APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 VOLTAGE-FEEDBACK AMPLIFIERS vs CURRENT-FEEDBACK AMPLIFIERS: BANDWIDTH AND DISTORTION CONSIDERATIONS by Tony Wang, (602) 746-7650 The primary determinants of AC performance for the current-feedback amplifier are the transresistance, RT, and the transcapacitance, CT. A well specified current-feedback amplifier’s data sheet will list these parameters. Figure 1 shows the current-feedback amplifier with a bipolar transistor as the input device. This is convenient because it has a low impedance inverting input (the emitter) and a high impedance noninverting input (the base). For this analysis, the bipolar transistor is considered ideal (i.e., infinite beta, zero base-emitter voltage, no base-collector capacitance). The collector terminates in RT, CT and an inverting buffer. The feedback network consists of RF and RG. Designers specify amplifiers based on certain key parameters, one of which is bandwidth. Traditionally, the gainbandwidth product of an amplifier told the user everything he needed to know about its small-signal AC performance. The useful bandwidth of an amplifier was determined by dividing the gain-bandwidth product (GBW) by the desired closed-loop gain. However, this simple formula cannot be used with current-feedback amplifiers. Current-feedback amplifiers have nearly constant bandwidth for varying closed-loop gains. The reason is that the user can adjust the open-loop gain of the current-feedback amplifier by changing the feedback network without affecting the open-loop pole. The concept can be more readily understood with the aid of Figure 1, which shows a simplified AC model for the current-feedback amplifier. Figure 2 shows the same circuit reconfigured for analysis. The feedback network is now the emitter load for the input transistor. Open-loop voltage gain can be determined by inspection to be: V+ RT = R T || ( j2πf C T ) R F || R G RF + RG RT 1 RG R F 1 + j2πf R T C T = A CL CT –1 –1 RT 1 R F 1 + j2πf R T C T VOUT Q1 V+ –In RT CT –1 RF +In VOUT Q1 RL = RF + RG –In RG RE = RF || RG FIGURE 1. Current-Feedback Amplifier Simplified for AC Analysis. © 1994 Burr-Brown Corporation FIGURE 2. Current-feedback Amplifier Simplified and Reconfigured for Analysis. AB-091 Printed in U.S.A. January, 1994 Application Bulletin Number 91 A OL = +In 91 The last expression shows that the open-loop gain for the current-feedback amplifier varies directly with closed-loop gain (for a given RF). This is why the current-feedback amplifier has a gain-independent bandwidth. This simplified analysis holds true for low to moderate gains, less than 25V/V, but becomes limited when second-order effects start to dominate. VIN VOUT OPA621 50Ω RF What this means is that the designer has to look more carefully at how the amplifier is specified. Gain-bandwidth is not meaningful when evaluating a current-feedback amplifier. However, it is an easy way to evaluate traditional voltage-feedback op amps. It is better for the designer to first determine the required gain and then make bandwidth comparisons. RG ACL +2V/V +10V/V RF RG 300Ω 300Ω 300Ω 33Ω VIN 50Ω 50Ω For instance, assume that the application requires processing a 10MHz signal and the amplifiers under consideration are the OPA621 and the OPA603. The OPA621 is a voltagefeedback op amp with 500MHz gain-bandwidth product. The OPA603 is a current-feedback amplifier that can be configured for a useful bandwidth of 100MHz. At first glance, both amplifiers appear adequate but this assumption neglects gain considerations. The circuit configurations of Figure 3 show resistor values for gains of +2V/V and +10V/V. For these gains, the OPA621 has closed-loop bandwidths of 250MHz and 50MHz, respectively. VOUT OPA603 50Ω RF RG CF ACL +2V/V +10V/V RF RG CF 1000Ω 1000Ω 0.5pF 1000Ω 110Ω NA FIGURE 3. Application Circuits for OPA621 and OPA603. With the aid of the data sheets for each of these products, a reasonable comparison of open-loop gain can be made. From the OPA621 data sheet, AOL = 60dB = 1,000V/V. This and the GBW are enough information to describe the openloop gain versus frequency: A OL = 50Ω 50Ω This provides a simple way to compare the useful bandwidths of voltage-feedback amplifiers and current-feedback amplifiers. First, determine the closed-loop gain required, then use data sheet specifications in the formulas presented above to compare the open-loop responses as an approximate indicator of the best op amp for lower distortion. 1, 000 1, 000 = 1 + jf / ( 500MHz / 1, 000 ) 1 + jf / 500kHz 80 Open-Loop Voltage Gain (dB) The OPA603 data sheet gives RT = 400kΩ and CT = 1.8pF. For these applications, the OPA603 was configured with RF = 1kΩ. The resulting open-loop gain curves are plotted in Figure 4. Loop gain is the area bounded above by the open-loop gain curve and below by the desired closed-loop gain. Loop gain is important because it provides a measure of an amplifier’s ability to reduce error and maintain fidelity with the original signal. For a gain of +2V/V (6dB), the OPA621 has 9dB more loop gain than the OPA603 at 10MHz. In a gain of +10V/V (20dB), the situation is reversed and the OPA603 has 5dB more loop gain than the OPA621. This is confirmed in the distortion figures tabulated below. ACL = +2V/V 2nd Harmonic 3rd Harmonic Effective Bits OPA621 OPA603 OPA621 –65dBc –78dBc 10.5 –68dBc < –90dBc 11 –63dBc –62dBc 10 –50dBc –70dBc 8 OPA621 60 50 OPA603 (ACL = +2V/V 40 30 ACL = +10 20 10 ACL = +2 0 104 105 106 107 108 Frequency (Hz) FIGURE 4. Open-Loop Gain Comparisons of the OPA621 and OPA603. ACL = +10V/V OPA603 OPA603 (ACL = +10V/V) 70 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. 2