# Society of Robots - Robot Forum

## Electronics => Electronics => Topic started by: annoyin_kid on February 16, 2007, 04:52:42 PM

Title: op amp questions
Post by: annoyin_kid on February 16, 2007, 04:52:42 PM
i knew this day would come so here goes. i dont know a thing about op amps ??? ??? ??? and i need it to know some things about them so that i can make a plug in speaker for a mp3 player. theres quite a few questions and any help will be appreciated.

1.) i tried my best to understand an op amp schematic but i just dont understand what pin on the ic all the lines from the triangle go to. can u guys please explain wot pin each goes to please? the op amp is a lm358

(http://www.techlib.com/electronics/graphics/audioa14.gif)

2.) what does "Total Supply Voltage (V)(Min)(+5V=5, +/-5V=10)" mean translated into english if it is equal to "7V"

3.) what does slew rate mean?

4.) what is offset?

5.) if gain=20 then does this mean that the volume is increased by 20 dB?

6.) wots the difference between harmonic distortion and distortion?

7.) in this site http://focus.ti.com/docs/prod/folders/print/tl084.html (http://focus.ti.com/docs/prod/folders/print/tl084.html) wot is the minimum voltage i have to provide to the ic so that i can run a 4ohm,6W or a 8ohm,5W speaker and how did you calculate it?
Title: Re: op amp questions
Post by: Admin on February 16, 2007, 06:31:55 PM
here is a basic tutorial on how each basic op-amp circuit works:
http://www.societyofrobots.com/schematics_voltamp.shtml
also useful:
http://en.wikipedia.org/wiki/Operational_amplifier

the op-amp on the left appears to be a non-inverting amplifier, and the one on the right looks like a differential amplifier.

the concept is that these devices take two signal input lines, two power supply lines, and one output line. depending on how you wire the inputs and output together, determines the amplification of the output.

ive never done audio amplification, but ive done amplification of many other signals for data analysis . . .

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i just dont understand what pin on the ic all the lines from the triangle go to. can u guys please explain wot pin each goes to please?
for reference, lm358 datasheet (http://www.national.com/images/pf/LM358/00778702.pdf)
it should be obvious to you with the datasheet . . . but if not . . .
looking at the datasheet, pin 4 goes to ground, looking at the schematic, pin 8 goes to 5V.
pins 2 and 6 are the lines that go to the - side, and pins 3 and 5 go to the + side. and of course 1 and 7 are the outputs.

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2.) what does "Total Supply Voltage (V)(Min)(+5V=5, +/-5V=10)" mean translated into english if it is equal to "7V"
That just means the lowest voltage you can apply to the supply lines of the op-amp. First, you need to know what voltage the amplified signal needs to be. For example, if your output is +15V/-15V, then you should supply greater than +15 to the positive supply, and less than -15V to the negative supply. In your case, you need +5V/0V.

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what does slew rate mean?
the slew rate is a nonlinear effect in amplifiers. it represents the maximum rate of change of signal at the amplifier output. basically for high bandwidth, you need a high slew rate. since the schematic specifies an op-amp for you, you dont need to worry about this.

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what is offset?
"Input offset voltage — the op amp will produce an output even when the input pins are at exactly the same voltage. For circuits which require precise DC operation, this effect must be compensated for; many practical op-amps have an offset compensation input." If you use a differential amplifier and short both lines, you still get a non-zero signal. this caused me hours of confusion back in the day . . .

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if gain=20 then does this mean that the volume is increased by 20 dB?
not exactly. gain is just the suggested maximum voltage amplification recommended.
input_voltage * 20 = output_voltage

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in this site http://focus.ti.com/docs/prod/folders/print/tl084.html wot is the minimum voltage i have to provide to the ic so that i can run a 4ohm,6W or a 8ohm,5W speaker and how did you calculate it?
no idea . . . i would check the specs on the speakers and see what kind of voltage inputs they require (i know almost nothing about audio amps). you probably need power amps if you require 6W+ . . . . i think that would fry any regular op-amp . . .
Title: Re: op amp questions
Post by: Hal9000 on February 17, 2007, 05:15:58 AM
And, as for circuits. Here are some more friendly looking sites that might be of use

http://www.techlib.com/electronics/audioamps.html#LM386

This is a cool site also

http://ourworld.compuserve.com/homepages/Bill_Bowden/homepage.htm
Title: Re: op amp questions
Post by: annoyin_kid on February 17, 2007, 03:26:49 PM

i got another question - what does low noise mean and how does it differ from something with normal noise?

