THS3061_16 Datasheet, PDF (19/33 Pages) Texas Instruments – LOW-DISTORTION, HIGH SLEW RATE, CURRENT-FEEDBACK AMPLIFIERS

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If a multiple-pole filter is required, a Sallen-Key filter
can work very well with CFB amplifiers. This is
because the filtering elements are not in the negative
feedback loop and stability is not compromised.
Because of their high slew rates and high
bandwidths, CFB amplifiers can pass very accurate
signals and help minimize distortion. An example is
shown in Figure 55.
C1
V_I
R1
+
R2
_
C2
R_f
R_g
R1 = R2 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
f–3dB
+
1
2pRC
( ) ^{Rg =}
R_f
1
2– _Q
Figure 55. 2-Pole Low-Pass Sallen-Key Filter
There are two simple ways to create an integrator
with a CFB amplifier. The first, shown in Figure 56,
adds a resistor in series with the capacitor. This is
acceptable, because at high frequencies, the resistor
is dominant and the feedback impedance never drops
below the resistor value. The second, shown in
Figure 57, uses positive feedback to create the
integration. Caution is advised because oscillations
can occur due to the positive feedback.
R_g
V_I
R_f
C1
–
+
V_O
THS306x
ȡ ȣ _VO
ǒ Ǔ_{ȧȢ ȧȤ VI}
+
Rf
Rg
S
)
1
RfC1
S
THS3061
THS3062
SLOS394B – JULY 2002 – REVISED NOVEMBER 2009
R_g
R_f
For Stable Operation:
–
THS306x ₊
R1
R2
V_I
R2
R1 || R_A ^≥
R_f
R_g
V_O
( ) ^{VO ≅ VI}
1+
R_f
R_g
sR1C1
R_A
C1
Figure 57. Noninverting CFB Integrator
The THS306x may also be employed as a very good
video-distribution amplifier. One characteristic of
distribution amplifiers is the fact that the differential
phase (dP) and the differential gain (dG) are
compromised as the number of lines increases and
the closed-loop gain increases. Be sure to use
termination resistors throughout the distribution
system to minimize reflections and capacitive loading.
R_g
R_f
75-Ω Transmission Line
–
75 Ω
V_I
+
V_O1
THS306x
75 Ω
N Lines
75 Ω
75 Ω
75 Ω
V_ON
Figure 58. Video Distribution Amplifier
Application
Figure 56. Inverting CFB Integrator
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Product Folder Link(s): THS3061 THS3062
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