LMH6723 Datasheet, PDF (17/28 Pages) National Semiconductor (TI) – Single/Dual/Quad 370 MHz 1 mA Current Feedback Op Amp

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LMH6723, LMH6724
SNOSA83I – AUGUST 2003 – REVISED AUGUST 2014
Driving Capacitive Loads (continued)
An alternative approach is to place R_OUT inside the feedback loop as shown in Figure 36. This will preserve gain
accuracy, but will still limit maximum output voltage swing.
R_IN
RG
51: 1.2k:
X1
+
+
R_OUT
51:
-
-
CL
10pF
R_L
1k:
R_F
1.2k:
Figure 36. Series Output Resistor Inside Feedback Loop
7.7 Inverting Input Parasitic Capacitance
Parasitic capacitance is any capacitance in a circuit that was not intentionally added. It is produced through
electrical interaction between conductors and can be reduced but never entirely eliminated. Most parasitic
capacitances that cause problems are related to board layout or lack of termination on transmission lines. See
Layout Considerations for hints on reducing problems due to parasitic capacitances on board traces.
Transmission lines should be terminated in their characteristic impedance at both ends.
High speed amplifiers are sensitive to capacitance between the inverting input and ground or power supplies.
This shows up as gain peaking at high frequency. The capacitor raises device gain at high frequencies by
making R_G appear smaller. Capacitive output loading will exaggerate this effect.
One possible remedy for this effect is to slightly increase the value of the feedback (and gain set) resistor. This
will tend to offset the high frequency gain peaking while leaving other parameters relatively unchanged. If the
device has a capacitive load as well as inverting input capacitance, using a series output resistor as described in
Driving Capacitive Loads will help.
C₁
R₁₁
680 pF 20:
e_in
+ ₍₁₎
R₃
-
50:
R₈
3.5 k:
R₄
-
1:
(2)
+
R₉
3.5 k:
-
R₅
1:
(3)
+
R₁₀
3.5 k:
-
R₆
1:
(4)
+
R₇
1:
R₁
750:
R₂
3 k:
OUTPUT
Figure 37. High Output Current Composite Amplifier
When higher currents are required than a single amplifier can provide, the circuit of Figure 37 can be used.
Careful attention to a few key components will optimize performance from this circuit. The first thing to note is
that the buffers need slightly higher value feedback resistors than if the amplifiers were individually configured.
As well, R₁₁ and C₁ provide mid circuit frequency compensation to further improve stability. The composite
amplifier has approximately twice the phase delay of a single circuit. The larger values of R₈, R₉ and R₁₀, as well
as the high frequency attenuation provided by C₁ and R₁₁, ensure that the circuit does not oscillate.
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