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I am making a practical audio amplifier with Input from the mobile headphone jack(though here it is a sine wave) . My speaker is Rl with resistance 8ohm and 0.5W. So, I made one on multisim (photos attached). Now, when I have made it on the breadboard, but now there is no voltage across the speaker. What is wrong in my amp? enter image description here

Edit : sorry for the late reply. I have taken my idea of the amplifier from this site: http://jamesoid.com/class_a+ab_amplifier_project/class_a+ab_amplifier.html So is this design correct?

shodan
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    You photographed the screen? Seriously!? Learn to use a computer. Also clean up the schematic. Those PNP transistors drawn sideways, then with wires crossing every which way make it difficult to see the circuit from the schematic. At first glance the two output transistors are biased to blow up, and these small signal transistors are inappropriate for the peak currents of about 1 A that you are expecting. But you need to first clean up the schematic and present it properly before we go any further. – Olin Lathrop Nov 25 '16 at 21:25
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    (1) The schematic is a mess. It needs to be redone, if you want to respect the time of others from whom you are asking their precious time. (2) Since you don't say you designed this, you should provide a link to where you got it; or else, (3) You should provide your design thinking here. (4) You breadboarded this but apparently didn't take any measurements except to say that there is no voltage across the speaker. You might at least provide some helpful data, if you seek help. We aren't going to breadboard it for you and make our own measurements. A photo would be nice, too. – jonk Nov 25 '16 at 23:43
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    You could start by drawing the schematic diagram correctly, Then you can proceed to whether the circuit itself is correct. Please refer to: http://electronics.stackexchange.com/questions/28251 – Richard Crowley Nov 25 '16 at 23:47

1 Answers1

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You are missing DC negative feedback to stabilize the operating point. The emitter of Q1 should be set at 1/2 of Vcc, or (1/2)*9 = 4.5V in your case.

Also, your simulator is using "model" transistors, not real ones. In the real design, you will probably not be using matched NPN-PNP transistors. Q1 and Q2 will need emitter-ballast resistors.

Your simulation might work OK, but the characteristics of real parts will vary. If your simulation included Monte-Carlo analysis, you could start to see the problem :-)

I would look at some other published transistor power amp designs to get more clues.

Rich S
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