Figure 7-5 shows the open-loop gain and phase response of the OPA818. The GBWP of an op amp is measured in the 20-dB/decade constant slope region of the A_OL magnitude plot. The open-loop gain of 60 dB for the OPA818 is along this 20-dB/decade slope and the corresponding frequency intercept is at 2.7 MHz. Converting 60 dB to linear units (1000 V/V) and multiplying by the 2.7-MHz frequency intercept gives the GBWP of the OPA818 as 2.7 GHz. The A_OL Bode plot shows that the second pole in the A_OL response occurs before the A_OL magnitude drops to less than 0 dB (1 V/V). This result shows a phase change of more than 180° at 0 dB A_OL, indicating that the amplifier can not be stable in a gain of 1 V/V. Amplifiers, such as the OPA818, that are not unity-gain stable are referred to as decompensated amplifiers. The decompensated architecture typically allows for higher GBWP, higher slew rate, and lower noise compared to a unity-gain stable amplifier with equivalent quiescent current. The additional advantage of the decompensated amplifier is better distortion performance at higher frequencies in high-gain applications for comparable quiescent current to a unity-gain stable amplifier.

The OPA818 is stable in a noise gain of 7 V/V (16.9 dB) or greater in conventional gain circuits (see Figure 7-1 and Figure 7-2). In the noise gain of 7 V/V, the OPA818 has 790 MHz of SSBW with approximately 50° phase margin.

The high GBWP and low voltage and current noise make the OPA818 an excellent amplifier choice for wideband, moderate-to-high transimpedance-gain applications. Transimpedance gains of 50 kΩ or greater benefit from the low-current-noise JFET input. In a typical transimpedance-amplifier (TIA) circuit (see also Figure 8-2), a unity-gain stable amplifier is not required. At low frequencies, the noise gain of the TIA is 0 dB (1 V/V), and at high frequencies, the noise gain is set by the ratio of the total input capacitance (C_TOT) and the feedback capacitance (C_F). To maximize TIA closed-loop bandwidth, the feedback capacitance is typically less than the total input capacitance. This configuration results in a ratio of total input capacitance to feedback capacitance greater than 1, which is ultimately the noise gain of the TIA at higher frequencies. The blog series, What you need to know about transimpedance amplifiers – part 1 and What you need to know about transimpedance amplifiers – part 2 describe TIA compensation techniques in greater detail.

RL = 100 Ω

Simulation

Figure 7-5 Open-Loop Gain Magnitude and Phase vs Frequency

RL = 100 Ω

Simulation

Figure 7-6 Open-Loop Gain Magnitude vs Temperature