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)

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

17

OPA818

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SBOS940A – MAY 2019 – REVISED MARCH 2020

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Feature Description (continued)

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 43

and Figure 44. 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 50 kΩ or higher benefit from the

low current noise JFET input. In a typical transimpedance (TIA) circuit as shown in Figure 50, 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 (CTOT) and the feedback capacitance (CF). 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 47. Open-Loop Gain Magnitude and Phase Vs

Frequency

RL = 100 Ω

Simulation

Figure 48. Open-Loop Gain Magnitude vs Temperature

8.3.4 Low Input Capacitance

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

the total output noise to maximize Signal-to-Noise Ratio (SNR). The total input capacitance (CTOT) of 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 higher CTOT thus increasing the noise gain at

lower frequency resulting in lower equivalent closed-loop bandwidth and higher total output noise compared to a

lower CTOT. By choosing an amplifier like OPA818 that features a low input capacitance (2.4 pF combined

common-mode and differential) for TIA application, the system designer can realize high closed-loop bandwidth

at low total output noise or have the flexibility to choose a photodiode with relatively higher capacitance. The

CTOT includes the input capacitance of the amplifier, the photodiode capacitance, and the PCB parasitic

capacitance at the inverting input.