100MHz voltage controlled current sink

Hi,

I am trying to build a voltage controlled current sink which can sink a current up to 300mA from DC to 100MHz. In my first draft i used one OpAmp in combination with an npn transistor to achieve the required current amplification. A Resistor 5Ohm (10R || 10R) between emitter and ground is used to build the negative feedback to close the loop. (see picture, C_in = 0pF)

There are two problems:     

  • transistor power disapation is to high (FZT651)
  • bandwidth only 10MHz (FZT651: ft = 150MHz; OpAmp: GBP = 600MHz)

Whats the matter for such a low bandwidth?

Any ideas for reducing power disapation?

Thank's

Parents
  • Hi,

    Please find the below attached circuit which I simulated in NI Multisim.

    The circuit uses Analog device's High speed Op amp AD8008 and MOSFET IRF530. This is almost linear voltage controlled current sink circuit which can operate upto 50MHz without any problem.

    C1, C2, R2, R4 is ued for the frequency stabilty compensation. You can fine tune these values according to your specs for settling time, noise limits etc.

    Please free to contact me for any clarifications.

    Thanks,

    AKHILESH K

Reply
  • Hi,

    Please find the below attached circuit which I simulated in NI Multisim.

    The circuit uses Analog device's High speed Op amp AD8008 and MOSFET IRF530. This is almost linear voltage controlled current sink circuit which can operate upto 50MHz without any problem.

    C1, C2, R2, R4 is ued for the frequency stabilty compensation. You can fine tune these values according to your specs for settling time, noise limits etc.

    Please free to contact me for any clarifications.

    Thanks,

    AKHILESH K

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