# Stabilize Your Transimpedance Amplifier

Abstract: Transimpedance amplifiers (TIAs) are widely used to translate the current output of sensors like photodiode-to-voltage signals, since many circuits and instruments can only accept voltage input. An operational amplifier with a feedback resistor from output to the inverting input is the most straightforward implementation of such a TIA. However, even this simple TIA circuit requires careful trade-offs among noise gain, offset voltage, bandwidth, and stability. Clearly stability in a TIA is essential for good, reliable performance. This application note explains the empirical calculations for assessing stability and then shows how to fine-tune the selection of the feedback phase-compensation capacitor.

*Electronic Design*magazine.

## Wild Oscillations: Why Do They Happen?

**Figure 1**is popularly used in dual-supply systems.

**Figure 2**is a minor modification of this circuit for single-supply applications. The resistive-divider formed by R1 and R2 ensures that the output node of the op amp is higher than the Output Voltage Low specification during a no-light condition when only a small dark current flows through the photodiode. By ensuring that the op amp's output stage operates in the linear region, this offset improves both photo-detection in low-light conditions and response speed. However, care must be taken to keep this bias voltage on the IN+ pin small. Otherwise reverse-leakage current in the photodiode can degrade linearity and increase offset drift over temperature. In some applications the circuit in

**Figure 3**is used where the photodiode is placed directly across the input terminals of the op amp. This circuit avoids the reverse bias across the photodiode, although it requires a buffered reference. The reference must be fast enough to sink the photodiode current as required by the application. This, in turn, implies that amplifier A1 must be as fast as amplifier A2.

*Figure 1. Basic TIA circuit (dual supply).*

*Figure 2. Basic TIA circuit of Figure 1 modified for a single supply.*

*Figure 3. Basic TIA circuit of Figure 2 modified for single supply.*

_{VOL}, and a feedback network comprised of the resistance and the photodiode.

**Figure 4**shows the equivalent circuit of the photodiode in Figures 1 to 3.¹ For most photodiodes, R

_{SERIES}= 0 and R

_{SHUNT}= Infinity is a fair approximation. Consequently, the simplified model reduces to the short-circuit current source in parallel with the junction capacitance. This simplified photodiode model will be used for subsequent stability analysis.

*Figure 4. Photodiode equivalent circuit: I*

_{P}= photocurrent; R_{SHUNT}= diode shunt junction resistance; C_{J}= junction capacitance; and R_{S}= series resistance.**Figure 5**plots the open-loop gain response of the op amp. It is constant from DC until the dominant-pole corner frequency; it decreases at 20dB per decade thereafter until it reaches the second-pole corner. Mathematically, the single-pole response can be represented as:

(Eq. 1) |

A

_{VOL}= DC open-loop gain

A

_{VOL}(jω) = open-loop gain corresponding to frequency, ω

ω

_{PD}= dominant-pole frequency in radians/seconds

_{F}, and the total input capacitance, C

_{i}(junction capacitance of the photodiode in parallel with the input capacitance of the op amp). The feedback factor is given as:

(Eq. 2) |

(Eq. 3) |

_{F}.

*Figure 5. Open-loop gain, A*

_{VOL}(jω), and the reciprocal of feedback factor, 1/β(jω), versus frequency. The rate of closure between the two curves determines the likelihood of oscillations/ringing._{VOL}(jω) response curve with the 1/β(jω) curve denotes a critical intercept fundamental for stability analysis. The phase margin at this intersection frequency can be determined by observing the rate of closure between the two response curves, A

_{VOL}(jω) and 1/β(jω). If the rate of closure of the two response curves is 40dB, as seen in Figure 5, the circuit will be unstable. There is another intuitive way to understand this. At lower frequencies the phase shift in the feedback signal is 180 degrees due to the inverting nature of the negative feedback. As the frequency increases well into the -20dB/dec slope region of A

_{VOL}, the dominant pole of the op amp can add up to 90 degrees of phase shift. Similarly, the pole introduced by the feedback network can add another 90 degrees of phase shift, thus producing a phase shift of about 360 degrees at Aβ = 1. If the phase shift is 360 degrees, self-sustaining oscillations will result. If the phase shift is close to 360 degrees, heavy ringing is observed. In either case, some form of phase compensation scheme will be required to stabilize the circuit.

## No Evil Is Without Its Compensation: Feedback Capacitor Calculations

**Figure 6**). It is important to calculate the value of the feedback capacitor required to provide optimal compensation. To account for the added phase-compensation capacitor, substitute Z

_{F}in Equation 2 with R

_{F}|| C

_{F}. The feedback factor now becomes:

(Eq. 4) |

_{F}introduces a zero in the feedback factor, besides modifying its pole. The zero compensates for the phase shift introduced by the feedback network. This can be seen graphically in

**Figure 7**. If the phase shift is overcompensated by choosing a large feedback capacitor, then the rate of closure can be reduced to 20dB per decade (90 degrees phase margin). However, overcompensation also reduces the usable bandwidth of the TIA. While a reduced bandwidth may not be an issue with low-frequency photodiode applications, high-frequency or low-duty-cycle pulsed photodiode circuits definitely need to maximize the available bandwidth. For such applications, the goal is to find the minimum value of the feedback compensation capacitor, C

_{F}, needed to eliminate oscillation and minimize ringing. However, it is always a good idea to overcompensate the TIA circuit slightly. Overcompensation is recommended to provide sufficient guardband to account for up to ±40% variation in an op amp's bandwidth over process corners and the tolerance of the feedback capacitor.

