Download Combining an Amplifier with the BUF634

APPLICATION BULLETIN
®
Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Tel: (520) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (520) 889-1510 • Immediate Product Info: (800) 548-6132
COMBINING AN AMPLIFIER WITH THE BUF634
By Uwe Vöhringer, Burr-Brown International GmbH
COMBINED OP AMP AND BUFFER ACHIEVE
HIGHER OUTPUT POWER AND MORE SPEED
V+
As long as amplifiers have existed, engineers have been
dreaming of an “ideal” op amp. As little noise as possible,
high bandwidth, great precision, unlimited input impedance,
and output impedance close to 0Ω—these are specifications
desirable for every application. Unfortunately, no op amp
can fulfill all of these requirements, particularly not while
remaining affordable. A good solution, therefore, is to combine two components, using the best of both parts to achieve
desired specifications.
C1
OPA
FIGURE 1. Composite Amplifier Using BUF634.
CALCULATING THE LOAD RESISTANCE (RLOAD)
FOR A 500mA OUTPUT CURRENT
The output voltage of the buffer was fixed at 15Vp-p for all
measurements to ensure that the op amp would remain
within its linear operating range. The circuit was configured
at gain 2 since the input is terminated at 50Ω for the high
frequency measurements. To achieve the 15Vp-p output
voltage at gain 2, the following rms-input voltage is required:
Possible applications for this combination include cable
drivers, virtual ground drivers for a dynamic load, or low
distortion end stages for both audio and video signal generators. In this circuit configuration, the work is divided so that
the op amp is responsible for precision while the buffer
provides the necessary current. An important advantage of
the combination is that the power dissipation is managed by
the buffer. The op amp is loaded only by the low input
current of the buffer amplifier. The temperature at the op
amp is only slightly higher than in the no-load mode. The
circuit parameters such as offset, drift, noise, and harmonic
distortion depend almost entirely upon the op amp used in
the circuit and have practically no influence on the configuration even when the temperature of the buffer rises. The
combination was tested using four different op amps. The
measurement diagrams in Figures 3 through 15 show the
performance of the various combinations.
V IN =
SBOA065
V OUT P−P
2 • 2 • Gain
=
15Vp−p
= 2.652Vrms
2• 2 •2
The load resistance for a peak output current of 500mA
equals:
15Vp−p
= 15Ω
2 • 500mA
For low-end audio circuits, the OPA604 is used for lownoise and low-distortion applications at frequencies of up to
about 100kHz. The OPA627, OPA671, and OPA603 are
used for higher frequency applications. As already mentioned, the buffer is located in the feedback loop of the op
amp. This configuration compensates the buffer’s internal
resistance so that the output resistance of the entire circuit is
close to zero. At high frequencies with high loads, however,
the internal resistance of the buffer increases, leading to a
rise in distortion as well. For this reason, the circuit contains
three BUF634T in parallel in order to achieve an output
current of 500mA, even though two of these components
would have sufficed for this current to be attained (see
Figure 2).
1995 Burr-Brown Corporation
VO
BW
V–
The following application note describes a combination
using an op amp with the high-speed buffer BUF634 located
in its feedback loop (see Figure 1). Depending upon the op
amp selected, large signals with output currents of over
500mA into the MHz range can be attained.
©
BUF634
VIN
The 50Ω series resistor at the buffer outputs provides reflection-free termination in the high-frequency range. No series
resistors were used between the output of op amp A1 and the
buffer inputs since they would form a low-pass filter in
combination with the input capacitance of the buffers. Any
phase shift resulting from this low-pass could cause the
entire circuit to oscillate, particularly when an op amp like
the OPA603 is used.
When selecting the value of resistors RF and R1, which
determine the gain, it should be noted that RF determines the
bandwidth and stability for current-feedback op amps, they
also determine the open-loop gain. Resistor values of 2.7kΩ
AB-101
Printed in U.S.A. September, 1995
RF
2.7kΩ
V+
Gain = 1 +
5
V+
10Ω
2
2.2µF
VIN
50Ω
V+
5
100nF
2
5
A1
4
2
100nF
G=1
BUF634T
4
50Ω
50Ω Cable
VO
1
BW
3
RLOAD
OPA604
OPA627
OPA671
OPA603
V–
+
R2
220Ω
BUF634T
4
V–
7
3
= 2
1
BW
3
+
R1
2.7kΩ
G=1
RF
R1
V+
2.2µF
5
10Ω
2
V–
G=1
BUF634T
4
1
BW
3
V–
FIGURE 2. Circuit Schematic of the Final Composite Amplifier.
a typical offset voltage of ±30mV, the compensation current
(IC) between the buffers equals the following:
have proven to be a good value for this circuit. When the two
resistors are lowered to 820Ω, the closed-loop gain still
remains the following:
IC =
820Ω 
G = 1+ 
=2
 820Ω 
60mV
= 3mA
2 •10Ω
The maximum offset voltage of 200mV results in a compensation current of:
200mV
IC =
= 10mA
2 •10Ω
The open-loop gain increases for the current-feedback amplifier, which would result in a higher chance of oscillation.
