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For my Electronics II class, I designed a discrete BJT amplifier circuit including a VGain, Buffer, and Power stage.

I succeeded in meeting the specs, and the next step is to improve our design in one of a few specific ways. I chose to improve the power output of my circuit. My plan has been to build two amplifier channels and bridge them.

To do so, I have added an inverter and a second buffer stage to the second channel. Currently I have the circuit set up in CircuitLab, and it simulates satisfactorily. The circuit is attached, along with a plot of the output.

My question is two fold.

The first arises because when the channels weren't balanced, I was getting an ever increasing static voltage at the output. I want to be able to tie the output to ground if possible. I found that adding a 1H inductor could do so, but I don't want to add a 1H (LOL) inductor to the output. Is this circuit safe to build, will I see real results like my simulation, and is there any way I can tie my output to ground so the DC offset is 0V?

Two, do you have any recommendations to decrease distortion, or increase power?

Circuit schematic of the amplifier stages

Plot of simulation results of signal asymmetry with some visible mid-scale distortion

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  • \$\begingroup\$ I've found that you can open the image in a new tab to view it better. \$\endgroup\$
    – reedjay
    Commented Feb 25, 2015 at 0:47

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You can't have a bridge tied load and then ground the load too.

To reduce distortion, first find out where the distortion is.
(Which stage to change first?) And then look what others have done...
or ask a more specific question here.

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  • \$\begingroup\$ Okay, then my only specific question is, is this circuit ok to build? Or is there some kind of fatal flaw? Thanks again! \$\endgroup\$
    – reedjay
    Commented Feb 25, 2015 at 2:46
  • \$\begingroup\$ Yeah it looks fine... (I didn't check values or things... the image was too small, only the topology) Build it a stage at a time and check that you understand the output. (Oh I should add that it is a nice schematic.. I should add +1 for an easy to read schematic.) \$\endgroup\$
    – George Herold
    Commented Feb 25, 2015 at 2:48
  • \$\begingroup\$ Thanks for the compliment. I've found I really enjoy using CircuitLab over LTSpice. I'm just going to take your response as an answer and breadboard the darn thing. I'll post OScope results once I've finished. \$\endgroup\$
    – reedjay
    Commented Feb 25, 2015 at 3:30
  • \$\begingroup\$ OK I looked at the schematic... one thing you might want to increase the supply voltage. 5V is not very much with diode drops here and there. \$\endgroup\$
    – George Herold
    Commented Feb 25, 2015 at 13:52
  • \$\begingroup\$ Haha, I would like to, but it's supposed to be USB powered. I suppose I should have said that. My roommate is actually building a buck boost converter to increase the voltage in his design. I may consider something similar. \$\endgroup\$
    – reedjay
    Commented Feb 26, 2015 at 0:58
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One caution with the bridge-tied load....

Your load should be considered 8 ohms in series with a 1000uF capacitor. This will be a polarized electrolytic capacitor, whose polarity must be carefully observed. (Reverse DC voltage across such a capacitor spells disaster).

It will be inevitable that one of your two power stages will output a DC voltage higher than the other. But which one? You could measure them, and then place the electrolytic capacitor polarity appropriately.

For the single-ended case where only one power stage is used, there is no polarity problem - the negative end of the capacitor faces the negative end of the USB supply.

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