what should be its capacity ?
2- Putting a cap am I not interfiring in some kind of way into the frequency (it is a relative of an high pass in that way) ?
3- Why and what should be the capacity in the one in the output ?
4- Assuming i connect a photodiode with a transinpedence amplifier instead of the
square wave generator should I also put a cap before the amplifier ?
5- Since the LM386 is single supply if I use it instead of the LM741, should I add some kind of virtual ground in addition to the cap ?
Thank you for the patient 😁
This is my first post so, sorry if I would not be proficient in discussing electronics or if maybe this is to demanding for the forum 😒
I am stuck in a practical experiment, I studied how amplifier for obtaining gain work. And I created a small circuit with an astable multivibrator, and operational, 2 resistors, two batteries of 9v:
Something like this:
Where the settings are :
non inverting input.
Rf = 300 K, Rg=150K
And the expected gain should be : Av= 1 + Rf/Rg = 3.
Doing a simulation I obtain reasonable results.
Where the output in volts of the operational is effectively augmented.
But when I build the real circuit it doesn't happen:
The oscillator generates a square wave that I can detect with
oscilloscope and even with a tester (between -4.80v +4.80v ), the output of the amplifier is instead stuck to 7.8v.
I have done this with a LM741, and also using a LM386 (one with
single supply) and nothing.
The crazy thing is that I obtain constant 7.8 v independently from the value of the feedback resistors (like they don't matter) and even if I completely disconnect the input.
So, it is like if whatever I do with this configuration
the amplifier saturates to 7.8v and end of the story.
I also tried the inverting input configuration and nothing.
Someone told me I need to insert a capacitor between the two part of the system and one in the output., but what is the logic in this ?
1- Why do I need a cap between the 555 and the op? and what should be its capacity ?
2- Putting a cap am I not interfiring in some kind of way into the frequency (it is a relative of an high pass in that way) ?
3- Why and what should be the capacity in the one in the output ?
4- Assuming i connect a photodiode with a transinpedence amplifier instead of the
square wave generator should I also put a cap before the amplifier ?
5- Since the LM386 is single supply if I use it instead of the LM741, should I add some kind of virtual ground in addition to the cap ?
Thank you for the patient 😁
Replies
-
freak16Okay i got your problem..
Well you apply a 9v battery so Opamp can never exceed the output more than baising voltage see acc to you output should be 4.8*3=14.4v
But you seee its dc characteristics you will get to know.. -
freak16Refer any good book to see its DC characteristics like D Roy chaudhry You maynot get on net..but a book will definitely clear youur concept for the same.
-
SyncopatorIf you look at the 555's data sheet you will see that its output, when low, doesn't go completely down to 0V. It can be as high as 30mV or so, so the amplifier's output will always be high.
It may be possible to overcome this by using the op-amp's offset pins and adjusting the potentiometer to see if the amplifier's output changes state correctly.
If not, then you will have to return the 150k in the feedback, not to 0V, but to a small positive voltage as shown here:
![[IMG]](proxy.php?image=http%3A%2F%2Fi225.photobucket.com%2Falbums%2Fdd27%2Fsyncopator%2F555outputcomp.gif&hash=8333d541236b34cbfabdbe0070ea690e)
Why have you set the gain of the amplifier to 3? The voltage at the 555's output should be near the supply rail, either at 9V or 0V. Since the amplifier is run from +/-9V its output can't be more than that.
You are reading an archived discussion.
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