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High BW amplifier design

Ajay Kumar
Prodigy 190 points
Other Parts Discussed in Thread: THS3091, THS3202

Hi,

I'm designing some kind of a voltage regulator for which I've to drive a LDMOS device (typical input capacitance : 100pF). I have used THS3091 in negative feedback configuration (with a gain of 10V/V) my first design. This configuration works well with a bandwidth of around 80MHz. 

Now, I want to increase the BW of my system to about 300MHz at least. A gain of 5V/V (or 10V/V max) is sufficient. Could anyone help/suggest me an opamp/configuration (single stage or double stage) to be used to achieve the purpose?

Thanks,

Ajay

  • 0
    TI__Genius 17905 points

    Ajay,

    What is the voltage swing you need at the LDMOS gate? Based on the 80MHz bandwidth achieved and 100pF load, I am guessing you are swinging around 10Vpp (5Vpeak). Is that about right?

    300MHz is a difficult requirement. You may consider the THS3202, which has 9000V/us slew rate (SR) and has a wider bandwidth than the THS3091. In this case, however, the slew rate is really going to be limited by that 100pF load, so you will need to parallel several THS3202 (each is a dual) amplifiers in parallel to increase your output current ability. If you intend to prototype, I would design a board with the capability to connect up to 4 THS3202 devices (8 amplifier channels in total) in parallel. Then you can try different numbers of parallel channels to see the improvement each channel provides.

    I've attached a TINA schematic showing how I might implement parallel amplifier channels. Note that I have a 25ohm resistor in series with each amplifier output before they connect together to the 100pF load. These resistors isolate each amplifier from the capacitive load as well as help with the current balancing to the load in this parallel configuration. With 8 channels, I used 25ohm resistors. If the channels were reduced to 4, I would use 12.5ohm resistors and so on. On a prototype board, these resistors would provide a way to disconnect amplifiers when trying different numbers of parallel amplifiers.

    THS3202 load sharing 100pF 6-2-2011.TSC
  • 0 in reply to Kris F
    Prodigy 190 points

    Dear Kristoffer,

    I need a swing of about 2-4V at LDMOS. Since, I use a PCI arbitrary waveform generator I need a gain of about 5 at least. 

    Thanks for the schematic and advice. I will try and use this in my design. I will make the board design myself for the prototype. I'll let you know if I encounter any problems.

    Regards,

    Ajay