8.3.3 Wide Gain-Bandwidth Product

Figure 6-10 shows the open-loop magnitude and phase response of the OPA858. Calculate the gain bandwidth

product of any op amp by determining the frequency at which the AOL is 60 dB and multiplying that frequency

by a factor of 1000. The second pole in the AOL response occurs before the magnitude crosses 0 dB, and

the resultant phase margin is less than 0°. This result 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 8-5 shows the open-loop magnitude (AOL) of the OPA858 as a function of temperature. The results show

minimal variation over temperature. The phase margin of the OPA858 configured in a noise gain of 7 V/V (16.9

dB) is close to 55° across temperature. Similarly Figure 8-6 shows the AOL magnitude of the OPA858 as a

function of process variation. The results show the AOL curve for the nominal process corner and the variation

one standard deviation from the nominal. The simulated results suggest less than 1° of phase margin difference

within a standard deviation of process variation when the amplifier is configured in a gain of 7 V/V.

One of the primary applications for the OPA858 is as a high-speed transimpedance amplifier (TIA), as Figure

9-4 shows. 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.

Frequency (Hz)

-15

0

15

30

45

60

75

90

100k

1M

10M

100M

1G

10G

D204

AOL at 40qC

AOL at 25qC

AOL at +125qC

Figure 8-5. Open-Loop Gain vs Temperature

Frequency (Hz)

-15

0

15

30

45

60

75

90

100k

1M

10M

100M

1G

10G

D205

AOL (

V

)

AOL (Typ.)

AOL (

V

)

Figure 8-6. Open-Loop Gain vs Process Variation

OPA858

SBOS629B – APRIL 2018 – REVISED MAY 2025

www.ti.com

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