For the voltage-feedback op amps (OPA604, OPA627 and
OPA671), the resistors are less important since they do not
influence the open-loop gain.
As expected, measurements using the four different op amps
showed that for the audio range, the op amps OPA627,
OPA671, and OPA604 produce lower harmonic distortion
than the OPA603. Since harmonic distortion rises with
frequency, the OPA604 should not be used above 50kHz,
and the OPA627 should not be used above 100kHz. Between
100kHz and 1MHz, the OPA671 has significantly lower
distortion than the OPA627 and the OPA604. Above 1MHz,
however, the high-speed op amp OPA603 is the best choice.
In composite amplifier circuits such as the one in Figure 1,
a capacitor (C1) is often located between the output of the op
amp and its inverted input. This capacitor, along with R1 and
RF, forms a low-pass filter which prevents high-frequency
circuit oscillation. The high bandwidth of the BUF634
(180MHz) keeps both the group delay time and the phase
shift low, avoiding the need for the capacitor. The advantage
of this configuration is that the cutoff frequency is determined solely by the op amp. In current-feedback op amps
such as the OPA603, a capacitor in the feedback loop could
lead to stability problems. The output resistance of the
BUF634 is about 10Ω. Therefore, series output resistors for
decoupling the individual buffers are no longer necessary.
At differing offset voltages, compensation currents flow
because the buffers are in parallel to each other. Assuming
Figure 3 through 15 show the harmonic distortion and
Figures 16 through 19 show the frequency responses of the
four op amps. Figure 3, 7, 11, and 14 show the harmonic
distortions of the sine generator. This distortion affects the
measurement diagrams as well, especially at frequencies of
1MHz and higher.
2
AC PERFORMANCE OF THE CIRCUIT
The AC performance of the circuit using the various op
amps was measured using a spectrum analyzer at a 15Ω
load.
the resulting buffer output voltage is 5.241Vrms. The peak
value is calculated as follows:
The analyzer could only deliver a maximum output of
0dBm at 50Ω, corresponding to a voltage of 223mVrms. For
this reason, the resistor R1 at the inverting input of the op
amp was reduced from 2.7kΩ to 120Ω, achieving a gain of:
When RLOAD is 15Ω, the peak current is 494mA. It is clear
that only the current-feedback op amp OPA603 can be used
for high frequencies (fg = 23MHz).
Vp = 5. 241V • 2 = 7. 4Vp (or 14.8Vp−p)
For higher outputs in the audio range, the OPA541 can be
used instead of the BUF634.
PROTECTION CIRCUITRY
Since the BUF634 is equipped with a short-circuit and a
thermal protection, no extra protection circuitry is necessary.
25
25
15
15
5
5
–5
–5
–15
–15
VO (dBm)
VO (dBm)
2. 7kΩ 
G = 1+ 
= 23. 5
 120Ω 
At an input voltage of 223mVrms and a gain factor of 23.5,
–25
–35
–25
–35
–45
–45
–55
–55
–65
–65
–75
–75
15
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
15
23.5
32
40.5
Frequency (kHz)
57.5
66
79.5
83
91.5 100
FIGURE 4. Spectrum of the BUF634T with the OPA604/
627 at 20kHz, G = 2.
FIGURE 3. Spectrum of the Sine Generator at 20kHz.
25
25
15
15
5
5
–5
–5
–15
–15
VO (dBm)
VO (dBm)
49
Frequency (kHz)
–25
–35
–25
–35
–45
–45
–55
–55
–65
–65
–75
–75
15
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
15
Frequency (kHz)
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
Frequency (kHz)
FIGURE 5. Spectrum of the BUF634T/OPA671 at 20kHz,
G = 2.
