Datasheet

Texas Instruments Incorporated
Amplifiers: Op Amps
28
Analog Applications Journal
Analog and Mixed-Signal Products www.ti.com/sc/analogapps 3Q 2003
Figure 11 shows the frequency responses
of several of these chips. Figure 12 shows
the results of using various ferrite chips
from the same AG family.
As expected, all of these graphs show
the same type of results obtained with
the noninverting configuration. Using a
low-Q ferrite chip with high impedance
will result in a stable system. Although the
noise plots for this configuration are not
presented here, they will show the same
type of results as the noninverting configu-
ration; using ferrite chips will have the
lowest output noise of any configuration.
Conclusion
Although this article shows only two con-
figurations with capacitors in the feedback
path, it shows the fundamental feasibility
of this compensation technique. While
resistors do work very well, producing the
most stable responses, the drawbacks of
the output noise coupled with the dc and
ac errors may limit some of the applications.
Using ferrite chips helps alleviate many
of these issues, producing the lowest noise
of all with no dc errors or in-band ac sig-
nal errors; and stability is almost as good
as when utilizing resistors. It is important
to choose the proper ferrite chip with the
amplifier; but this is considered normal
procedure for any circuit design and is no
more difficult than selecting the right
amplifier for the system.
This simple technique helps eliminate
one of the major drawbacks of using the
CFB amplifier while allowing any system
to enjoy many of its benefits. Designers of
multiple feedback filters, for example, once
limited to the use of VFB amplifiers, can
now take advantage of the superior slew
rates and lack of gain-bandwidth product
characteristics found in the CFB amplifier.
References
For more information related to this article,
you can download an Acrobat Reader file
at www-s.ti.com/sc/techlit/litnumber and
replace “litnumber” with the TI Lit. #
for the materials listed below.
Document Title TI Lit. #
1. “Voltage Feedback Vs. Current Feedback
Op Amps,” Application Report . . . . . . . . . . . . . .slva051
2. “The Current-Feedback Op Amp: A High-
Speed Building Block,” Application Bulletin . . .sboa076
3. “Current Feedback Amplifiers: Review,
Stability Analysis, and Applications,”
Application Bulletin . . . . . . . . . . . . . . . . . . . . . . .sboa081
4. “Low-Noise, High-Speed Current Feedback
Amplifiers,” Data Sheet . . . . . . . . . . . . . . . . . . . .slos385
5. “Effect of Parasitic Capacitance in Op Amp
Circuits,” Application Report . . . . . . . . . . . . . . .sloa013
Related Web sites
analog.ti.com
www.ti.com/sc/device/partnumber
Replace partnumber with OPA685, THS3112, THS3202 or
THS4012
10 M 100 M 1 G
Frequency (Hz)
V (dB)
OUT
10
5
0
–5
–10
–15
–20
–25
Z = 681 Ω
Z = BLM18HD601SN1
Z = BLM18HG601SN1
Z = BLM18AG601SN1
Figure 11. Frequency responses above 10 MHz
with ferrite chips (gain = –5)
10 k 100 k 1 M 10 M 100 M 1 G
Frequency (Hz)
V (dB)
OUT
15
10
5
0
–5
–10
–15
–20
–
25
xxx = 221
xxx = 471
xxx = 601
xxx = 102
Z = Ferrite Chip
BLM18AGxxxSN1 Series
Figure 12. Frequency responses with AG series
ferrite chips (gain = –5)