Improving an Op Amp + Quasi-complementary amplifier

Discussion in 'The Projects Forum' started by wponline, Oct 3, 2012.

  1. wponline

    Thread Starter New Member

    Oct 3, 2012
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    Hi Circuit Gurus,

    I would like to ask your inputs and recommendations to improve this amplifier circuit driving a 4 ohms motor(Op amp_Quasi-complementary.jpg). A DAC circuit outputs a 0 to +/-10V stepped sinusoidal wave and goes to the inverting input of the op amp. A quasi-complementary provides the current drive to the motor. A +/-10V corresponds to a +/-1.5A load current.

    I've searched for more information regarding quasi-complementary amplifiers and found an improved quasi-complementary amplifier. Very, very similar but the upper and lower pairs are reversed. I have spare transistors so it would be nice to use the ones I already have in improving this amplifier.

    I hope adapting the improved circuit to the existing amplifier is as easy as plug and play.

    Would really appreciate your inputs and recommendations.

    Thanks in advance guys. Nuki
     
  2. crutschow

    Expert

    Mar 14, 2008
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    What's wrong with those circuits?

    How do you want to "improve" them?
     
  3. wponline

    Thread Starter New Member

    Oct 3, 2012
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    Hi Crutschow,

    The improved quasi-complementary, as I had read improves the linearity of the amplifier by way of the Baxandall diode. The darlington pair is in the positive half and the CFP is in the negative half. The existing amplifier has each pair in the opposite locations. How can I insert the Baxandall diode? Also, I'm wondering why the 33 ohms location is not symmetrical in each pair?

    The webage on the improved quasi-amp says the bias adjust Q5 reduces the crossover distortion plus the Q6/Q7 over-current protection is nice since i encounter blown 2n6259, 33 ohms, 2n5320/2n5322 and sometimes the 100 ohms and 2n2222/2n2907 transistors.

    How can I integrate the improved quasi to the existing amplifier?

    Thanks.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Why do you think you need ultra-low harmonic distortion at high frequencies when driving a motor?
     
  5. wponline

    Thread Starter New Member

    Oct 3, 2012
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    Hi Ron,

    I don't now the answer to your question but since this is a "precision motion control system" in the late 70's and now we're talking about 1 micron resolution. I can only speculate at this time (don't have the knowledge and skill to quantify) that at the crossover point and this may relate to minute positions that is not readily measured. The motor is a linear motor and there is no encoder that says it got to the position it was commanded to. Simply put, if there is crossover error, this "could" translate to positional errors where the amplifier is not very linear, proportionally speaking.

    Is there no harmonic distortion at low frequencies? Related to crossover distortion?

    Thanks.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't have immediate answers to your questions, but do you know what the required bandwidth of your amplifier?
     
  7. wponline

    Thread Starter New Member

    Oct 3, 2012
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    The input op amp is direct coupled and there is a 100pf capacitor between base and collector of the driver transistors. I've read somewhere in one of the links here at the forum (another topic) that the input capacitor is responsible for the low frequency cutoff and the capacitor at the driver limits the maximum frequency. Since there is no capacitor, then I think that this amplifier's bandwidth is limited to 0 Hz to whatever the formula (which I don't know) that you can use the 100pF capacitor. I maybe wrong though...


    Had retraced my previous searches, here's the link: http://electronics-diy.com/60W_2N3055_Power_Amplifier.php
     
    Last edited: Oct 3, 2012
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Sorry, I meant the high frequency bandwidth. You mentioned a stepped sine wave input. What is the maximum sine wave frequency?
     
  9. wponline

    Thread Starter New Member

    Oct 3, 2012
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    Hi Ron,

    I need to check the DAC output and will get back to you on that.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Also, how many bits in the DAC input? Or, how many steps in the ±10V range?
     
  11. wponline

    Thread Starter New Member

    Oct 3, 2012
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    Hi Ron,

    10-bit DAC and 1 sign bit. For a complete wave, there would be 200 steps.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    10 bits is 2^10=1024 steps. With sign, that will be 2048 steps.

    Do you have the sine wave frequency yet?
     
  13. wponline

    Thread Starter New Member

    Oct 3, 2012
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    Surprisingly, that would be the case but there is a look up table contained in an EPROM and the output data is fed to the DACs. The gap in the bits allows the EPROM to tailor the waveform specifically to the motor.

    I had measured the output of the DAC and the frequency of the sinusoidal output is 375 Hz.
     
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