8.3.3 Wide Gain-Bandwidth Product

Figure 6-10 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 AOL 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 AOL 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 8-5 shows the open-loop magnitude (AOL) 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 8-6 shows

the AOL magnitude of the OPA855 as a function of process variation over time. The results show the AOL 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.

Frequency (Hz)

-15

0

15

30

45

60

75

90

100k

1M

10M

100M

1G

10G

D604

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

D605

AOL ( 1 V)

AOL (Typ.)

AOL (+1 V)

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

OPA855

SBOS622D – JULY 2018 – REVISED MAY 2025

www.ti.com

18

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