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OPA622 Datasheet(PDF) 12 Page - Texas Instruments |
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OPA622 Datasheet(HTML) 12 Page - Texas Instruments |
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12 / 19 page  OPA622 12 ® CONFIGURATIONS VOLTAGE-FEEDBACK AMPLIFIER The OPA622’s internal design differs from a “classical” operational amplifier structure, but it can nevertheless be used in all traditional operational amplifier applications. As with conventional op amps, the feedback network connected to the inverting input controls closed-loop gain (GCL). But with the OPA622, the resistor ROG is simultaneously adapted to the closed-loop gain, optimizing the frequency response and stability. The “classical” differential input stage consists of two identical transistors with an emitter degeneration resistor, two current sources, and an active load diode. However, the classical configuration limits the current through the gain transistor to that supplied by the current sources. In the new design, a complementary push-pull buffer (emit- ter follower) replaces one side of the differential stage without the 0.7V offset. The feedback buffer as a second complementary emitter follower and the open-loop gain resistor ROG connected between the outputs recreate the differential stage without the disadvantages of the classical design. The current charging the parasitic capacitance at the base of the gain transistor is no longer limited to the fixed current of the current sources and is proportional to the input signal. This improvement results in an approximately 10-times better slew rate. The amplified current through the gain transistor of one of the buffers is mirrored and becomes the output current. The high-impedance output of the OTA is now buffered by the high current output stage, which is designed to drive long cables or low-impedance loads at full power. The identical input buffers reduce the input offset to typi- cally less than ±7µV. Closed-loop output offset is typically due to mismatch of the NPN and PNP transistors in the OTA mirror ±100µV after the output bias current is trimmed. Figure 5 illustrates the circuit configuration of the voltage- feedback op amp in a complementary circuit design. The feedback buffer and the OTA input buffer form the differential input. Inserting the feedback buffer section trans- forms the current feedback shown in Figure 4 into the voltage feedback shown in Figure 5. The resistor ROG sets the open-loop gain and corresponds to the emitter degeneration resistor in a classical differential stage. Because the ROG resistor can be varied externally, a flat frequency response can be achieved over a wide range of applications without the need to compensate the amplifier with a capacitor. In contrast to a current-feedback amplifier, it is possible to adjust the closed-loop gain using the feedback resistors and to adjust the open-loop gain indepen- dently using ROG to optimize the frequency response. Unlike “classical” operational amplifier structures, the OPA622 configuration makes it possible to attain a nearly constant bandwidth for varying closed-loop gains, as well as improved frequency response and large-signal behavior. In addition—and also unlike current-feedback op amps—it provides two identical high-impedance inputs, lower input offset values, and improved CMRR. CURRENT-FEEDBACK AMPLIFIER Figure 4 shows the current-feedback configuration. The feedback loop is closed from the output to the BUF+ terminal of the OTA section. The shorter feedback loop FIGURE 5. Voltage-Feedback Amplifier. 9 6 11 V OUT + V CC OUT R 1 10 +In – V CC OUT OB FB R OG R 2 C OTA –In 3 8 13 – V CC + V CC 4 OTA 5 12 C OTA: Sets the first open-loop pole R OG: Sets the open-loop gain G CL = 1 + R 2 R 1 |
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