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lm6152 bandwidth

jacob sarusi
Prodigy 120 points
Other Parts Discussed in Thread: LM6152, LM7171, LM7372

Hi,

I am using lm6152  non inverting, single supply (15v) unity gain design, input 30MHZ 2v pp + changing dc offset.

Outout assumed to be same as input 2v pp +dc offset.

Yet dc Offset ok, ac signal drop dramatically @ 1MHZ

This is a rail to rail op amp, and I expected it to be able to deliver 2v pp output.

What am I missing ?

  • 0
    TI__Mastermind 49906 points

    Hello Jacob,

    What is your load? What is the amplifier driving?

    A highly capacitive load, like a long cable, is difficult to drive at >1MHz and may be loading the output down.

    Also, the supplies need to be properly bypassed (0.1uF ceramic and 4.7uF tantalum) on the supply pin.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 120 points

    Well this is a test bench, and the load is > 10K ohm. capacitence less then 4pF.

    Any how, 1MHz is not the problem. As mentioned I need a 30MHz with 2V pp output.

    Thanks

  • 0 in reply to jacob sarusi
    TI__Mastermind 49906 points

    Hello Jacob,

    Simply...The amp is not fast enough for 2Vpp at 30MHz. You are hitting slew rate limiting, also known as the "Power Bandwidth Limit".

    The amplifiers 75MHz is a small-signal BW.  When the output starts moving many volts, slew rate becomes involved.

    See Linear Brief 19 for more info - while it is from the 70's - the concept is just as true today.

     LB-19 Predicting OP Amp Slew Rate Limited Response

     Fmax = SR/(2*pi*Vp) = 30/(2*3.14*2) = 30/12.56 = 2.38MHz....

    Uh,oh...You are not the first to be caught in this trap!

    So..working things in reverse...if you want 30MHz, then 12.56 * 50 = 628V/us. You would need a device with a minimum slew rate of ~650V/us. The more the better.

    This would be in the LMH or THS series of high speed op-amps. Since 2Vpp was the standard for video, this is easy for these devices.

    But beware, these high bandwidth devices do not like and direct capacitance on their output. They are usually run into a 50 or 75 ohm back-terminated applications (resistor in series with the output). Also, many of these high speed devices are low voltage (<10V), so that may limit your selection.

    For your application, you could use either a current feedback or voltage feedback device. Current feedback devices have faster slew rates and low distortion - but higher bias currents and "sloppier" DC specifications.

    Try a device like the LM7171, LM7372 or the THS3xxx series of high speed op-amps - just make sure the slew rate is >650V/us.

    Regards,