Figure 7-10 shows the open-loop magnitude and phase response of the OPA855. Calculate the gain bandwidth product of any op amp by determining the frequency at which the A_OL is 40 dB and multiplying that frequency by a factor of 100. The open-loop response shows the OPA855 to have approximately 62° of phase-margin in a noise gain of 7 V/V. The second pole in the A_OL response occurs before the magnitude crosses 0 dB, and the resultant phase margin is less than 0°. This indicates instability at a gain of 0 dB (1 V/V). Amplifiers that are not unity-gain stable are known as decompensated amplifiers. Decompensated amplifiers typically have higher gain-bandwidth product, higher slew rate, and lower voltage noise, compared to a unity-gain stable amplifier with the same amount of quiescent power consumption.

Figure 9-5 shows the open-loop magnitude (A_OL) of the OPA855 as a function of temperature. The results show approximately 5° of phase-margin variation over the entire temperature range in a noise gain of 7 V/V. Semiconductor process variation is the naturally occurring variation in the attributes of a transistor (Early-voltage, β, channel-length and width) and other passive elements (resistors and capacitors) when fabricated into an integrated circuit. The process variation can occur across devices on a single wafer or across devices over multiple wafer lots over time. Typically, the variation across a single wafer is tightly controlled. Figure 9-6 shows the A_OL magnitude of the OPA855 as a function of process variation over time. The results show the A_OL curve for the nominal process corner and the variation one standard deviation from the nominal. The simulated results show less than 2° of phase-margin difference within a standard deviation of process variation in a noise gain of 7 V/V.

One of the primary applications for the OPA855 is as a high-speed transimpedance amplifier (TIA). The low-frequency noise gain of a TIA is 0 dB (1 V/V). At high frequencies the ratio of the total input capacitance and the feedback capacitance set the noise gain. To maximize the TIA closed-loop bandwidth, the feedback capacitance is typically smaller than the input capacitance, which implies that the high-frequency noise gain is greater than 0 dB. As a result, op amps configured as TIAs are not required to be unity-gain stable, which makes a decompensated amplifier a viable option for a TIA. What You Need To Know About Transimpedance Amplifiers – Part 1 and What You Need To Know About Transimpedance Amplifiers – Part 2 describe transimpedance amplifier compensation in greater detail.

Figure 9-5 Open-Loop Gain vs Temperature

Figure 9-6 Open-Loop Gain vs Process Variation