Audio Amp Help - Basic

Thread Starter

RobD

Joined Dec 14, 2013
12
Heyo!

First time trying to use the forums here. Anywho, I've been toying around with learning op-amp audio amps and am having some trouble. I found this schematic online and whipped it up in multisim, but am only getting square waves through the output with a sine input. Is this because it is acting as a comparator? Would that be because there is no feedback resistor? Any help/advice is greatly appreciated!


alternative image link: http://imgur.com/HZoFvjm
 

shteii01

Joined Feb 19, 2010
4,644
Heyo!

First time trying to use the forums here. Anywho, I've been toying around with learning op-amp audio amps and am having some trouble. I found this schematic online and whipped it up in multisim, but am only getting square waves through the output with a sine input. Is this because it is acting as a comparator? Would that be because there is no feedback resistor? Any help/advice is greatly appreciated!


alternative image link: http://imgur.com/HZoFvjm
That does not look like amplifier.

Forget the audio for now. Just do simple amplifier, say Gain of 2, put 1 volt in, get 2 volts out.
 

Thread Starter

RobD

Joined Dec 14, 2013
12
Ok, so if I put a feedback resistor between the output and the non-inverting input, it would then become an amplifier? As for the power, what would having a positive and negative power supply alter? Forgive me for being a little naive
 

#12

Joined Nov 30, 2010
18,224
Just the basic principle that waves go positive and negative. If the reference point of the amplifier is zero volts and the audio voltage goes negative, how is it going to output a negative voltage with no negative voltage available? You have to either give the amplifier a negative voltage supply or build a circuit to keep the audio centered between zero and the positive supply voltage, then make a circuit on the output to remove the DC from the audio.
 

Thread Starter

RobD

Joined Dec 14, 2013
12
That makes perfect sense, thanks for clearing up why negative power is needed! So if there is only positive power supplied with an AC input, could you bias the output with a negative DC power source to make the output after the op amp consisted both positive and negative voltages? And what do you mean by removing the DC from the output?
 

shteii01

Joined Feb 19, 2010
4,644
Ok, so if I put a feedback resistor between the output and the non-inverting input, it would then become an amplifier? As for the power, what would having a positive and negative power supply alter? Forgive me for being a little naive
Two problems.

Problem 1.
You obviously do not understand how to setup amplifier. I suggest a quick read of the wiki. It is not the best, but it does give you the basic "meat and potatoes" of the setup.

Problem 2.
You input signal is centered on zero. For example, the sine wave would vertically go from -1 volt to +1 volt. Your power supplies are 0 volt (ground at Pin 4), and 9 volt (at Pin 7?). So. When your input is 0 to 1 volts, that part will be amplified and will be the output. When your input is any negative voltage (from -1 to 0 volts), this part will be blocked by the op amp and you will see a simple zero at the output. The two power supplies (0 volt and 9 volt) that you use only allow half of your input to be worked on and be outputted. If you want to see the negative half of your input, you need to have a negative power supply instead of zero. For example, you could setup -9 volt at Pin 4. This way you have two power supplies, -9 volts at Pin 4, +9 volts at Pin 7.
 

shteii01

Joined Feb 19, 2010
4,644
That makes perfect sense, thanks for clearing up why negative power is needed! So if there is only positive power supplied with an AC input, could you bias the output with a negative DC power source to make the output after the op amp consisted both positive and negative voltages? And what do you mean by removing the DC from the output?
The dc bias moves the ac signal up or down.

Let say you have dc bias that is 0 volts. Your ac signal is from -1 to +1 volts. So the highest point of ac signal is +1 volt. The lowerst point of ac signal is -1 volt.

Now we apply dc bias of 1 volt. What happens to ac signal? It is shifted up by 1 volt. Now your ac signal is from 0 volt to +2 volts. The highest point of the ac signal is now 2 volts. The lowest point of ac signal is now 0 volts.
 

MikeML

Joined Oct 2, 2009
5,444
All you guys failed to notice that the pictured amplifier is a LM386, and it is wired as shown on its data sheet.

If you used a real simulator, instead of that toy, you would see that the stupid function generator is overdriving the amplifier which as configured has a gain of 200.
 
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#12

Joined Nov 30, 2010
18,224
I just looked again and still totally missed the LM386 label, but I saw the 741 label. That's probably why everybody thinks it's an LM741.
 
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