THS3202 Datasheet, PDF (25/32 Pages) Texas Instruments – 2-GHZ, LOW DISTORTION, CURRENT FEEDBACK AMPLIFIERS

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THS3202
SLOS242D − SEPTEMBER 2002 − REVISED JANUARY 2004
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 (see Figure 96).
Rg
Rf
VI
R1
−
+
C1
f–3dB
+
1
2pR1C1
VO
ǒ Ǔǒ Ǔ ^VO
VI
+
1
)
Rf
Rg
1
1 ) sR1C1
Figure 96. 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 97.
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 97. 2-Pole Low-Pass Sallen-Key Filter
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