8.3.3 Wide Gain-Bandwidth Product

Figure 6-10 shows the open-loop magnitude and phase response of the OPA855-Q1. Calculate the gain

that frequency by a factor of 100. The open-loop response shows the OPA855-Q1 to have approximately 62°

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.

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

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-Q1 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

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

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

OPA855-Q1

SBOSA57A – FEBRUARY 2021 – REVISED MARCH 2021

www.ti.com

18

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