Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd.

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Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd.

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Burr-Brown International GmbH
Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd. Tucson, 85706 Tel: (520) 746-1111 Twx: 910-952-111 Telex: 066-6491 (520) 889-1510 Immediate Product Info: (800) 548-6132
COMBINING AMPLIFIER WITH BUF634
COMBINED BUFFER ACHIEVE HIGHER OUTPUT POWER MORE SPEED long amplifiers have existed, engineers have been dreaming "ideal" amp. little noise possible, high bandwidth, great precision, unlimited input impedance, output impedance close 0-these specifications desirable every application. Unfortunately, fulfill these requirements, particularly while remaining affordable. good solution, therefore, combine components, using best both parts achieve desired specifications. following application note describes combination using with high-speed buffer BUF634 located feedback loop (see Figure Depending upon selected, large signals with output currents over 500mA into range attained. Possible applications this combination include cable drivers, virtual ground drivers dynamic load, distortion stages both audio video signal generators. this circuit configuration, work divided that responsible precision while buffer provides necessary current. important advantage combination that power dissipation managed buffer. loaded only input current buffer amplifier. temperature only slightly higher than no-load mode. circuit parameters such offset, drift, noise, harmonic distortion depend almost entirely upon used circuit have practically influence configuration even when temperature buffer rises. combination tested using four different amps. measurement diagrams Figures through show performance various combinations. low-end audio circuits, OPA604 used lownoise low-distortion applications frequencies about 100kHz. OPA627, OPA671, OPA603 used higher frequency applications. already mentioned, buffer located feedback loop amp. This configuration compensates buffer's internal resistance that output resistance entire circuit close zero. high frequencies with high loads, however, internal resistance buffer increases, leading rise distortion well. this reason, circuit contains three BUF634T parallel order achieve output current 500mA, even though these components would have sufficed this current attained (see Figure
BUF634
FIGURE Composite Amplifier Using BUF634. CALCULATING LOAD RESISTANCE (RLOAD) 500mA OUTPUT CURRENT output voltage buffer fixed 15Vp-p measurements ensure that would remain within linear operating range. circuit configured gain since input terminated high frequency measurements. achieve 15Vp-p output voltage gain following rms-input voltage required:
Gain 15Vp-p 2.652Vrms
load resistance peak output current 500mA equals: 15Vp-p 500mA series resistor buffer outputs provides reflection-free termination high-frequency range. series resistors were used between output buffer inputs since they would form low-pass filter combination with input capacitance buffers. phase shift resulting from this low-pass could cause entire circuit oscillate, particularly when like OPA603 used. When selecting value resistors which determine gain, should noted that determines bandwidth stability current-feedback amps, they also determine open-loop gain. Resistor values 2.7k
AB-101 Printed U.S.A. September, 1995
1995 Burr-Brown Corporation
2.7k BUF634T
Gain
2.2µF
2.7k
100nF OPA604 OPA627 OPA671 OPA603
BUF634T RLOAD Cable
100nF
BUF634T
2.2µF
FIGURE Circuit Schematic Final Composite Amplifier. have proven good value this circuit. When resistors lowered 820, closed-loop gain still remains following:
typical offset voltage ±30mV, compensation current (IC) between buffers equals following:
60mV
