Spice problem or Design issue with differential Amplifier - diyAudio
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Old 19th June 2012, 03:58 AM   #1
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Default Spice problem or Design issue with differential Amplifier

Greetings everyone! I am not sure if this is the forum I should put this in, but since Spice is involved I figured it wouldn't be too out of place!

I've been putting together an audio amp for college project and I've run into a bit of a problem but I am not sure if it's a Spice problem or a design problem. The amp is essentially a booster for MP3 players for small speakers. The output stage is essentially variation of a differential amp, or at least that is what I was taught in the Senior Level Electronics class.

The idea behind the output is that you take 2 AC signals 180 degrees apart and put the load across them hence doubling the Peak-to-Peak voltage. Take a Wall outlet for example. If it's a 110/120v RMS then its roughly 170v to -170V peak to peak. BUT Those voltages are in reference to ground or 0 (Zero) volts... If you had another Wall outlet that was 180 degrees out of phase and you hooked a load across both Wall outlets instead of 0v to 170v for the first half of the sine wave you would get -170 to 170v. One of the lines acts like varying ground to double the voltage you see. This is how some houses get 220/240 RMS outlets.

With that in mind here are two amp configurations with almost identical output amplitudes. There is are 180 degree phase difference.

Click the image to open in full size.

The colors represent what the output waves below are. The Blue color is the upper amp's output before the capacitor and the green color is the lower amp's output before the capacitor. The red line is for the difference between both points.

Click the image to open in full size.

As you can see the voltage swing(peak-to-peak) of each amp by itself is about 4 volts. However, the swing between the two points is 8 volts; double either amp by itself.

Now, when I connect the load across both amps as shown below, the output swing gets LOWERED down to about 3.2 volts. The Black line between both green blocks represents the two points the voltage is being measured at.

Click the image to open in full size.

Output:

Click the image to open in full size.

Why don't I get 8 volts from Peak-to-Peak? Why is the output of the two amps so much lower when they are connected than when they are measured while being connected individualy?

Thanks for your time.
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Old 19th June 2012, 04:29 AM   #2
kevinkr is offline kevinkr  United States
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Loss across your 5.6 ohm emitter resistors.

Note that I expect if you are building this you aren't actually using 2N3904/6 devices as outputs..
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Old 19th June 2012, 04:44 AM   #3
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I was actually. I heard they are good up to 500mW and I am looking to get a maximum of 350mW so I figured they should be fine... That and I've never had to pick parts out before so they are literately the only transistor models I know. Kinda like the only OP-Amp I am familiar with is the LM741...

Edit: Holy Hell I think I found the problem. I just noticed the signal loss across the output capacitor is over 95% on the first two amps. I thought AC is supposed to pass through Capacitors unhindered?!

EDIT2: yea it appears you are right I got most of my signal back from changing the 5.6 resistors to 2 Ohm.

EDIT3: FU*&^#$%#&! I was hoping to use this amp for headphones and speakers but it looks like the full bridge model can't work for headphones because headphones use 3 wires. They have a common ground. I don't think this will work if two speakers you are using for 2 sets of these amps have a common ground.

EDIT4: Yea, it definitely won't work with a common ground between the speakers. Here is what happens if you have 2 Amp circuits and the first one has an input at 300 Hertz and the other input is at 3000 hertz:
Click the image to open in full size.

Back to the drawing board!

Last edited by Nubcakes; 19th June 2012 at 05:14 AM.
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Old 19th June 2012, 07:26 PM   #4
macboy is offline macboy  Canada
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You had two 8 ohm loads (to ground) first, then replaced those with a single load (floating, not grounded). It should be 16 ohms (two 8 in series) but you made it 8 ohms. It pulled twice the current from the amps, and this reduced the voltage swing, since you have a non-feedback amp topology.
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Old 19th June 2012, 08:27 PM   #5
macboy is offline macboy  Canada
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As for the capacitors, you specified "0.00001" which is 10 uF (microFarads). For an 8 ohm load, you should use something like 2200 uF or more. Basically, the impedance of that capacitor must be much less than the impedance of the load (8 Ohm) at the lowest frequency you will use (< 20 Hz), in order to minimize bass roll-off. You know that Z=1/(2*pi*f*C).

(You can specify "10u" instead of "0.00001" in any variant of spice).
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Old 20th June 2012, 01:10 AM   #6
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Here is the next question; Assuming you have two bridged amps for a stereo audio signal similiar to the one I posted above, there is no common ground for the outputs with the configuration I have. How could you create one? I was potentially thinking of using one of those small audio transformers but the frequency response is pretty bad below 500ish hertz.
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Old 20th June 2012, 12:29 PM   #7
macboy is offline macboy  Canada
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Quote:
Originally Posted by Nubcakes View Post
Here is the next question; Assuming you have two bridged amps for a stereo audio signal similiar to the one I posted above, there is no common ground for the outputs with the configuration I have. How could you create one? I was potentially thinking of using one of those small audio transformers but the frequency response is pretty bad below 500ish hertz.
You can't.
Short of using a transformer as you suggested, there is no way. You can't have a common connection between the two channels.
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Old 20th June 2012, 01:45 PM   #8
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It may be possible to use three amps with one being a virtual ground and still have a bridged configuration.
But the virtual ground amp would have to have twice the current handling capability of that of one of the single amp stages.
But this would basically be a waste of circuitry as it would be easier to just increase the power supply voltage in order to get the required voltage swing that you are after.

FWIW

jer
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Old 20th June 2012, 01:54 PM   #9
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Quote:
Originally Posted by geraldfryjr View Post
It may be possible to use three amps with one being a virtual ground and still have a bridged configuration.
But the virtual ground amp would have to have twice the current handling capability of that of one of the single amp stages.
But this would basically be a waste of circuitry as it would be easier to just increase the power supply voltage in order to get the required voltage swing that you are after.

FWIW

jer

Hmm... I may end up trying this if it doesn't murder the efficiency because increasing the Voltage Rail beyond 7.1 is out of the question. The source I am using has high current but is only 0.9 voltage. I already have to use multiple stages of boost converters just to get to 7.1v and the efficiency already suffers pretty badly (67%) because of it. Merely increasing the rail to 7.5v drops the efficiency to 55ish%
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Old 20th June 2012, 02:44 PM   #10
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It has been a while,So if I did this right, This drawing is basically the configuration that you should have.
I didn't work out any of the gains of the stages but it should give you an idea as to where to start.



jer
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