# Trying to learn about opamps

Joined Mar 4, 2020
41
Hi everyone,
I'm reading about opamps and trying to understand how they work. The subject is a bit over my head but it's a good way to learn.

Consider my use case: I have a digital circuit that generates two square waves from 0v to 2.5v that I would like to feed into an amplified speaker or headphones.
1) So I understand how to sum these 2 signals. But do I even need to amplify if I'm already going to an amplified speaker?

2) Let's assume that I do need to amplify. I read everywhere about inverting amplifiers using an op-amp. Using an inverting amp seems a bit complicated since I need to feed a negative voltage to the op-amp. My circuit is mostly digital so this is kinda exotic to me. Why is the inverting amp so popular? Would there be any downside for me to use the non-inverting version?

3) And if I do end up using the inverting amp, can I just connect the +vcc of the opamp to 0v and the -vcc to +5?

Thanks for the help

#### BobTPH

Joined Jun 5, 2013
7,562
You can combine the two signals with just resistors. At 2.5V, the signal id actually high for an amplified speaker.

Bob

#### Ramussons

Joined May 3, 2013
1,356
1) So I understand how to sum these 2 signals. But do I even need to amplify if I'm already going to an amplified speaker?
No, there is no need to amplify if it is an Ampli speaker. In fact, you may have to reduce that 2.5 volts by about 10 times to avoid overloading the ampli speaker input.

... since I need to feed a negative voltage to the op-amp...
Not too sure what you mean by a negative voltage. There will be a negative voltage fed to the opamp (depending on the configuration) whether or not it is an Inverting or Non inverting setup.
If you mean a Negative Feedback, again, it will be required whether or not it is an Inverting or Non inverting setup.

That aside, since your signals are all Digital, simply use a AND (NAND) gate to mix them, use a FET (mainly current gain) to drive the stand alone speaker.

If it is an Ampli speaker, make a combiner using two 10 K Ohm and one 1 K Ohm resistors and feed the speaker.

#### elvil

Joined Jul 11, 2019
1
Sergio Franco has an excellent book :

#### Papabravo

Joined Feb 24, 2006
20,400
After you combine the two signals, you can feed the output to the amplified speaker. The amplified speaker should have the volume control on a low setting. As you increase the volume setting you should find a comfortable level. Square waves into a speaker sound pretty awful if youve never listened to them before.

#### ericgibbs

Joined Jan 29, 2010
18,059

E

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Joined Mar 4, 2020
41
@Ramussons , I'm refering to the negative voltage supply for pin4 on the LM741. The designs I see are providing +12, -12 to pin7,4

What I'm wondering is if this would work if I feed 0,+5 to pin 7,4 insteads.

Using a simulator, I see that If I feed a 0-5v signal on pin2, then I would get upside down square wave on pin 6. But I am getting 0 because I give 0v to pin4
So I see 3 options:
1) I properly feed -12, +12 to the chip
2) I feed my signal to the non-inverting pin (but why isn't everybody doing that? why is that everything I see is using the inverting input?)
3) I feed 0, +5 to pin 7,4. But would that even work?

Joined Mar 4, 2020
41
Thanks to everyone posting links to documents and resources, but I am currently full of very good documentation and good EE books. I've resorted to asking questions on a forum because it seems that I am missunderstanding a basic concept somewhere but all books assume that I have well understood the prior material. If I was a few years younger, this would be the point where I'd go ask a question to the teacher

#### Ya’akov

Joined Jan 27, 2019
8,180
I don't know what your current level of understanding is, but this video from Dave Jones might clear some things up and make it easier to understand the written material. Dave is very good and knows his stuff. I have a little trouble with his relatively high pitched voice and some things about his presentation but I have found the content worth the discomfort most of the time.

#### AnalogKid

Joined Aug 1, 2013
10,589
You are going to get a lot of responses about inverting cirucits, non-inverting circuits, how they are different, how they are the same, etc. . . . . . . But to answer your question:

do I even need to amplify if I'm already going to an amplified speaker?
Here is the very short form:

1) No.