*Figure 6. Phase compensation capacitor C*

_{F}helps improve stability.*Figure 7. Phase response with the phase-compensation capacitor, C*

_{F}._{VOL}(jω) and 1/β(jω) curves. This margin requires the optimum value of C

_{F}to be calculated so that the added zero in the feedback factor, β(jω), is located at the frequency corresponding to Aβ = 1, as shown in Figure 7. One equation for the intercept frequency is:

(Eq.5 ) |

_{i}, and the feedback capacitor, C

_{F}. To solve for C

_{F}, we need to find another simultaneous equation. One way to obtain the second equation is to equate the A

_{VOL}(jω

_{i}) and 1/β(jω

_{i}) curves. The resulting equation is complicated and does not lend itself to an easy solution. The graphical approach for solving C

_{F}is a more convenient alternative.² Observing Figure 7, both curves have a slope of 20dB/dec. Therefore, the approximate triangle formed by both curves with the horizontal axis is isosceles. Hence, the intercept frequency, f

_{i}, is the average of the other two vertices. Since the frequency is plotted in the logarithmic scale, we have:

(Eq. 6) |

(Eq. 7) |

_{GBWP}= unity gain bandwidth of the op amp. To account for the variation in unity-gain bandwidth over process corners, select f

_{GBWP}to be 60% of the value specified on the op amp's data sheet.

_{GBWP}to equal 60% of the frequency at which the projection of the -20dB A

_{VOL}(jω

_{i}) slope intersects the 0dB x-axis line.

(Eq. 8) |

_{i}, is equal to the geometric mean of the unity-gain bandwidth, f

_{GBWP}, and the pole-corner frequency, f

_{F}, of β(jω). Substituting for f

_{F}from Equation 7, we get:

(Eq. 9) |

_{F}:

(Eq. 10) |

_{F}is valid for both large-area and small-area photodiodes.

## Alright...Give Us the Scope Now: Design Example

ON time duration = 50µs

Duty cycle = 5%

R

_{F}= 100kΩ

BPW46 photodiode is used

**Table 1**lists some low-noise, CMOS input, Maxim op amps are popularly used in TIA circuits in a wide variety of applications. For this design example, we select the MAX9636 op amp. The MAX9636 is also suitable for other battery-powered, portable equipment since its design is a good trade-off between lower quiescent current and noise performance. For higher-bandwidth applications, op amps like the MAX4475 and MAX4230 might be more suitable.

Table 1. Maxim Op Amps Suitable for Transimpedance Amplifier Circuits | ||||||

Part | Input Bias Current (pA) | Input Voltage Noise (nV/sqrt(Hz)) | Supply Current (µA) | Unity Gain Bandwidth (MHz) | Smallest Package | Features |

MAX9636 | < 0.8 | 38 at 1kHz | 36 | 1.5 | SC70 | Low power, low bias current, high GBW to supply current ratio, low cost |

MAX9620 | < 80 | 42 at 1kHz | 59 | 1.5 | SC70 | Precision, low power, high GBW-to-supply current ratio |

MAX9613 | < 1.55 | 28 at 10kHz | 220 | 2.8 | SC70 | Low bias current at V_{CM} = V_{EE}, V_{OS} self-calibration |

MAX4475 | < 1 | 4.5 at 1kHz | 2200 | 10 | SOT23, TDFN | Ultra-low noise |

MAX4230 | < 1 | 15 at 1kHz | 1100 | 10 | SC70 | High bandwidth, low noise |

MAX9945 | < 0.15 | 16.5 at 1kHz | 400 | 3 | TDFN | High voltage, low power |

MAX4250 | < 1 | 8.9 at 1kHz | 400 | 3 | SOT23 | Low noise and low distortion |

MAX4238 | < 1 | 30 at 1kHz | 600 | 1 | SOT23, TDFN | Precision and low drift |

MAX4400 | < 1 | 36 at 10kHz | 320 | 0.8 | SC70 | Low cost |

C_{i} |
= junction capacitance of photodiode (70pF) + 2pF input capacitance of the MAX9636 |

= 72pF |

f

_{GBWP}= 0.9MHz.

_{F}= 100kΩ.Therefore, the calculated value of C

_{F}= 15.6pF. The next highest standard value of the capacitor is 18pF.

**Figure 8**shows the output of the TIA without any compensation feedback capacitor and using the circuits in Figures 1 to 3. As expected, oscillation is observed with no phase compensation capacitor. If C

_{F}= 10pF is used, then ringing stops, although an overshoot is still visible as seen in

**Figure 9**. Next the feedback capacitor value is increased to the recommended calculated value of 18pF.

**Figure 10**shows that no ringing or oscillation is observed for the C

_{F}= 18pF case, thus validating the theoretical analysis above. Figure 11 shows the corresponding small signal-step response with 50nA amplitude of photodetector current.

*Figure 8. MAX9636 output with R*

_{F}= 100kΩ, C_{F}not installed, and a 10µA input current pulse.*Figure 9. MAX9636 output with R*

_{F}= 100kΩ, C_{F}= 10pF, and a 10µA input current pulse.*Figure 10. MAX9636 output with R*

_{F}= 100kΩ, C_{F}= 18pF, C_{i}= 72pF, and a 10µA input current pulse.*Figure 11. MAX9636 output with R*

_{F}= 100kΩ, C_{F}= 18pF, C_{i}= 72pF, and a 50nA input current pulse. Waveform is AC-coupled in order to zoom in.#### References

- Jiang, H., and Yu, P. K. L., "Equivalent Circuit Analysis of Harmonic Distortions in Photodiode," IEEE® Photonics Technology Letters, vol. 10, no. 11, November 1998, pp. 1608–1610.
- Graeme, Jerald, "Photodiode Amplifiers: Op amp Solutions," The McGraw-Hill Companies, Inc., ISBN 0-07-024247-X, pp. 47–50.