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OP279GRU Datenblatt(PDF) 6 Page - Analog Devices |
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OP279GRU Datenblatt(HTML) 6 Page - Analog Devices |
6 / 16 page OP179/OP279 –6– REV. F Typical Performance Graphs THEORY OF OPERATION The OP179/OP279 is the latest entry in Analog Devices’ ex- panding family of single-supply devices, designed for the multi- media and telecom marketplaces. It is a high output current drive, rail-to-rail input /output operational amplifier, powered from a single +5 V supply. It is also intended for other low supply voltage applications where low distortion and high out- put current drive are needed. To combine the attributes of high output current and low distortion in rail-to-rail input/output operation, novel circuit design techniques are used. For example, Figure 1 illustrates a simplified equivalent circuit for the OP179/OP279’s input stage. It is comprised of two PNP differential pairs, Q5-Q6 and Q7-Q8, operating in parallel, with diode protection networks. Diode networks D5-D6 and D7-D8 serve to clamp the applied differential input voltage to the OP179/OP279, thereby protecting the input transistors against avalanche damage. The fundamental differences between these two PNP gain stages are that the Q7-Q8 pair are normally OFF and that their inputs are buffered from the operational amplifier inputs by Q1-D1-D2 and Q9-D3-D4. Operation is best under- stood as a function of the applied common-mode voltage: When the inputs of the OP179/OP279 are biased midway be- tween the supplies, the differential signal path gain is controlled by the resistively loaded (via R7, R8) Q5-Q6. As the input common-mode level is reduced toward the negative supply (VNEG or GND), the input transistor current sources, I1 and I3, are forced into saturation, thereby forcing the Q1-D1-D2 and Q9-D3-D4 networks into cutoff; however, Q5-Q6 remain active, providing input stage gain. On the other hand, when the common-mode input voltage is increased toward the positive supply, Q5-Q6 are driven into cutoff, Q3 is driven into satura- tion, and Q4 becomes active, providing bias to the Q7-Q8 dif- ferential pair. The point at which the Q7-Q8 differential pair becomes active is approximately equal to (VPOS – 1 V). I2 R5 4k D7 I1 R6 4k D8 D5 D6 R3 2.5k R4 2.5k Q4 Q3 Q2 Q5 Q6 Q9 Q1 R1 6k R2 3k VPOS VNEG R7 2.2k R8 2.2k I3 D1 D2 D3 D4 VO –+ IN– IN+ Q8 Q7 Figure 22. OP179/OP279 Equivalent Input Circuit The key issue here is the behavior of the input bias currents in this stage. The input bias currents of the OP179/OP279 over the range of common-mode voltages from (VNEG + 1 V) to (VPOS – 1 V) are the arithmetic sum of the base currents in Q1- Q5 and Q9-Q6. Outside of this range, the input bias currents are dominated by the base current sum of Q5-Q6 for input signals close to VNEG, and of Q1-Q5 (Q9-Q6) for input signals close to VPOS. As a result of this design approach, the input bias currents in the OP179/OP279 not only exhibit different ampli- tudes, but also exhibit different polarities. This input bias cur- rent behavior is best illustrated in Figure 3. It is, therefore, of paramount importance that the effective source impedances connected to the OP179/OP279’s inputs are balanced for opti- mum dc and ac performance. 100 60 0 10 10k 1k 100 1 40 20 80 FREQUENCY – Hz VS = +5V TA = +25 C Figure 19. Voltage Noise Density vs. Frequency 120 60 0 1k 1M 100k 10k 100 40 20 80 100 FREQUENCY – Hz TA = +25 C VS 2.5V Figure 21. Common-Mode Rejection vs. Frequency COMMON-MODE VOLTAGE – Volts 60 0 5 30 10 1 20 0 50 40 4 3 2 VS = +5V TA = +25 C FREQUENCY = 1kHz Figure 20. Voltage Noise Density vs. Common-Mode Voltage |
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Ähnliche Beschreibung - OP279GRU |
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