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op amp questions

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annoyin_kid:
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



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 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?

Admin:
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 . . .


--- Quote ---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?
--- End quote ---

for reference, lm358 datasheet
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.


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

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.


--- Quote ---what does slew rate mean?
--- End quote ---

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.


--- Quote ---what is offset?
--- End quote ---

"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 . . .


--- Quote ---if gain=20 then does this mean that the volume is increased by 20 dB?
--- End quote ---

not exactly. gain is just the suggested maximum voltage amplification recommended.
input_voltage * 20 = output_voltage


--- Quote ---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?
--- End quote ---

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 . . .

Hal9000:
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

annoyin_kid:
alrite thanks admin

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


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

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

lol that is where i got the circuit diagram that i posted.

Admin:

--- Quote ---what does low noise mean and how does it differ from something with normal noise?
--- End quote ---
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

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