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 BOOST AMPLIFIER OUTPUT SWING WITH SIMPLE MODIFICATION By R. Mark Stitt and Rod Burt (602) 746-7445 In many applications it is desirable for the output of an amplifier to swing close to its power supply rails. Most amplifiers only guarantee an output swing of ±10V to ±12V when operating on standard ±15V power supplies. With the addition of four resistors and a pair of garden-variety transistors, the INA105 or INA106 difference amplifiers can be modified to provide nearly a full ±15V output swing on ±15V supplies. Even though there is gain in the feedback of the INA105, the circuit is stable as shown by the small-signal response of the amplifier as seen in the scope photo, Figure 5. Since a unitygain difference amplifier operates in a noise gain of two, gain can be added in its feedback loop without causing instability with the following restrictions: 1) the added gain is less than 2V/V, 2) the op amp in the difference amplifier is unity gain stable, and 3) the phase shift added by the gain buffer is low at the unity gain frequency of the op amp. All stability requirements are met when using the INA105. Figure 1 shows the modified circuit for the INA105. The combined INA105 quiescent current and output current flowing from its power-supply pins drives external transistors Q1 and Q2 through base-emitter connected resistors R 3 and R4. Q1 and Q2 are arranged as common-emitter amplifiers in a gain of approximately 1.7V/V (1 + 750Ω/1kΩ) so that the INA105’s output only needs to swing about ±9V for a ±15V swing at the buffer output. Figure 4 shows the boosted INA105 driving a 1kΩ load to within a fraction of a volt of its ±15V power supplies. Figure 4 is a multiple exposure scope photo showing the composite amplifier output and the power-supply voltages. To understand the details of the composite amplifier, consider the block diagram, Figure 2. Resistors R 1 and R2 set the gain of the buffer amplifier A 2. The buffer amplifier is a current-feedback op amp formed from the output transistors in the INA105 and the external transistors, Q 1 and Q2. The current feedback amplifier gives wide bandwidth and low phase shift. Figure 3 shows one of two complementary current-feedback amplifiers formed from the NPN output transistor in the INA105 and the external PNP transistor, Q 1. This current-feedback amplifier section is active for positive swings of the composite amplifier output. A complementary current-feedback amplifier, using external transistor Q 2, is active for negative output swings of the composite amp. +15V R3 200Ω Q1 2N3906 7 –In INA105 –In 2 R1 A1 R2 +In 3 R4 R3 4 (1) R1 1000Ω VO 1 NOTE: Gain of the boost circuit is approximately 1 + R 2/R1. FIGURE 2. Block Diagram Showing Boost Circuit Feedback Arrangement. Because the maximum gain in the feedback of an INA105 is limited to 2V/V, the boosted circuit works best with power supplies of ±12V or more. The INA105 doesn’t have enough output swing on lower supplies to drive a gain-of-2 buffer to the power supply rails. For boosted output swing on lower supplies, consider the INA106 gain-of-10 difference amplifier. Although the op amp in the INA106 is not unity gain stable, the INA106 is stable with added gain in its feedback of up to 3V/V. This allows full output boost on lower voltage supplies. Scope photograph Figure 6 shows the boosted Q2 2N3904 –15V NOTES: (1) R2 = 750Ω for INA105. R 2 = 1kΩ for INA106 on ±12V to ±18V power supplies. R 2 = 2kΩ for INA106 on ±8V to ±18V power supplies. FIGURE 1. External Transistors Q 1 and Q2 Add Output Boost so the Difference Amplifier Can Drive Loads Close to Its Power-Supply Rails. 1990 Burr-Brown Corporation R2 750Ω R1 1000Ω R4 200Ω VO INA105 R2 750Ω 6 A2 +In 5 AB-016 Printed in U.S.A. September, 1990 INA106 driving a 1kΩ load to within a fraction of a volt on ±8V supplies. Figure 6 is a multiple exposure scope photo showing the composite amplifier output and the powersupply voltages. Scope photograph Figure 7, shows the small signal response of the INA106 with a gain-of-3 feedback buffer. A word of caution: To obtain the boosted output swing, output protection circuitry was eliminated. There is no current limit in the output buffer. A short circuit at the output may destroy the external output transistors. Still, this simple modification is an effective means to obtain wide output swing. So long as the stated stability requirements are observed, this technique can be applied to other op amp circuits. FIGURE 5. Small-Signal Response of Composite Amplifier Using INA105 and Buffer Amplifier with 750Ω, 1kΩ Feedback Resistors. V+ R3 200Ω (1) Q1 7 (2) Q3 VO +In CFB Amp –In CFB Amp 6 INA105 Circuit Fragment To Q2 Collector R2 R1 NOTES: (1) Q 1 is the external 2N3906 PNP transistor. (2) Q3 is the INA105 output NPN transistor. FIGURE 3. Circuit Detail Showing One-Half of the Symmetrical Current-Feedback Amplifier Output Stage, A2 in Figure 2. FIGURE 6. Triple Exposure Showing ±8V Power Supplies and Composite Amplifier Output Driving 1kΩ Load. FIGURE 4. Triple Exposure Showing ±15V Power Supplies and Composite Amplifier Output Driving 1kΩ Load. FIGURE 7. Small-Signal Response of Composite Amplifier Using INA106 and Buffer Amplifier with 3kΩ, 1kΩ Feedback Resistors. 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