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The response scale for "FLATNESS" is on the right-hand vertical axis, at a scaling of 0.1dB/division in this case. This allows the 0.1dB bandwidth to be determined, which is about 65MHz in this case. There is the general point to be noted here, and that is the major difference in the applicable bandwidth between the 3dB and 0.1dB criteria. It requires a 400MHz bandwidth amplifier (as conventionally measured) to provide the 65MHz 0.1dB flatness rating.

**Frequency Response— Voltage Feedback Op amps, Gain-Bandwidth Product**
Current feedback op amps do not behave in the same way as voltage feedback types. They are not stable with capacitive feedback, nor are they so with a short circuit from output to inverting input. With a CFB op amp, there is generally an optimum feedback resistance for maximum bandwidth. Note that the value of this resistance may vary with supply voltage— consult the device data sheet. If the feedback resistance is increased, the bandwidth is reduced. Conversely, if it is reduced, bandwidth increases, and the amplifier may become unstable.

Figure 1-62: Frequency response for current feedback op amps

In a CFB op amp, for a given value of feedback resistance (R2), the closed-loop bandwidth is largely unaffected by the noise gain, as shown in Figure 1-62 above. Thus it is not correct to refer to gain-bandwidth product, for a CFB amplifier, because of the fact that it is not constant. Gain is manipulated in a CFB op amp application by choosing the correct feedback resistor for the device (R2), and then selecting the bottom resistor (R1) to yield the desired closed loop gain. The gain relationship of R2 and R1 is identical to the case of a VFB op amp (Fig. 1-14, again).

Typically, CFB op amp data sheets will provide a table of recommended resistor values, which provide maximum bandwidth for the device, over a range of both gain and supply voltage. It simplifies the design process considerably to use these tables.

**Bandwidth Flatness**

In demanding applications such as professional video, it is desirable to maintain a relatively flat bandwidth and linear phase up to some maximum specified frequency, and simply specifying the 3dB bandwidth isn't enough. In particular, it is customary to specify the 0.1dB bandwidth, or 0.1dB bandwidth flatness. This means there is no more than 0.1dB ripple up to a specified 0.1dB bandwidth frequency.

Video buffer amplifiers generally have both the 3dB and the 0.1dB bandwidth specified. Figure 1-63 below shows the frequency response of the AD8075 triple video buffer.

Figure 1-63: 3dB and 0.1dB bandwidth for the AD8075, G = 2, triple video buffer, RL = 150Ω

Note that the 3dB bandwidth is approximately 400MHz. This can be determined from the response labeled "GAIN" in the graph, and the corresponding gain scale is shown on the left-hand vertical axis (at a scaling of 1dB/division).

The response scale for "FLATNESS" is on the right-hand vertical axis, at a scaling of 0.1dB/division in this case. This allows the 0.1dB bandwidth to be determined, which is about 65MHz in this case. There is the general point to be noted here, and that is the major difference in the applicable bandwidth between the 3dB and 0.1dB criteria. It requires a 400MHz bandwidth amplifier (as conventionally measured) to provide the 65MHz 0.1dB flatness rating.

It should be noted that these specifications hold true when driving a 75Ω source and load terminated cable, which represents a resistive load of 150Ω. Any capacitive loading at the amplifier output will cause peaking in the frequency response, and must be avoided.

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