Texas Instruments | Matching operational amplifier bandwidth with application | Application notes | Texas Instruments Matching operational amplifier bandwidth with application Application notes

Texas Instruments Matching operational amplifier bandwidth with application Application notes
Amplifiers: Op Amps
Texas Instruments Incorporated
Matching operational amplifier bandwidth
with applications
By Ron Mancini
Senior Application Specialist, Operational Amplifiers
Introduction
Selecting the correct op amp for an application requires
investigation of many different parameters. Voltage offset,
bias currents and similar parameters are easy to evaluate
because they are DC parameters that do not vary with
frequency. Accuracy, on the other hand, is hard to specify
and comply with because it is a function of frequency;
hence, accuracy specifications involve the knowledge of
frequency-dependent feedback circuits that are bandwidthdependent.
The bandwidth (BW) problem is complicated by the op
amp’s feedback because it hides decreasing BW until
accuracy problems become apparent. If op amps had a
constant open-loop gain, the accuracy of an op amp circuit would remain constant. The open-loop gain of any op
amp decreases with increasing frequency. Except for a
phenomenon called “peaking,” all op amps lose accuracy
at high frequencies. The designer’s problem is selecting
an op amp that has an acceptable accuracy loss at the
frequencies of interest. Proper analysis of this problem
requires an understanding of feedback, loop gain, and
frequency dependence.
Preserving the signal integrity or accuracy during
amplification is an essential part of the design, but in
order to preserve the signal one must define the signal.
Defining the signal sounds like a simple task, but it is
complicated and must be performed in several different
ways. Various methods used for defining the frequency
content of the signal are examined in detail in this article
because no single method works for every case.
Feedback and accuracy
The basic feedback circuit is shown in Figure 1, where E
is the error voltage, β is the feedback factor, and A is the
forward gain. Equations 1 and 2 govern the circuit
performance.
(1)
V
= EA
OUT
(2)
E = VIN − βVOUT = VIN − βEA
Figure 1. Basic feedback loop
VIN
+
E
Σ
A
–
β
VOUT
The accuracy equation (Equation 3) is obtained by
combining Equations 1 and 2.
E
1
=
VIN 1 + Aβ
(3)
Equation 4, which is the circuit gain, is also obtained
from Equations 1 and 2 and is shown for completeness.
VOUT
A
=
VIN
1 + Aβ
(4)
The quantity Aβ appears in both equations and is called
loop gain because it has a special significance in feedback
circuits. The loop gain determines the stability of a feedback circuit as shown in Equation 4 (instability occurs
when Aβ = –1), and it determines the accuracy as shown
in Equation 3. Accuracy and stability are inversely
related—i.e., stability decreases as accuracy increases,
and vice versa. The loop gain is calculated with the voltage inputs grounded (current inputs open), so the input
signal and position (plus or minus input) have no effect
on the loop gain. This means that the loop gain for a noninverting, inverting, or differential op amp is the same.
Three op amp circuits are shown in Figure 2, and the loop
gain for all circuits is given in Equation 5.
Aβ =
aRG
RF + RG
(5)
The parameter “a” is the open-loop gain of the op amp,
and it is often confused with the forward gain, “A.” The
op amp open-loop gain decreases with frequency, hence
the error increases with frequency, as Equation 3 illustrates.
A more in-depth analysis of stability and feedback is
found in References 1, 2, and 3.
Defining a signal to determine its BW
The simplest case exists when the amplifier circuit specifications are included in the system specifications or given to
the amplifier designer. A good amplifier specification shows
the low-frequency gain at one frequency and the highfrequency gain at a second frequency. Sometimes the rate
of the gain decrease (gain roll-off) is specified in dB/decade.
When a complex signal is applied to the amplifier input,
and only a distortion or fidelity specification is given, the
systems designer has passed the signal definition problem
to the circuit designer. The circuit designer must determine
what portion of the signal can be sacrificed because of loop
gain reduction (“a” decreases with frequency) while meeting
the distortion or fidelity specification. The first step in this
procedure is to divide the signal into segments and analyze
each segment using a Fourier series. An arbitrary maximum
frequency is chosen; frequencies exceeding the maximum
frequency are discarded, and the signal is reconstructed
from the remnants. If the signal meets the specification,
36
Analog and Mixed-Signal Products
February 2000
Analog Applications Journal
Amplifiers: Op Amps
Texas Instruments Incorporated
the maximum frequency equals the BW requirement. If
the signal does not meet the specification, a new maximum frequency is chosen and this procedure is repeated
until the required distortion or fidelity specification is met.
