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R

F

402

Ω

C

S

13pF

OPA699

+5V

-5V

V

O

V

I

C

F

2.8pF

200

Ω

R

G

402

Ω

V

H

V

L

S0052-01

Z

O +

GBP

NG2

1

1

*

N

G1

N

G2

*

1

* 2

N

G1

N

G2

(1)

C

F +

1

2pR

F

Z

O

N

G2

(+ 3 pF)

(2)

C

S +

N

G2 *

1 C

F

(3)

f

*3dB 5

Z

O *

GBP

(4)

OPA699M

SGLS289A – MAY 2005 – REVISED MARCH 2006

APPLICATION INFORMATION (continued)

Figure 51. Broadband, Low-Inverting Gain External Compensation

To choose the values for both CS and CF, two parameters and only three equations need to be solved. The first

parameter is the target high-frequency noise gain (NG2), which should be greater than the minimum stable gain

for the OPA699. Here, a target of NG2 = 26 is used. The second parameter is the desired low-frequency signal

gain, which also sets the low-frequency noise gain (NG1). To simplify this discussion, we target a maximally flat

2nd-order low-pass Butterworth frequency response (Q = 0.707). The signal gain shown in Figure 51 sets the

low-frequency noise gain to NG1 = 1 + RF/RG (= 2 in this example). Then, using only these two gains and the gain

bandwidth product for the OPA699 (1000 MHz), the key frequency in the compensation is set by Equation 1.

Physically, this ZO (22.3 MHz for the values shown above) is set by 1/[2πRF(CF + CS)] and is the frequency at

which the rising portion of the noise gain would intersect the unity gain if projected back to a 0 dB gain. The

actual zero in the noise gain occurs at NG1 × ZO and the pole in the noise gain occurs at NG2 × ZO. That pole is

physically set by 1/(RFCF). Since GBP is expressed in Hz, multiply ZO by 2π and use to get CF by solving

Equation 2.

Finally, since CS and CF set the high-frequency noise gain, determine CS using Equation 3 (solving for CS by

using NG2 = 6):

which gives CS = 15 pF.

Both of these calculated values have been reduced slightly in Figure 51 to account for parasitics. The resulting

closed-loop bandwidth is approximately equal to Equation 4.

For the values shown in Figure 51, f–3 dB is approximately 149 MHz. This is less than that predicted by simply

dividing the GBP product by NG1. The compensation network controls the bandwidth to a lower value, while

providing the full slew rate at the output and an improved distortion performance due to increased loop gain at

frequencies below NG1 × ZO.

20

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