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How to improve the linearity of LM13700 transconductance amp?

Shardul Adkar
Prodigy 10 points
Other Parts Discussed in Thread: LM13700

I am using LM13700 transconductance op-amp to generate a sawtooth waveform by connecting a capacitor at the buffer output. Firstly, it is very difficult to improve the linearity of this IC. I tried to use the linearizing diodes by giving them +15V bias ( and a series resistor of 15k to limit the diode current).  I am only able to achieve linearity till 60 mV. I want to achieve a better linearity say 10 mV to 100 mV atleast. Please suggest me some ways to improve the linearity of this IC.

Also, in the datasheet it has been mentioned that the IC has 2MHz bandwidth. However, I am not able to achieve the sawtooth frequency over 500 kHz. There is no gain vs frequency curve given in the datasheet so it is really difficult to determine whether this IC is suitable for this high frequency range. 

  • 0
    Prodigy 190 points

    The bandwidth spec is the open-loop bandwidth. A sawtooth waveform has even and odd harmonic components, so to get a good representation of a sawtooth at 500kHz you might need the first 10 harmonics. 10*500kHz = 5MHz.

    On Pg. 10 of the datasheet, it is suggested that, when using the Linearizing Diodes, your signal current Is must be less than your diode current divided by two.

    |Is| < Id/2

    It is also suggested that implementing Id with a current-source may help to increase linearity as well (but does suggest the increased performance from using a current source will be minimal).

    Table 6.1 specifies Id(max) of 2mA. So perhaps a strategy for increasing linearity would be to bias these diodes at their maximum rated current with a current source.