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OPA622 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA622 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 19 page  OPA622 11 ® Application circuits generally do not show the resistor RQ, but it is required for proper operation. With a fixed RQ, the quiescent current increases with tem- perature (see Typical Performance Curves.) This variation of the quiescent current with temperature keeps the band- width and AC behavior relatively constant with temperature. It is also possible to vary the quiescent current by an external control signal or circuitry. Figure 3 shows a circuit to disable the OPA622 with TTL-compatible logic levels. 0V/5V logic levels are converted into a 1mA/0mA current connected to Pin 2. The current flowing in RQ increases the voltage at Pin 2 to approximately 1V above the –VCC rail, thus reducing IQ to near zero and disabling the OPA622. OTA AND OUTPUT BUFFER SECTIONS An Operational Transconductance Amplifier (OTA) and an output buffer are the basic building blocks of a current- feedback amplifier. The current-feedback configuration of the OPA622 is illustrated in Figure 4. The OTA consists of a complementary emitter follower and a subsequent comple- mentary current mirror. The voltage at the high-impedance +In terminal is transferred to the BUF+ input/output termi- nal at a low impedance. If a current flows into or out of the BUF+ terminal, the complementary mirror reflects the current to the OTA terminal. The current flow at the high- impedance OTA terminal is determined by the product of the voltage between the +In and BUF+ terminals and the transconductance. The output buffer section is an open-loop buffer consisting of complementary emitter followers. It is designed to drive cables or low-impedance loads. The buffer output is not current-limited or -protected. As can be seen in Figure 4, the feedback network for a current-feedback amplifier is applied between the VOUT and BUF+ terminals. Figure 8 illustrates the bandwidth for various output volt- ages of the current feedback configuration. FIGURE 3. Logic-Controlled Disable Circuit. FIGURE 4. Current-Feedback Amplifier. 100k Ω 100 Ω R Q 430 Ω –5V Internal Current Source Circuitry OPA622 2N2907 +5V 0/5V Logic In 5V: OPA622 On 25 I C I C = 0: OPA622 On I C = 1mA: OPA622 Off 4.7k Ω C OTA 9 6 11 –V CC OUT V OUT + V CC OUT R 1 R 2 10 13 12 +V CC 5 –V CC +In 4 BUF+ OTA OB FEEDBACK BUFFER SECTION This section of the OPA622 is a complementary emitter follower identical to the input buffer of the OTA section. It is designed for interstage buffering, not for driving long cables or low-impedance loads. A minimum load resistance of 500 Ω is recommended when using the feedback buffer as a stand-alone device. The feedback buffer output is not current-limited or -protected. The bandwidth of the feedback buffer is shown in Figure 7. |
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