THS3001 Datasheet, PDF (26/32 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS

THS3001, THS3002
420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS
SLOS217A – JULY 1998 – REVISED JUNE 1999
APPLICATION INFORMATION
general configurations
A common error for the first-time CFB user is the creation of a unity gain buffer amplifier by shorting the output
directly to the inverting input. A CFB amplifier in this configuration will oscillate and is not recommended. The
THS300x, 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 57).
RG
RF
VI
R1
–
+
C1
VO
ǒ Ǔǒ Ǔ ^{+ f–3dB}
1
2pR1C1
+ ) ₎ ^VO
VI
1
RF
RG
1
1
sR1C1
Figure 57. 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 58.
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 58. 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 59, 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 60, uses
positive feedback to create the integration. Caution is advised because oscillations can occur due to the positive
feedback.
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