LMH6628 Datasheet, PDF (10/11 Pages) National Semiconductor (TI) – Dual Wideband, Low Noise, Voltage Feedback Op Amp

Application Section (Continued)
(4)
To build a boost circuit, use the design equations Eq. 6 and
Eq. 7.
(5)
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FIGURE 6.
A current source, built around Q1, provides the necessary
bias current for the second amplifier and prevents saturation
when power is applied. The resistor, R, closes the loop while
diode D2 prevents negative saturation when V_IN is less than
V_C. A MOS-type switch (not shown) can be used to reset the
capacitor’s voltage.
The maximum speed of detection is limited by the delay of
the op amps and the diodes. The use of Schottky diodes will
provide faster response.
ADJUSTABLE OR BANDPASS EQUALIZER
A "boost" equalizer can be made with the LMH6628 by
summing a bandpass response with the input signal, as
shown in Figure 7.
(6)
Select R₂ and C using Eq. 6. Use reasonable values for high
frequency circuits - R₂ between 10Ω and 5kΩ, C between
10pF and 2000pF. Use Eq. 7 to determine the parallel com-
bination of R_a and R_b. Select R_a and R_b by either the 10Ω to
5kΩ criteria or by other requirements based on the imped-
ance V_in is capable of driving. Finish the design by determin-
ing the value of K from Eq. 8.
(7)
Figure 8 shows an example of the response of the circuit of
Figure 9, where f_o is 2.3MHz. The component values are as
follows: R_a=2.1kΩ, R_b = 68.5Ω, R₂ = 4.22kΩ, R = 500Ω, KR
= 50Ω, C = 120pF.
FIGURE 7.
The overall transfer function is shown in Eq. 5.
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FIGURE 8.
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