LM13700 VCO Design help

Discussion in 'Analog & Mixed-Signal Design' started by MachineHum, Jul 15, 2016.

  1. MachineHum

    Thread Starter Member

    Nov 3, 2014
    70
    4
    Hey guys/girls,

    I'm designing a VCO using TI's LM13700 (Datasheet below)
    http://www.ti.com/lit/ds/symlink/lm13700.pdf

    On page 19 there is a pretty good looking schematic. Working the math is no issues, but all the formulas are using Ic and Ia, and to get these I need to know the voltage at the two points I've marked below. Obviously I can't assume their high impedance inputs because that would make the current zero.

    Selection_001.png

    If anyone has an answer that would be very helpful, bonus points for resources to learn about TC amps.

    Thanks

    MH
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    From the data sheet, page 11:
    Next, set IBIAS. The Bias Input pins (pins 1 or 16), are 2 diode drops above the negative supply, and therefore
    VBIAS = 2(VBE) + V- , which for this application is -13.6 V. To set IBIAS to 1ma when VC = 15 V requires a 28.6-kΩ;
    30-kΩ is a standard value and is used for this application. The gain will be linear with the applied voltage.
     
    GopherT and MachineHum like this.
  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Hola Albert,

    You can claim his bonus points as yours.
     
  4. MachineHum

    Thread Starter Member

    Nov 3, 2014
    70
    4
    Thanks Albert, much appreciated :)
     
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