THS3061 Datasheet, PDF (20/25 Pages) Texas Instruments – LOW DISTORTION HIGH SLEW RATE CURRENT FREEBACK AMPLIFIERS

THS3061
THS3062
SLOS394A – JULY 2002 – OCTOBER 2002
www.ti.com
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 THS306x, 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 (see Figure 54).
Rg
Rf
VI
R1
–
+
C1
f_–3dB
+
1
2pR1C1
VO
ǒ Ǔǒ Ǔ ^VO
VI
+
1
)
Rf
Rg
1
1 ) sR1C1
Figure 54. 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 55.
C1
VI
R1
+
R2
_
C2
Rf
Rg
R1 = R2 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
f_–3dB
+
1
2pRC
( ) ^{Rg =}
Rf
1
2– _Q
Figure 55. 2-Pole Low-Pass Sallen-Key Filter
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