AD823A

Data Sheet

Rev. | Page 16 of 20

WIDEBAND PHOTODIODE PREAMP

–

+

VOUT

VB

CD

CM

CM

AD823A

RSH = 10

11

Ω

CS

IPHOTO

CF

RF

Figure 42. Wideband Photodiode Preamp

The AD823A is an excellent choice for photodiode preamp

application. Its low input bias current minimizes the DC error

at the preamp output. In addition, its high gain bandwidth

product and low input capacitance maximizes the signal

bandwidth of the photodiode preamp. Figure 42 shows the

AD823A as a current-to-voltage (I/V) converter with an

electrical model of a photodiode.

The transimpedance gain of the photodiode preamp can be

described by the basic transfer function:

F

F

F

PHOTO

OUT

R

sC

R

I

V

+

×

=

1

(1)

where IPHOTO is the output current of the photodiode, and the

parallel combination of RF and CF sets the signal bandwidth (see

the I to V gain curve in Figure 43). Note that one should set RF

such that the maximum attainable output voltage corresponds

to the maximum diode current IPHOTO. This allows one to utilize

the full output swing.

The signal bandwidth that is attainable with this preamp is a

function of RF, the gain bandwidth product (fu) of the amplifier,

and the total capacitance at the amplifier summing junction,

including CS and the amplifier input capacitance CD and CM. RF

and the total capacitance produce a pole with loop frequency (fp).

S

F

p

C

R

f

π

2

1

=

(2)

With the additional pole from the amplifier’s open loop

response, the two-pole system results in peaking and instability

due to an insufficient phase margin (Figure 43(A), Without

Compensation).

Adding CF creates a zero in the loop transmission that compensates

for the effect of the input pole. This stabilizes the photodiode

preamp design because of the increased phase margin. It also sets

the signal bandwidth (Figure 43(B), With Compensation). The

signal bandwidth and the zero frequency are determined by

F

F

z

C

R

f

π

2

1

=

(3)

Setting the zero at the frequency fx maximizes the signal

bandwidth with a 45° phase margin. Since fx is the geometric

mean of fp and fu, it can be calculated by

u

p

x

f

f

f

×

=

(4)

Combining Equation 2, Equation 3 and Equation 4, the value of

CF that produces fx is defined by

u

F

S

F

f

R

C

C

×

×

=

π

2

(5)

The frequency response in this case shows about 2 dB of

peaking and 15% overshoot. Doubling CF and cutting the

bandwidth in half results in a flat frequency response with

about 5% transient overshoot.

B