THS3202_16 Datasheet, PDF (24/37 Pages) Texas Instruments – 2-GHz, LOW DISTORTION, DUAL CURRENT-FEEDBACK AMPLIFIERS

THS3202
SLOS242F – SEPTEMBER 2002 – REVISED JANUARY 2010
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GENERAL CONFIGURATIONS
A common error for the first-time CFB user is creating a unity-gain buffer amplifier by shorting the output directly
to the inverting input. A CFB amplifier in this configuration oscillates and is not recommended. The THS3202, like
all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing capacitors directly
from the output to the inverting input is not recommended. This is because, at high frequencies, a capacitor has
a very low impedance. This results in an unstable amplifier and should not be considered when using a
current-feedback amplifier. Because of this, integrators and simple low-pass filters, which are easily implemented
on a VFB amplifier, have to be designed slightly differently. If filtering is required, simply place an RC-filter at the
noninverting terminal of the operational amplifier, as shown in Figure 83.
R_G
R_F
f_–3dB =
1
2pR1C1
-
V_I
+
R1
C1
V_O
V_{O =}
R_F
1+
V_I
R_G
1
1 + sR1C1
Figure 83. Single-Pole Low-Pass Filter
If a multiple-pole filter is required, the use of 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 create very accurate signals and help minimize
distortion. An example is shown in Figure 84.
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 84. 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 85, 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 86, 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
THS3202
ȡ ȣ _VO
ǒ Ǔ_{ȧȢ ȧȤ VI}
+
Rf
Rg
S
)
1
RfC1
S
Figure 85. Inverting CFB Integrator
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