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APPLICATION BULLETIN CALCULATING THE LOAD RESISTANCE (R
LOAD
)
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 50 for the high
frequency measurements. To achieve the 15Vp-p output
voltage at gain 2, the following rms-input voltage is re-
quired:
The load resistance for a peak output current of 500mA
equals:
The 50 series resistor at the buffer outputs provides reflec-
tion-free termination in the high-frequency range. No series
resistors were used between the output of op amp A
1
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 R
F
and R
1
, which
determine the gain, it should be noted that R
F
determines the
bandwidth and stability for current-feedback op amps, they
also determine the open-loop gain. Resistor values of 2.7k ©
1995 Burr-Brown Corporation
AB-101
Printed in U.S.A. September, 1995
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
COMBINED OP AMP AND BUFFER ACHIEVE
HIGHER OUTPUT POWER AND MORE SPEED
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 0 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 com-
bine two components, using the best of both parts to achieve
desired specifications.
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.
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 genera-
tors. 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 configu-
ration 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.
For low-end audio circuits, the OPA604 is used for low-
noise and low-distortion applications at frequencies of up to
about 100kHz. The OPA627, OPA671, and OPA603 are
used for higher frequency applications. As already men-
tioned, the buffer is located in the feedback loop of the op
amp. This configuration compensates the buffers 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).
FIGURE 1. Composite Amplifier Using BUF634.
COMBINING AN AMPLIFIER WITH THE BUF634
By Uwe Vöhringer, Burr-Brown International GmbH
OPA
BUF634
V
IN
V
V
O
C
1
BW
V+
V
IN
=
V
OUT
P P
2 2 Gain
=
15Vp p
2 2 2
=
2. 652Vrms
15Vp p
2 500mA
=
15
2
a typical offset voltage of
±
30mV, the compensation current
(I
C
) between the buffers equals the following:
The maximum offset voltage of 200mV results in a compen-
sation current of:
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.
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.
FIGURE 2. Circuit Schematic of the Final Composite Amplifier.
G
=
1
+
820 820


=
2
I
C
=
60mV
2 10
=
3mA
I
C
=
200mV
2 10
=
10mA
have proven to be a good value for this circuit. When the two
resistors are lowered to 820 , the closed-loop gain still
remains the following:
The open-loop gain increases for the current-feedback am-
plifier, 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.
In composite amplifier circuits such as the one in Figure 1,
a capacitor (C
1
) is often located between the output of the op
amp and its inverted input. This capacitor, along with R
1
and
R
F
, 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 deter-
mined 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 10 . 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
G = 1
V+
BW
1
2
3
5
4
V
BUF634T
G = 1
V+
BW
1
2
5
3
5
4
V
BUF634T
G = 1
V+
BW
1
2
3
5
4
V
BUF634T
50 2
3
R
F
2.7k R
2
220 R
1
2.7k 50 V
IN
4
7
V
100nF
2.2
µ
F
10 10 V+
100nF
A
1
2.2
µ
F
OPA604
OPA627
OPA671
OPA603
Gain = 1 +
R
F
R
1
= 2
50 Cable
R
LOAD
V
O
+
+ 3
AC PERFORMANCE OF THE CIRCUIT
The AC performance of the circuit using the various op
amps was measured using a spectrum analyzer at a 15 load.
The analyzer could only deliver a maximum output of
0dBm at 50 , corresponding to a voltage of 223mVrms. For
this reason, the resistor R
1
at the inverting input of the op
amp was reduced from 2.7k to 120 , achieving a gain of:
At an input voltage of 223mVrms and a gain factor of 23.5,
the resulting buffer output voltage is 5.241Vrms. The peak
value is calculated as follows:
When R
LOAD
is 15 , the peak current is 494mA. It is clear
that only the current-feedback op amp OPA603 can be used
for high frequencies (f
g
= 23MHz).
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.
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 users 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.
FIGURE 4. Spectrum of the BUF634T with the OPA604/
627 at 20kHz, G = 2.
FIGURE 3. Spectrum of the Sine Generator at 20kHz.
FIGURE 6. Spectrum of the BUF634T/OPA603 at 20kHz,
G = 2.
FIGURE 5. Spectrum of the BUF634T/OPA671 at 20kHz,
G = 2.
G
=
1
+
2. 7k 120


=
23. 5
Vp
=
5. 241V 2
=
7. 4Vp (or 14.8Vp p)
25
15
5
5
15
25
35
45
55
65
75
15
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
V
O
(dBm)
Frequency (kHz)
25
15
5
5
15
25
35
45
55
65
75
15
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
V
O
(dBm)
Frequency (kHz)
25
15
5
5
15
25
35
45
55
65
75
15
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
V
O
(dBm)
Frequency (kHz)
25
15
5
5
15
25
35
45
55
65
75
15
23.5
32
40.5
49
57.5
66
79.5
83
91.5 100
V
O
(dBm)
Frequency (kHz) 4
FIGURE 8. Spectrum of the BUF634T/OPA627 at 100