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Old 12th July 2003, 02:30 AM   #1
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Default Why wouldn't this work?

so I am playing around with an op-amp based power amp. Here it is.

The design goal is to use CFP OPS to expand the output voltage swing. This way, it wouldn't be limited to the +/- 15v of the op amp. or so I thought.

This is based on the op amp concept of ESP. Changes I have made are:

1) It originally used TL074 and I substituted it with LT1057. Why? LTSpice doesn't have TL074, .
2) two zeners used to supply +- 15v to the op amp but 26v to the ops.
3) the original "compound pair" ops was replaced with CFP ops with 10x voltage gain (as determined by r2/r12, and r10/r9).
4) I used just 2n5550 and 2n5401 as drivers and output devices to prove the concept.

here is what I got.

the amp works just fine if the overall voltage gain is less 15x, thus staying within the output voltage limit of the opamp.

Higher gains will result in clipping. Namely, the cfp is not providing any voltage gain.

Why?

Thanks in advance.
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Old 12th July 2003, 03:19 AM   #2
andy_c is offline andy_c  United States
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Millwood,

The CFP has a voltage gain of one because of the 100% feedback from second stage collector to first stage emitter. You're seeing performance that's consistent with what the actual circuit will do.

You'll need something along the lines of the schematic in the post referenced below. Note the CE amp driven by the current drawn from the power supply rails of the op amp.

http://www.diyaudio.com/forums/showt...570#post191570
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Old 12th July 2003, 03:38 AM   #3
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Quote:
Originally posted by andy_c
Millwood,

The CFP has a voltage gain of one because of the 100% feedback from second stage collector to first stage emitter.

that's what I had suspected. Basically, the feedback pick up is from the emitter of the driver, so that would give a 1x voltage gain. I will toy around with it a little bit to see if I can make it work.

the circuitry you referred is the basic bootstrapping on op amp to expand their output voltage. Part of the reason for me to try a cfp ops is to avoid bootstrapping..
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Old 12th July 2003, 03:49 AM   #4
andy_c is offline andy_c  United States
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If you do end up going with a design using the bootstrapping approach, the op amp models in LTSpice don't handle this case correctly at all. They are the old Boyle macromodels for which the supply current doesn't vary with changes in the load current. It's a constant - totally unrealistic. This can be fixed by downloading Analog Devices models and incorporating them into LTSpice. These models handle the supply current variation with load correctly.
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Old 12th July 2003, 07:11 AM   #5
sajti is offline sajti  Hungary
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"Namely, the cfp is not providing any voltage gain"

Of course not.

If You need any gain just connect the pin between R10, and R12 to the ground, and You can get gain.
But I think I can make some modification with Your schematic:
1, The bias at the output stage is not so stable with this schemaitc. Use small emitter resistors to set the bias.
2, Set the gain of the output stages, and set the bias too. Use small collector resistors for the output devices, and make nfb. from the collectors back to the driver's emitter. Set the gain -say- 2, and the result will be excellent.
3, Use current regulator instead of R11, and R1. This means lower load for the opamp, and lower distortion.

Check this amp:
Click the image to open in full size.

Sajti
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Old 12th July 2003, 07:26 AM   #6
AJT is offline AJT  Philippines
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despite rails of +/-26 volts for the output devices, your output will still be limited to less than +/-15 volts, the capability of the op-amp, unless you design some voltage gain at the output stage, daniel meyer did it in the late 60's....
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Old 12th July 2003, 07:37 AM   #7
PMA is offline PMA  Europe
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millwood,

here is the example of CFP with voltage gain of +1. Please see the difference to your circuit.

Pavel
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Old 12th July 2003, 07:38 AM   #8
sajti is offline sajti  Hungary
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Quote:
Originally posted by PMA
millwood,

here is the example of CFP with voltage gain of +1. Please see the difference to your circuit.

Pavel

As I understand millwood want to make some gain with his cfp output. In that case he need to put two more resistors series into this schematic.
Two - say 47ohm- resitors. One between the collector os the Q5 and the emiiter of Q3, and one between the emitter of Q3, and the ground. With this action he has the gain about 2.

Sajti
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Old 12th July 2003, 07:54 AM   #9
PMA is offline PMA  Europe
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sajti,

you are right, but the way you describe you will not have power amplifier (47R resistors in a current path). Your recommendation is good for a preamp. And output should be taken from collector of Q5 again, how to couple with bottom power branch?

Pavel
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Old 12th July 2003, 08:03 AM   #10
PMA is offline PMA  Europe
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sajti,

did you mean this?
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