LM13700 and transconductance transconfusion

Discussion in 'General Electronics Chat' started by JingleJoe, Feb 27, 2012.

  1. JingleJoe

    Thread Starter Member

    Jul 23, 2011
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    Hello people of Earth,
    I come to you from a distant and remote satelite called Kenphuscien.

    I've been trying to understand the LM13700 OTA (operational transconductance amplifier) (datasheet) in order to make a voltage controlled amplifier (VCA) I get the basics of operation; current applied to the amp bias input controls conductance however I don't quite understand the first VCA circuit in the datasheet (figure 2) or why there is diode bias on the inputs?

    Why don't they use the ordinary arrangement for a differential amplifier or even an inverting amplifier?

    By the By I'll be running my circuits on a single supply (+12V) ordinarily I'd apply half Vs to the non-inverting input of an op amp to make a single supply, non-inverting amplifier ... but the arrangement suggested in the LM13700 datasheet is strange and confusing :confused:

    I'm reading and re-reading the datasheet and it slowly making more sense (but why don't they just use an ordinary non-confusing op amp arrangement!?!?!? :()

    My point is; another person's explanation would help me understand more, another wording or a different take on the description of the VCA would shed more light.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    They don't "use an ordinary non-confusing op amp arrangement" because it isn't an ordinary op amp. It's a transconductance amp, which is useful for adjustable gain applications and has only a passing resemblance to a voltage controlled op amp.

    It states the input diode bias is to improve linearity so that's why it's there. As to how it improves linearity, read the Circuit Description for more detail on that.

    So I would recommend just building the amp using one of their cookbook circuits and let it go at that. The Circuit Description provides information on its operation and you need to further study that if you want a better understanding of how it works.
     
  3. #12

    Expert

    Nov 30, 2010
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    I go through this every time I need an OTA. They are so different that I have to read the datasheet at least 5 times before I resume being able to think in those terms.

    I'd really like to see the whole thing arranged differently so I could think in terms of a VCA, but you know how nerds are. They don't bother to make it easy for everybody else. So self centered! ;)
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    why would one need to improve the linearity? What if I don't care about linearity? Can I just leave the diodes disconnected?



    I am glad I am not alone :) The only schematics I have are the VCA circuit from the datasheet, so far.

    I had a look at that one but it just raises more questions for me, why has Mr Wilson cinluded a voltage divider on the buffers? I assumed they didn't need biasing like a transistor amplifier?


    I'm not looking forward to all the reading I have to do about this :( due to autism it goes in about as well as sand into a seive. I could really do with more diagrams, pictures and consice captions ... I wonder if falstad has anything on this ...
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Linearity affects the amount of distortion generated by the amp. If you don't care how distorted your signal is then you don't have to connect the diodes.
     
  7. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
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    It would help if they included some scope drawings or photos in the datasheet, so if the amp is non-linear does that mean the wave gets a bit stretched or squashed at the top?
     
  8. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
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