open-loop gain increases current-feedback amplifier, which would result higher chance oscillation. voltage-feedback amps (OPA604, OPA627 OPA671), resistors less important since they influence open-loop gain. composite amplifier circuits such Figure capacitor (C1) often located between output inverted input. This capacitor, along with forms low-pass filter which prevents high-frequency circuit oscillation. high bandwidth BUF634 (180MHz) keeps both group delay time phase shift low, avoiding need capacitor. advantage this configuration that cutoff frequency determined solely amp. current-feedback amps such OPA603, capacitor feedback loop could lead stability problems. output resistance BUF634 about Therefore, series output resistors decoupling individual buffers longer necessary. differing offset voltages, compensation currents flow because buffers parallel each other. Assuming
maximum offset voltage 200mV results compensation current 200mV 10mA expected, measurements using four different amps showed that audio range, amps OPA627, OPA671, OPA604 produce lower harmonic distortion than OPA603. Since harmonic distortion rises with frequency, OPA604 should used above 50kHz, OPA627 should used above 100kHz. Between 100kHz 1MHz, OPA671 significantly lower distortion than OPA627 OPA604. Above 1MHz, however, high-speed OPA603 best choice. Figure through show harmonic distortion Figures through show frequency responses four amps. Figure show harmonic distortions sine generator. This distortion affects measurement diagrams well, especially frequencies 1MHz higher.
PERFORMANCE CIRCUIT performance circuit using various amps measured using spectrum analyzer load. analyzer could only deliver maximum output 0dBm corresponding voltage 223mVrms. this reason, resistor inverting input reduced from 2.7k 120, achieving gain
input voltage 223mVrms gain factor 23.5,
resulting buffer output voltage 5.241Vrms. peak value calculated follows:
241V 14.8Vp-p)
When RLOAD peak current 494mA. clear that only current-feedback OPA603 used high frequencies 23MHz). higher outputs audio range, OPA541 used instead BUF634. PROTECTION CIRCUITRY Since BUF634 equipped with short-circuit thermal protection, extra protection circuitry necessary.
(dBm)
23.5 40.5 57.5 79.5 91.5
(dBm)
Frequency (kHz)
23.5 40.5 57.5 79.5 91.5 Frequency (kHz)
FIGURE Spectrum Sine Generator 20kHz.
FIGURE Spectrum BUF634T with OPA604/ 20kHz,
(dBm) (dBm)
23.5 40.5 57.5 79.5 91.5 23.5 40.5 57.5 79.5 91.5 Frequency (kHz) Frequency (kHz)
FIGURE Spectrum BUF634T/OPA671 20kHz,
FIGURE Spectrum BUF634T/OPA603 20kHz,
information provided herein believed reliable; however, BURR-BROWN assumes responsibility inaccuracies omissions. BURR-BROWN assumes responsibility this information, such information shall entirely user's risk. Prices specifications subject change without notice. patent rights licenses circuits described herein implied granted third party. BURR-BROWN does authorize warrant BURR-BROWN product life support devices and/or systems.
(dBm)
(dBm)
1000 Frequency (kHz) 1000 Frequency (kHz)
FIGURE Spectrum Sine Generator 100kHz.
FIGURE Spectrum BUF634T/OPA627 100kHz,
(dBm)
1000 Frequency (kHz)
(dBm)
1000 Frequency (kHz)
FIGURE Spectrum BUF634T/OPA671 100kHz,
FIGURE Spectrum BUF634T/OPA603 100kHz,
(dBm) (dBm) 1.82 2.84 3.86 4.88 6.92 7.94 8.96 9.98 Frequency (MHz)
1.82 2.84 3.86 4.88 6.92 7.94 8.96 9.98 Frequency (MHz)
FIGURE Spectrum Sine Generator 1MHz.
FIGURE Spectrum BUF634T/OPA671 1MHZ,
(dBm)
1.82 2.84 3.86 4.88 6.92 7.94 8.96 9.98 Frequency (MHz)
(dBm)
11.2 14.8 18.4 25.6 29.2 32.8 36.4 Frequency (MHz)
FIGURE Spectrum BUF634T/OPA603 1MHz.
FIGURE Spectrum Sine Generator 5MHz.
(dBm)
11.2 14.8 18.4 25.6 29.2 32.8 36.4 Frequency (MHz)
(dBm)
Frequency (kHz) 282kHz
FIGURE Spectrum BUF634T/OPA603 5MHz,
FIGURE Frequency Response OPA604 23).
(dBm)
542kHz
(dBm)
Frequency (MHz)
Frequency (MHz) 984kHz
FIGURE Frequency Response OPA627 23).
FIGURE Frequency Response OPA671 23).
(dBm)
0.5dB 13.36MHz OPA671 OPA603
Frequency (MHz) 23.08MHz
(dBm)
OPA604
OPA627
Frequency (MHz)
FIGURE Frequency Response OPA603 23).
FIGURE Frequency Responses Four Amps 23).

Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd.

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