Computer programs best implement the Fourier series
procedure, but the procedure is complicated and laborious,
so many engineers take the easy path of looking at test
results. It is common for engineers to use the video screen
or data error rate to evaluate amplifiers. They will solder
the test units into working circuit boards and evaluate the
video op amp performance by observing the screen.
Likewise, data transmission amplifiers are often evaluated
by in-circuit testing. The Fourier procedure must be used
in many designs because in-circuit testing does not allow
for manufacturing tolerances, it is not as accurate as the
Fourier procedure, and it is hard to use where the results
are not easily observable.
At this point it may seem that the easiest and safest
path is to select an op amp with a BW much larger than
required, but that isn’t an option in most cases because
extra BW is costly and amplifies noise. The extra cost is
prohibitive in multiple op amp or high-volume applications; thus, in-circuit testing or Fourier series analysis is
used to evaluate the BW requirements of op amps. Extra
BW can’t multiply the signal, and “Murphy’s Law” guarantees that it will multiply noise. When the only op amp that
fits the BW requirement has extra BW, the designer
should consider putting a passive filter in the signal chain
to limit the noise passed by the system.
There is an extra requirement imposed on op amps that
are used in active filter circuits. These op amps must have
adequate BW to support the signal and to function as an
active filter at noise frequencies. Often active filter op amps
have their BW set by the noise frequencies rather than by
the signal frequencies. Circuit designers must predict the
highest noise frequency by calculation, measurement, or
experience if they want to design good workable filters.
Voltage feedback op amps
The gain versus frequency plot of a typical voltage feedback
amplifier (VFA) is shown in Figure 3. The loop gain-plusone separates the closed-loop gain and forward gain plots.
The closed-loop gain is down 3 dB at the intersection point.
The loop gain decreases at –20 dB/decade beginning at low
frequencies. The error increases as the frequency increases.
The open-loop gain plot of the TLV2472 is shown in
Figure 4. This plot defines the op amp open-loop gain,
“a,” which is not necessarily the forward gain, “A.” The
error equation for the op amp circuits shown in Figure 2
is given in Equation 6.
E
1
=
=
VIN 1 + Aβ
1
aRG
1+
RF + RG
(6)
Continued on next page
Figure 3. Plot of op amp equation
Gain in dB
20 log A
Figure 2. Op amp circuits
RG
VIN
–20 dB / Decade
Intersection Point
RF
20 log VOUT
VIN
VOUT
a
log f
0 dB
Frequency in Hz
(a) Inverting Op Amp
VOUT
a
VIN
Figure 4. Open-loop gain plot of the TLV247x
A VD - Differential Voltage Gain (dB)
100
(b) Non-inverting Op Amp
RG
RF
VIN –
RG
VIN +
a
VOUT
RF
80
60
45
0
-45
40
-90
20
-135
0
-180
-20
-225
-40
100
(c) Differential Op Amp
VDD = ± 2.5 V
R L = 600Ω
CL = 0
TA = 25°C
1k
10k
100k
1M
10M
Phase (Degrees)
RF
RG
-270
100M
Frequency (Hz)
37
Analog Applications Journal
February 2000
Analog and Mixed-Signal Products
Amplifiers: Op Amps
Texas Instruments Incorporated
The loop gain equation contains the closed-loop gain
equation; thus, the error is dependent on the closed-loop
gain and the amplifier frequency response.
For a non-inverting circuit with a closed-loop gain of 2
(6 dB), the open-loop gain is approximately 61 dB at
1 kHz; therefore, the non-inverting circuit built with a
TLV2472 op amp has about 0.18% error at 1 kHz. For a
non-inverting circuit with a closed-loop gain of 10 (20 dB),
the open-loop gain is approximately 43 dB at 1 kHz;
therefore, the non-inverting circuit built with a TLV2472
op amp has about 7.9% error at 1 kHz.
