This has the interesting effect of doubling the

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This has the interesting effect of doubling the equivalent GBP for the amplifier. This can be seen in comparing the G = +12 and G = –20 small-signal frequency response curves. Both show approximately 500MHz bandwidth with 3dB peaking, but the inverting configuration of Figure 2 is giving 4.4dB higher signal gain. The noise gains are ap- proximately equal in this case. If the signal source is actually the low impedance output of another amplifier, R G should be increased to be greater than the minimum value allowed at the output of that amplifier and R F adjusted to get the desired gain. It is critical for stable operation of the OPA687 that this driving amplifier show a very low output impedance through frequencies exceeding the expected closed-loop bandwidth for the OPA687. WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE DESIGN The high Gain Bandwidth Product (GBP) and low input voltage and current noise for the OPA687 make it an ideal wideband transimpedance amplifier for low to moderate transimpedance gains. Very high transimpedance gains (> 100k ) will benefit from the low input noise current of a FET-input op amp such as the OPA655. Unity gain stability in the op amp is NOT FIGURE 2. Inverting G = –40 Specifications and Test Circuit.
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9 OPA687 SBOS065A required for application as a transimpedance amplifier. Fig- ure 3 shows one possible transimpedance design example that would be particularly suitable for the 155Mbit data rate of an OC-3 receiver. Designs that require high bandwidth from a large area detector with relatively low transimpedance gain will benefit from the low input voltage noise for the OPA687. The amplifier’s input voltage noise is peaked up, at the output, over frequency by the diode source capaci- tance and can, in many cases, become the dominant output noise contribution. The key elements to the design are the expected diode capacitance (C D ) with the reverse bias volt- age (–V B ) applied, the desired transimpedance gain, R F , and the GBP for the OPA687 (3600MHz). With these three variables set (and including the parasitic input capacitance for the OPA687 added to C D ), the feedback capacitor value (C F ) may be set to control the frequency response. FIGURE 3. Wideband, High Sensitivity, OC-3 Transimpedance Amplifier. To achieve a maximally flat 2nd-order Butterworth fre- quency response, the feedback pole should be set to: 1/(2 π R F C F ) = (GBP/(4 π R F C D )) Adding the common-mode and differential-mode input ca- pacitance (1.2 + 2.5)pF to the 1pF diode source capacitance of Figure 3 (C D ), and targeting a 12k transimpedance gain using the 3600MHz GBP for the OPA687, will require a feedback pole set to 71MHz to get a maximum bandwidth design. This will require a total feedback capacitance of 0.16pF. Using this maximum bandwidth, maximally flat frequency response target will give an approximate –3dB bandwidth set by: f –3dB = (GBP/2 π R F C D )Hz The example of Figure 3 will give approximately 100MHz flat bandwidth using the 0.16pF feedback compensation capacitor. This bandwidth will easily support an OC-3 re- ceiver with exceptional sensitivity.
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