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Bandwidth of current feedback amplifiers

Jason Ensher
Prodigy 10 points
Other Parts Discussed in Thread: OPA695, LMH6714

I'm trying to understand the bandwidth and gain tradeoffs of the current feedback amplifiers. I've read a number of application notes, such as slva051, and the application details in datasheets for CFBs such as the OPA695 or even the THS family. They all indicate that the bandwidth of the amplifier circuit decreases as the feedback resistor increases. However, when I read the detailed Electrical Specifications or examine the signal response vs frequency curves, they indicate the opposite behavior is true.

Can someone shed light on this apparent discrepancy? Thank-you.

  • 0
    TI__Genius 13935 points

    I'm one of the newly acquired National employees, so I'm going to refer you to a part that I worked on years ago.  The LMH6714 is a classic current feedback amplifier.  If you look at the datasheet on page 9 figure 1 there is a chart that shows bandwidth over a range of feedback resistor values. 

    http://www.national.com/ds/LM/LMH6714.pdf

     

    The chart shows that smaller feedback resistors give more bandwidth, and as the loop gain goes up phase margin goes down, so at Rf = 147 Ohms there is noticable peaking.  You'll also notice that if you make a buffer from a current feedback amplifier you can't just connect the inverting input to the output - that usually makes an oscillator. 

    What you may be noticing is that as gain goes up for a given value of Rg may get so small that it begins to compete with the internal inverting input impedance.  Att his point the current feedback amp becomes gain bandwidth limited just like a voltage feedback amp.  Because of this, current feedback op amps are not usually used in very high gain configurations.  While a voltage feedback op amp can support a gain of 100V/V by the time you get that much gain in a current feedback amp, the resistors just aren't working in your favor.  Either Rf is very large and the loop gain is very low, or Rg gets very small and the inverting input impedance is dominating the response.