7.3.3 Decompensated Architecture With Wide Gain-Bandwidth Product

Figure 10 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 it with the 2.7 MHz frequency intercept gives the GBWP of OPA818 as 2.7 GHz. As can be inferred from the A_OL Bode plot, the second pole in the A_OL response occurs before A_OL magnitude drops below 0 dB (1 V/V). This results in phase change of more than 180° at 0 dB A_OL indicating that the amplifier will not be stable in a gain of 1 V/V. Amplifiers like 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.

OPA818 is stable in noise gain of 7 V/V (16.9 dB) or higher in conventional gain circuits as shown in Figure 6 and Figure 7. It has 790 MHz of SSBW in this gain configuration with approximately 50° phase margin.

The high GBWP and low voltage and current noise of OPA818 make it a very suitable amplifier for wideband moderate to high transimpedance gain applications. Transimpedance gains of 50kΩ or higher benefit from the low current noise JFET input. In a typical transimpedance (TIA) circuit as shown in Figure 13, unity-gain stable amplifier is not a requirement. At low frequencies, the noise gain of 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 generally smaller than the total input capacitance. This results in the ratio of total input capacitance to the feedback capacitance to be 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 10. Open-Loop Gain Magnitude and Phase Vs Frequency

RL = 100 Ω

Simulation

Figure 11. Open-Loop Gain Magnitude vs Temperature