LMH6629MFSLASHNOPB datasheet(21/45 Pages) TI

21 / 45 page

VIN

a) Non-inverting

b) Inverting

pin =

COMP

Constraint

Costraint

MHz

V/V

)

)(

(

p +

LMH6629

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SNOSB18I – APRIL 2010 – REVISED DECEMBER 2014

Feature Description (continued)

Figure 50. External Compensation

This circuit operates by increasing the Noise Gain (NG) beyond the minimum stable gain of the LMH6629 while

maintaining a positive loop gain phase angle at 0 dB. There are two constraints shown in Figure 50: “Constraint

1” ensures that NG has increased to at least 10 V/V when the loop gain approaches 0dB, and “Constraint 2”

places an upper limit on the feedback phase lead network frequency to make sure it is fully effective in the

frequency range when loop gain approaches 0dB. These two constraints allow one to estimate the “starting

value” for Rc and Cc which may need to be fine tuned for proper response.

Here is an example worked out for more clarification:

•

Assume that the objective is to use the SOT-23-5 version of the LMH6629 for a closed loop gain of +3.7 V/V

using the technique shown in Figure 50.

•

Selecting Rf = 249 Ω → Rg = 91 Ω → REQ= 66.6 Ω.

•

For 50-

Ω source termination (Rs= 50 Ω), select RT= 50 Ω → Rp = 25 Ω.

•

Using “Constraint 1” (= 10V/V) allows one to compute Rc

≊ 56 Ω. Using “Constraint 2” (= 90 MHz) defines the

appropriate value of Cc ≊ 33 pF.

•

The frequency response plot shown in Figure 51 is the measured response with Rc and Cc values computed

above and shows a -3 dB response of about 1 GHz.

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