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OP184FP 데이터시트(PDF) 10 Page - Analog Devices |
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OP184FP 데이터시트(HTML) 10 Page - Analog Devices |
10 / 20 page REV. 0 –10– OP184/OP284/OP484 FREQUENCY – Hz 0.1 0.010 0.0005 20 100 10k 1k 0.001 20k VO = ±0.75V VO = ±2.5V VO = ±1.5V AV = 1000 VS = ±2.5V RL = 2kΩ Figure 40. Total Harmonic Distortion vs. Frequency 10 0% 100 90 500ns 100mV VS = ±1.5V AV = 1 NO LOAD TA = +25°C 200mV 0V –200mV Figure 38. Small Signal Transient Response 200mV 0V –200mV 10 0% 100 90 1µs 100mV VS = ±0.75V AV = 1 NO LOAD TA = +25°C Figure 39. Small Signal Transient Response APPLICATIONS Functional Description The OP284 and OP484 are precision single-supply, rail-to-rail operational amplifiers. Intended for the portable instrumenta- tion marketplace, the OP184/OP284/OP484 combines the at- tributes of precision, wide bandwidth, and low noise to make it a superb choice in those single supply applications that require both ac and precision dc performance. Other low supply voltage applications for which the OP284 is well suited are active filters, audio microphone preamplifiers, power supply control, and tele- com. To combine all of these attributes with rail-to-rail input/ output operation, novel circuit design techniques are used. D1 D2 Q4 R2 4k VPOS I1 R1 4k Q3 Q1 Q2 R4 3k I2 R3 3k V01 –IN V02 VNEG +IN Figure 41. OP284 Equivalent Input Circuit For example, Figure 41 illustrates a simplified equivalent circuit for the OP184/OP284/OP484’s input stage. It is comprised of an NPN differential pair, Q1-Q2, and a PNP differential pair, Q3-Q4, operating concurrently. Diode network D1-D2 serves to clamp the applied differential input voltage to the OP284, thereby protecting the input transistors against avalanche dam- age. Input stage voltage gains are kept low for input rail-to-rail operation. The two pairs of differential output voltages are con- nected to the OP284’s second stage which is a compound folded cascode gain stage. It is also in the second gain stage where the two pairs of differential output voltages are combined into a single-ended output signal voltage used to drive the output stage. A key issue in the input stage is the behavior of the input bias currents over the input common-mode voltage range. Input bias currents in the OP284 are the arithmetic sum of the base currents in Q1-Q3 and in Q2-Q4. As a result of this design approach, the input bias currents in the OP284 not only exhibit different amplitudes, but also exhibit different polarities. This effect is best illustrated in Figure 8. It is, therefore, of para- mount importance that the effective source impedances con- nected to the OP284’s inputs be balanced for optimum dc and ac performance. In order to achieve rail-to-rail output, the OP284 output stage design employs a unique topology for both sourcing and sinking current. This circuit topology is illustrated in Figure 42. As previously mentioned, the output stage is voltage-driven from the second gain stage. The signal path through the output stage is inverting; that is, for positive input signals, Q1 provides the base current drive to Q6 so that it conducts (sinks) current. For negative input signals, the signal path via Q1-Q2-D1-Q4-Q3 provides the base current drive for Q5 to conduct (source) cur- rent. Both amplifiers provide output current until they are forced into saturation which occurs at approximately 20 mV from negative rail and 100 mV from the positive supply rail. VPOS I2 I1 Q1 Q3 Q4 Q2 VNEG Q5 VOUT Q6 R6 R3 R2 R1 R4 INPUT FROM SECOND GAIN STAGE R5 D1 Figure 42. OP284 Equivalent Output Circuit |
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