For an inverting circuit with a closed-loop gain of 2
(6 dB), the open-loop gain is approximately 61 dB at
1 kHz; therefore, the non-inverting circuit built with a
TLV2472 op amp has about 0.26% error at 1 kHz. For a
non-inverting circuit with a closed-loop gain of 10 (20 dB),
the open-loop gain is approximately 43 dB at 1 kHz;
therefore, the non-inverting circuit built with a TLV2472
op amp has about 8.7% error at 1 kHz.
Although the advertised gain-BW product of the
TLV2472 is 2.8 MHz, circuits built with this IC can show
gain errors at much lower frequencies because the amplifier gain starts falling off at much lower frequencies.
Current feedback op amps
The loop gain for a current feedback amplifier (CFA) is
given in Equation 7, where Z is the transimpedance (sometimes called transresistance) and ZB is the input buffer’s
output impedance. Z functions in a CFA like the amplifier
gain, “a,” does in a VFA. Both are very large quantities, so
they are very hard to measure. Transimpedance measurements must be made at very high frequencies and in the
presence of noise, so many manufacturers do not include
transimpedance plots in their data sheets.
Aβ =
Z

ZB 
RF  1 +

 RF + RG 
(7)
The input buffer’s output impedance is made very small
by design, and when it is neglected, Equation 8 results.
Aβ =
Z
RF
(8)
The closed-loop gain is not contained in the loop gain;
thus, the CFA BW and error are independent of closedloop gain. The manufacturer seldom plots the transimpedance, hence open-loop gain plots cannot be used to
calculate the error. The manufacturers do plot amplitude
versus frequency as a function of feedback resistance,
supply voltage, and closed-loop gain. These plots are used
to determine the accuracy of the circuit.
Figure 5 is the closed-loop response plot of the THS3001;
notice that the response can be peaked, flat, or rolled off.
The peaked response (RF = 750 Ω) creates distortion in a
perfect signal because it emphasizes the high-frequency
components in the signal. Sometimes the peaked response
is chosen because it compensates for high-frequency gain
lost due to stray capacitances or cables. Some CFAs have
external leads that enable peaking control so that the
overall response can be made flat.
Figure 5. Closed-loop response plot of
the THS3001
3
2
1
Output Amplitude (dB)
Continued from previous page
Gain = 1
VCC = ± 5 V
R L = 150Ω
VI = 200 mVrms
RF = 750Ω
0
-1
R F = 1 kΩ
-2
-3
R F = 1.5 kΩ
-4
-5
-6
100k
1M
10M
100M
1G
Frequency (Hz)
The rolled-off response (RF = 1.5 kΩ) is used only
when a less expensive op amp having the correct BW
can’t be found. When the signal requires a 10-MHz BW,
and a lower-cost op amp can’t be found, the designer
often makes RF = 1.5 kΩ or slightly more to roll off the
gain so that the circuit cannot amplify high-frequency
noise. The RF = 1 kΩ response is usually chosen because
it amplifies the signal with the best fidelity.
Conclusions
Determining the amplifier’s required BW can be as simple as
in-circuit testing or as complicated as using Fourier series
analysis. The VFA loop gain contains the closed-loop gain;
thus, the error is related to the closed-loop gain and amplifier
frequency response. Selecting the proper BW VFA consists
of using the op amp open-loop gain plot to calculate the
error at the operating frequency. Selecting the proper CFA
consists of reviewing the closed-loop gain plots and calculating the error based on these plots. In either case, excess
BW is detrimental to good circuit performance because it
contributes to instability, increases cost, and amplifies noise.
References
For more information related to this article, visit the TI Web
site at www.ti.com/ and look for the following materials by
entering the TI literature number into the quick-search box.
Document Title
TI Lit. #
1. “Feedback Amplifier Analysis Tools” . . . . . .SLOA017
2. “Stability Analysis of Voltage Feedback
Op Amps, Including Compensation
Techniques” . . . . . . . . . . . . . . . . . . . . . . . . .SLOA020
3. “Current Feedback Amplifier Analysis
and Compensation” . . . . . . . . . . . . . . . . . . .SLOA021
Related Web sites
amplifier.ti.com
analog.ti.com
Get product data sheets at:
www.ti.com/sc/docs/products/analog/device.html
Replace device with ths3001 or tlv2472
38
Analog and Mixed-Signal Products
February 2000
Analog Applications Journal
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