p =

1

1 + sCFRF

´

¶

z =

1

1 + sRF (CF + CIN)

´

¶

F =

1 + sCFRF

1 + sRF (CF + CIN)

-

+

CF

RF

IDIODE

VOUT

LMV793

VA

CIN

-

+

2.5V

CF

RF

DPHOTO

CD

CCM

-2.5V

2.5V

VOUT

LMV793

LMV793, LMV794

www.ti.com

SNOSAX6D – MARCH 2007 – REVISED MARCH 2013

Figure 56. Transimpedance Amplifier

Figure 56 is the complete schematic for a transimpedance amplifier. Only the supply bypass capacitors are not

shown. CD represents the photo diode capacitance which is given on its datasheet. CCM is the input common

mode capacitance of the op amp and, for the LMV793 it is shown in the last drawing of the Typical Performance

Characteristics section of this datasheet. In Figure 56 the inverting input pin of the LMV793 is kept at virtual

ground. Even though the diode is connected to the 2.5V line, a power supply line is AC ground, thus CD is

connected to ground.

Figure 57 shows the schematic needed to derive F, the feedback factor, for a transimpedance amplifier. In this

figure CD + CCM = CIN. Therefore it is critical that the designer knows the diode capacitance and the op amp input

capacitance. The photo diode is close to an ideal current source once its capacitance is included in the model.

What kind of circuit is this? Without CF there is only an input capacitor and a feedback resistor. This circuit is a

differentiator! Remember, differentiator circuits are inherently unstable and must be compensated. In this case CF

compensates the circuit.

Figure 57. Transimpedance Feedback Model

Using feedback theory, F = VA/VOUT, this becomes a voltage divider giving the following equation:

(12)

The noise gain is 1/F. Because this is a differentiator circuit, a zero must be inserted. The location of the zero is

given by:

(13)

CF has been added for stability. The addition of this part adds a pole to the circuit. The pole is located at:

(14)

To attain maximum bandwidth and still have good stability the pole is to be located on the open loop gain curve

which is A. If additional compensation is required one can always increase the value of CF, but this will also

reduce the bandwidth of the circuit. Therefore A = 1/F, or AF = 1. For A the equation is:

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Product Folder Links: LMV793 LMV794