THS3001IDR Datasheet, PDF (25/39 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIER

THS3001
www.ti.com................................................................................................................................................. SLOS217H – JULY 1998 – REVISED SEPTEMBER 2009
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
THS3001, 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 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 56).
R_G
R_F
V_I
R1
−
+
C1
f_{*3 dB}
+
1
2pR1C1
V_O
ǒ Ǔǒ Ǔ ^VO
VI
+
1
)
RF
RG
1
1 ) sR1C1
Figure 56. Single-Pole Low-Pass Filter
If a multiple-pole filter is required, the use of a Sallen-Key filter can work 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 accurate signals and help minimize
distortion. An example is shown in Figure 57.
C1
V_I
R1
+
R2
_
C2
R_F
R_G
R1 = R2 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
f_{*3 dB}
+
1
2pRC
( ) ^{RG =}
R_F
1
2− _Q
Figure 57. 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 58, 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 59, 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
THS3001
ȡ ȣ _VO
ǒ Ǔ_{ȧȢ ȧȤ VI}
+
RF
RG
S
)
1
RFC1
S
Figure 58. Inverting CFB Integrator
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