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Signal Chain Basics #115: The impact of the feedback resistance value on gain, bandwidth, and slew rate in current feedback amplifiers

Editor’s note: I am pleased to bring you another excellent Analog tech insight from Texas Instruments in their Planet Analog Signal Chain Basics series by Loren Siebert, Applications Engineer, Texas Instruments

Current feedback (CFB) amplifiers have been available since the 1990s, but they are still a small percentage of the overall operational amplifier (op amp) market and may not be covered in basic electrical engineering classes. Like voltage feedback (VFB) amplifiers, CFB amplifiers are nearly always used in a closed-loop configuration. Fortunately, this makes implementing CFB amplifiers nearly identical to the traditional circuits normally used with VFB amplifiers. Although CFB amplifiers are used in similar circuits to VFB amplifiers, there are some key differences.

Figure 1 shows the internal structure of a CFB amplifier, and the associated gain equation is shown in equation 1. This equation shows that as Z(s) become very large, the gain of the current feedback amplifier becomes 1+ Rf/Rg – just like a voltage feedback amplifier. There are three key parameters to non-ideal behavior: namely RF, RI and Z(s). Of these three, only RF is under the control of the amplifier user.

Figure 1

Current feedback amplifier block diagram.

Current feedback amplifier block diagram.

At first glance the CFB gain equation could lead one to believe that the ideal RF is equal to zero. However, this is not the case for most CFB amplifiers. Just as a VFB amplifier is designed to have ideal compensation for a particular gain configuration, a CFB amplifier is designed with compensation for a particular value of RF. This value will be clearly spelled out in the amplifier data sheet. In this article, we will briefly explore how changes to the value of the feedback resistor change the amplifier performance. The first consideration for choosing a feedback resistor for a CFB amplifier will be the amplifier closed-loop gain.

As shown in Figure 2 , the best feedback resistor value is dependent on gain (LMH6514). For example the resistor value for a gain of 1 is very large compared to the higher gain settings. Also, note that at a gain of 6 the recommended resistor value begins to increase. For gain settings of 7 or higher, the amplifier bandwidth will begin to decrease with higher gain settings in a manner similar to the gain bandwidth product of a VFB amplifier. This is one reason that CFB amplifiers are not normally used at gain settings higher than 20 V/V (26 dB).

Figure 2

Recommended RF versus non-inverting gain.

Recommended RF versus non-inverting gain.

Once the gain is selected, the recommended value of RF may be adjusted based upon the circuit needs. For example, in a circuit where settling time is very important, the recommended value of feedback resistor will be very close to the recommended value. This is visible in Figure 3 , which shows the amplifier response for a gain of 2 and various values of RF. As shown in this figure, the recommended value of RF at a gain of 2 is 300 Ohms. Using a smaller feedback resistor such as 147 Ohms would give peaking while the higher feedback resistor values would lead to a slower rise time.

Figure 3

Frequency response versus feedback resistor value.

Frequency response versus feedback resistor value.

Another example is that of a cable driver. In this case, maximum signal bandwidth is desired because the extra bandwidth at higher frequencies is a benefit. This is because cable losses are higher at higher frequencies, so a peaked-frequency response provides pre-emphasis on the higher frequencies and results in a flatter response after cable losses.

While Figure 3 shows a small signal frequency response, the behavior for large signals is similar in a CFB amplifier. While VFB amplifiers have a fixed slew rate, the slew rate of a CFB amplifier is also a function of the feedback resistor. This is why CFB amplifiers are said to have no gain/bandwidth limitation. By adjusting the value of RF, the bandwidth of a CFB amplifier will not drop proportionately with gain.

Conclusion

We can see that with CFB op amps there is significant flexibility in small signal bandwidth, large signal bandwidth, and slew rate independent of the gain chosen. This flexibility is accomplished by adjusting the value of the feedback resistor. This flexibility allows CFB op amps to drive large signals with high bandwidth and good linearity.

Please join us next time when we discuss combining multiple devices for improved performance.

Additional information

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7 comments on “Signal Chain Basics #115: The impact of the feedback resistance value on gain, bandwidth, and slew rate in current feedback amplifiers

  1. sharps_eng
    August 19, 2016

    The term Z(s) is mentioned but not defined.

    The text states that only Rf is under the control of the user, but Rg is drawn as an external resistor. 

    My immediate reaction was to stop reading the article. No logical framework for this device could be constructed in my head as I read on; so will this increase the number of people using these devices?

     

  2. LorenDSiebert
    August 19, 2016

    Thanks for your feedback.  Z(s) is the open loop gain. Because it is defined by an input current and an output voltage it is a transimpedance gain.  It is fixed for a given amplifier.  

     

    While Rg is also an external resistor once you select Rf the value of Rg is fixed for a given gain selection.  I could have been more clear.

     

    I hope this response helps.

  3. pierree
    August 21, 2016

    the calculations do not necessarily give the result that we want, and indeed the term Z (s) is not defined.

  4. sroochi
    August 24, 2016

    It is my understanding that the effect of Zs is a function of α(s).  As I interpret the block diagram, an idea, infinitely widebandwidth buffer with infinite input impedance will let Z(s) go to ∞ so the equation (1) simplifies to the standard ideal op amp voltage follower equation. 

  5. Sergio Franco
    August 24, 2016

    …you may be interested in Sections 6.7 and 8.6 of the book “Design with Operational Amplifiers and Analog ICs”, by Sergio Franco. ISBN 0-07-232084-2

     

  6. Thomas7
    October 27, 2016

    Thank you, I've been seeking for info about this subject matter for ages and yours is the best I have discovered so far.

  7. Parreco
    December 15, 2016

    Z(s) is obviously a function of frequency.  (You.ve stated that it's fixed for a given amplifier, but I think you meant it's frequency response is fixed).  But my question is in regards to Ri….is Ri independent of frequency, or does that vary with frequency, as does Z(s)?

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