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 (CTOT) and the feedback

capacitance (CF). 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.

Frequency (Hz)

0

-180

8

-165

16

-150

24

-135

32

-120

40

-105

48

-90

56

-75

64

-60

72

-45

80

-30

88

-15

96

0

104

15

1k

10k

100k

1M

10M

100M

1G

D027

AOL Magnitude (dB)

AOL Phase (q)

RL = 100 Ω

Simulation

Figure 7-5. Open-Loop Gain Magnitude and Phase

vs Frequency

Frequency (Hz)

0

10

20

30

40

50

60

70

80

90

100

110

120

1k

10k

100k

1M

10M

100M

1G

D106

TJ = 40qC

TJ = 27qC

TJ = 85qC

RL = 100 Ω

Simulation

Figure 7-6. Open-Loop Gain Magnitude vs

Temperature

7.3.4 Low Input Capacitance

Often, the two primary considerations for TIA applications are maximizing the TIA closed-loop bandwidth and

minimizing the total output noise to maximize the signal-to-noise ratio (SNR). The total input capacitance (CTOT)

of the TIA circuit causes a zero in the noise gain in combination with the transimpedance gain (feedback

resistor, RF) at frequency 1 / (2πRFCTOT). For a fixed RF, this zero is at a lower frequency for a higher CTOT,

thus increasing the noise gain at the lower frequency. This configuration results in lower equivalent closed-

loop bandwidth and higher total output noise compared to a lower CTOT. The OPA818 features a low input

capacitance (2.4 pF combined common-mode and differential). The OPA818 also provides high closed-loop

bandwidth at low total output noise, or provides the flexibility to choose a photodiode with relatively higher

capacitance for the TIA application. The CTOT includes the input capacitance of the amplifier, the photodiode

capacitance, and the PCB parasitic capacitance at the inverting input.

OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

www.ti.com

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