2) See #1.

3) See #1.

ak

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#### drjohsmith

Joined Dec 13, 2021
793
@Ramussons , I'm refering to the negative voltage supply for pin4 on the LM741. The designs I see are providing +12, -12 to pin7,4
View attachment 258152
What I'm wondering is if this would work if I feed 0,+5 to pin 7,4 insteads.

Using a simulator, I see that If I feed a 0-5v signal on pin2, then I would get upside down square wave on pin 6. But I am getting 0 because I give 0v to pin4
So I see 3 options:
1) I properly feed -12, +12 to the chip
2) I feed my signal to the non-inverting pin (but why isn't everybody doing that? why is that everything I see is using the inverting input?)
3) I feed 0, +5 to pin 7,4. But would that even work?
Using an op amp ,
a few basics to help understand , as a first degree this will help

1) input impedance is infinate
2) output impedance is zero
3) voltage difference between the + and - input is zero.

Try working out the equation for the gain of the inverting and none inverting amp circuit.

#### MrChips

Joined Oct 2, 2009
29,251
Opamps are generally used in low power applications, not for driving a loud speaker.

The basic opamp has moderately high input impedance. You want your input source to have a lower impedance.
The rule of thumb is that the driving (source) impedance should be ten times lower than the receiving (load) impedance.
Thus, if the input impedance is 100kΩ, the driving impedance should be 10kΩ or lower.

(Here, we are talking about the input side of the opamp, not its output.)

Now we can sum the two input signals using a summing amplifier, using the criteria given above. You very likely can use a voltage gain of unity.

Inverting Summing Amplifier

If you make R1 = R2 = R3 = 100kΩ you will have a voltage gain of -1 with 100kΩ input impedance.

The output of the opamp must be fed to a loudspeaker amplifier. You cannot drive the loudspeaker directly from the opamp.

Joined Mar 4, 2020
41
Using an op amp ,
a few basics to help understand , as a first degree this will help

1) input impedance is infinate
2) output impedance is zero
3) voltage difference between the + and - input is zero.

Try working out the equation for the gain of the inverting and none inverting amp circuit.
Yes, I've seen those rules in pretty much every documentation about opamps so far. But I still can't figure out, based on that, what reversing the polarity on Vs and -Vs (pin 7 and 4 of lm741) would do.

#### MrChips

Joined Oct 2, 2009
29,251
Yes, I've seen those rules in pretty much every documentation about opamps so far. But I still can't figure out, based on that, what reversing the polarity on Vs and -Vs (pin 7 and 4 of lm741) would do.
You cannot reverse the polarity.
It is difficult for electrons to cross an N-P semiconductor barrier when reverse biased.

#### AnalogKid

Joined Aug 1, 2013
10,589
I still can't figure out, based on that, what reversing the polarity on Vs and -Vs (pin 7 and 4 of lm741) would do.
Destroy the part, almost immediately.

ak

#### Audioguru again

Joined Oct 21, 2019
6,169
The 741 opamp is an antique that is 53 years old and its datasheet shows that it works ONLY when its power supply is +15V and -15V but some of them work with a +5V and -5V supply. It will not work properly with a power supply that is only +5V and 0V.

Many new opamps work perfectly from a power supply that is +5V and 0V but not when they are inverting and have a positive voltage as the input since then the output must be negative (a positive input is inverted into a negative output).

#### MrChips

Joined Oct 2, 2009
29,251
I still can't figure out, what reversing the polarity on Vs and -Vs (pin 7 and 4 of lm741) would do.
You can try this simple experiment yourself.

Reverse the battery connection and see what happens.

#### sparky 1

Joined Nov 3, 2018
735
Another video the author gets out his trusty clipboard working step by step at the bench. He uses the multimeter and scope.
The 1.3Vpp shown on the scope was close to the gain that he calculated. He could always adjust or trim is not confined by only fixed value.
Yes, there there should be a preamplifier at least a voltage follower stage but in this an oscilloscope will be enough.

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