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And, as for circuits. Here are some more friendly looking sites that might be of use

http://www.techlib.com/electronics/audioamps.html#LM386 (http://www.techlib.com/electronics/audioamps.html#LM386)
lol that is where i got the circuit diagram that i posted.
Title: Re: op amp questions
Post by: Admin on February 17, 2007, 04:03:06 PM
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what does low noise mean and how does it differ from something with normal noise?
All wires are also antennas. As such, all wires can pick up radio signals from cellphones, lightbulbs, and even alien spaceships. This noise can also come from your own circuit. Normally this interference is so low is doesnt affect anything, but when you amplify tiny signals, you also amplify the interference. This amplified interference could possibly cause problems, such as reduced quality in your audio amp. Low noise would just mean low interference from unwanted signals.

As a side note, digital systems inherently will reduce signal interference (noise), which is why cellphones are all digital now :P
Title: Re: op amp questions
Post by: annoyin_kid on February 17, 2007, 10:22:55 PM
how come digital systems reduce noise?
Title: Re: op amp questions
Post by: Admin on February 18, 2007, 07:41:24 AM
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how come digital systems reduce noise?
ok you just opened up a can of worms . . .

analog signals look like a squiggley sinusoidal like line. if the signal is between 0 and 5V, at any point the signal could be like 4.35V, or 1.32V, or 4.99V.

digital signals look like a staircase. if the signal is between 0 and 5V, and its a 5bit system, that means you can only get 5 steps -> 1V, 2V, 3V, 4V, and 5V.

but when the digital signal is transmitted, noise goes in doing something like 1.2V, 2.1V, 2.8V, 3.9V, and 5.1V. when the digital signal is recieved, the reciever knows its only 5 bit, so it knows that 1.2V must be 1V, and that 2.8V must be 3V, etc. the signal is instantly cleaned. this cannot happen with an analog signal. because of this, digital signals can be transmitted significantly further with less energy, and yet still have better quality too - perfect for cellphones.

have you ever heard an mp3 encoded in 64bit, and another in 128bit, and noticed the quality difference? the higher bit number means more steps, so it mimics the analog signal much better.

the problem with too high of a bit rate is, if the noise is greater than the step, the reciever would clean the signal to the wrong step. higher bit rates also require more memory than lower bit rates (more steps to encode).

are we digitally clear? :P
Title: Re: op amp questions
Post by: JesseWelling on February 18, 2007, 09:36:24 AM
111100111001011110011 ;D
Title: Re: op amp questions
Post by: annoyin_kid on February 19, 2007, 11:25:14 AM
oh so digital systems dont actually send sine waves  :o :o :o . i should be alrite now thanks for the help admin ;D ;D ;D ;D
Title: Re: op amp questions
Post by: ComputerGeek on February 19, 2007, 10:52:14 PM
NO!!! it's important to note a square wave form is a sin wave of sort. It is a signal that can be read translated resent and everything else you can think of with a sin wave. What Admin said is that digital signals produce a square wave which is 'cleaner' or easier to read and more importantly correct when recieved. Correct me if I'm wrong but sinusoidial is a mathmatical discription of what is known as an analouge signal. Sorry about stealing the light Admin but I'm trying to leave my mark here and I believe this makes a nice round 5 to finish out the day  ;D . Good Night World
Title: Re: op amp questions
Post by: trigger on February 19, 2007, 11:47:12 PM
NO!!! it's important to note a square wave form is a sin wave of sort.
If it's implemented as such, yes. But you can implement a square wave without resorting to the Fourier series (sums of sine waves). For example, if you set a digital pin out put high and then low, there's your square wave.

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Correct me if I'm wrong but sinusoidial is a mathmatical discription of what is known as an analouge signal.
Many analog signals have mathematical representations that aren't sinusoidal. The Fourier series / transform is just one convenient way to represent signals (including digital signals!). It's not the only way. You can use wavelets, for example. In fact, on a more abstract level, sinusoids are only one orthonormal basis function out of many that can be used to construct a signal.