FIGURE 6. Spectrum of the BUF634T/OPA603 at 20kHz,
G = 2.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
3
25
15
15
5
5
–5
–5
–15
–15
VO (dBm)
VO (dBm)
25
–25
–35
–25
–35
–45
–45
–55
–55
–65
–65
–75
–75
80
80
172 264 356 448 590 632 724 816 908 1000
FIGURE 8. Spectrum of the BUF634T/OPA627 at 100kHz,
G = 2.
25
25
15
15
5
5
–5
–5
–15
–15
VO (dBm)
VO (dBm)
FIGURE 7. Spectrum of the Sine Generator at 100kHz.
–25
–35
–25
–35
–45
–45
–55
–55
–65
–65
–75
–75
80
172 264 356 448 590 632 724 816 908 1000
80
Frequency (kHz)
172 264 356 448 590 632 724 816 908 1000
Frequency (kHz)
FIGURE 10. Spectrum of the BUF634T/OPA603 at 100kHz,
G = 2.
FIGURE 9. Spectrum of the BUF634T/OPA671 at 100kHz,
G = 2.
25
25
15
15
5
5
–5
–5
–15
–15
VO (dBm)
VO (dBm)
172 264 356 448 590 632 724 816 908 1000
Frequency (kHz)
Frequency (kHz)
–25
–35
–25
–35
–45
–45
–55
–55
–65
–65
–75
–75
0.8 1.82 2.84 3.86 4.88 5.9 6.92 7.94 8.96 9.98
11
0.8 1.82 2.84 3.86 4.88 5.9 6.92 7.94 8.96 9.98
Frequency (MHz)
11
Frequency (MHz)
FIGURE 11. Spectrum of the Sine Generator at 1MHz.
FIGURE 12. Spectrum of the BUF634T/OPA671 at 1MHZ,
G = 2.
4
25
15
15
5
5
–5
–5
–15
–15
VO (dBm)
VO (dBm)
25
–25
–35
–25
–35
–45
–45
–55
–55
–65
–65
–75
–75
0.8 1.82 2.84 3.86 4.88 5.9 6.92 7.94 8.96 9.98
11
4
7.6 11.2 14.8 18.4
Frequency (MHz)
25.6 29.2 32.8 36.4
40
FIGURE 14. Spectrum of the Sine Generator at 5MHz.
25
22
15
21
5
20
–5
19
–15
18
VO (dBm)
VO (dBm)
FIGURE 13. Spectrum of the BUF634T/OPA603 at 1MHz.
G = 2.
–25
–35
fg = 282kHz
17
16
–45
15
–55
14
–65
13
–75
12
4
7.6 11.2 14.8 18.4
22
25.6 29.2 32.8 36.4
40
0
50
100 150 200 250 300 350 400 450 500
Frequency (MHz)
Frequency (kHz)
FIGURE 16. Frequency Response of the OPA604 (G = 23).
FIGURE 15. Spectrum of the BUF634T/OPA603 at 5MHz,
G = 2.
22
22
21
21
20
20
19
19
18
VO (dBm)
VO (dBm)
22
Frequency (MHz)
fg = 542kHz
17
16
18
17
15
15
14
14
13
13
12
fg = 984kHz
16
12
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
Frequency (MHz)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Frequency (MHz)
FIGURE 17. Frequency Response of the OPA627 (G = 23).
FIGURE 18. Frequency Response of the OPA671 (G = 23).
5
22
22
0.5dB
21
20
f = 13.36MHz
19
19
18
18
VO (dBm)
VO (dBm)
20
fg = 23.08MHz
17
16
15
14
13
13
12
12
8
12
16
20
24
28
32
36
40
fg
fg
fg
OPA604
0
Frequency (MHz)
fg
16
14
4
OPA671
17
15
0
OPA603
21
.4
OPA627
.8
1.2
1.6
2
16
20
24
28
32
Frequency (MHz)
FIGURE 19. Frequency Response of the OPA603 (G = 23).
FIGURE 20. Frequency Responses of the Four Op Amps
(G = 23).
6
IMPORTANT NOTICE
Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
any product or service without notice, and advise customers to obtain the latest version of relevant information
to verify, before placing orders, that information being relied on is current and complete. All products are sold
subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those
pertaining to warranty, patent infringement, and limitation of liability.
TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent
TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily
performed, except those mandated by government requirements.
Customers are responsible for their applications using TI components.
In order to minimize risks associated with the customer’s applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other
intellectual property right of TI covering or relating to any combination, machine, or process in which such
semiconductor products or services might be or are used. TI’s publication of information regarding any third
party’s products or services does not constitute TI’s approval, warranty or endorsement thereof.
Copyright  2000, Texas Instruments Incorporated