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enter image description here

I'm need of an amp that could charge small capacitors 'fast'. The capacitors are actually a disc with one side grounded and the other side coated in oxide so effectivly this is a series of small caps as the disc keeps moving. By charging the caps to different voltages, data is stored on the disc.

The circuit is not very complex, there is a non-inverting JFET input op amp that gets an audio signal and outputs it into a class AB BJT amp. The transistor bias is set using a textbook trick for self biasing of sorts. The negative feedback for the op amp is taken from the output of the transistor amp through a resistor that sets the gain together with R8 (it's actually a 50K trimmer). The problem I am having is that the output is distorted at any gain setting. Is there something fundamentally flawed in the design?

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2 Answers 2

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There is no DC feedback in this amplifier at all so the LF347 will hit the end stops and distortion will prevail: -

enter image description here

Where did you get this crappy design from? Also, why are you wanting to use a class AB amplifier to charge capacitors? Why not just use a MOSFET switch?

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  • \$\begingroup\$ True, I did not notice there is no DC feedback, so I need to add some feedback from the output of the op amp before the 100uF cap? How can I determine the value of this resistor, but is should be a fixed value right? The same value should work for whatever gain setting I have there? This is sourced from a user manual of an echo unit. How can I use a mosfet switch to charge in co-relation to a modulating waveform? \$\endgroup\$
    – user34920
    May 18, 2014 at 10:48
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    \$\begingroup\$ Before I give more advice on your amplifier you should explain and justify why you need to use the amp to achieve what you want. \$\endgroup\$
    – Andy aka
    May 18, 2014 at 10:48
  • \$\begingroup\$ fair enough, I simply start with a given design because I don't want to re-invent the wheel sort of speak. An op amp by itself won't be able to to drive the low capacitive load (I tried). \$\endgroup\$
    – user34920
    May 18, 2014 at 10:51
  • \$\begingroup\$ Then don't use that op-amp - you can get op-amps that can drive several amps OR add a simple one-transistor buffer onto an op-amp. You don't need a push-pull class AB stage for charging a capacitor dude. \$\endgroup\$
    – Andy aka
    May 18, 2014 at 10:59
  • \$\begingroup\$ OK, so you don't want to reinvent the wheel. ti.com/lit/an/snoa600b/snoa600b.pdf \$\endgroup\$
    – WhatRoughBeast
    May 18, 2014 at 11:13
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You may be able to save this design by making a few changes to the circuit.

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Whether this meets your needs will be impossible to tell because it is not known just exactly what your parametric requirements are. At least this minimizes the distortion at the output.

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  • \$\begingroup\$ that's great and very elegant :) \$\endgroup\$
    – user34920
    May 18, 2014 at 15:54
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    \$\begingroup\$ @Michael - How about, just for fun, you add a 1uF cap to ground to Vout? \$\endgroup\$
    – WhatRoughBeast
    May 18, 2014 at 17:57
  • \$\begingroup\$ @WhatRoughBeast - Yeah, I know. Try even a 50 or 100 ohm load to GND from Vout. I didn't put a load and this because the original poster never even showed where he intended the output to be then yet what the load would be - other than his vague description about some disks. It's still a crappy circuit. \$\endgroup\$
    – Michael Karas
    May 18, 2014 at 19:30
  • \$\begingroup\$ @MichaelKaras - Yup. But notice that if you didn't increase the gain, it would do okay for 50 ohm loads. \$\endgroup\$
    – WhatRoughBeast
    May 18, 2014 at 20:55

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