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SA5230N Datasheet(PDF) 7 Page - NXP Semiconductors |
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SA5230N Datasheet(HTML) 7 Page - NXP Semiconductors |
7 / 17 page Philips Semiconductors Product specification NE/SA5230 Low voltage operational amplifier 1994 Aug 31 7 Output Stage Processing output voltage swings that nominally reach to less than 100mV of either supply voltage can only be achieved by a pair of complementary common-emitter connected transistors. Normally, such a configuration causes complex feed-forward signal paths that develop by combining biasing and driving which can be found in previous low supply voltage designs. The unique output stage of the NE5230 separates the functions of driving and biasing, as shown in the simplified schematic of Figure 3, and has the advantage of a shorter signal path which leads to increasing the effective bandwidth. This output stage consists of two parts: the Darlington output transistors and the class AB control regulator. The output transistor Q3 connected with the Darlington transistors Q4 and Q5 can source up to 10mA to an output load. The output of NPN Darlington connected transistors Q1 and Q2 together are able to sink an output current of 10mA. Accurate and efficient class AB control is necessary to insure that none of the output transistors are ever completely cut off. This is accomplished by the differential amplifier (formed by Q8 and Q9) which controls the biasing of the output transistors. The differential amplifier compares the summed voltages across two diodes, D1 and D2, at the base of Q8 with the summed voltages across the base-emitter diodes of the output transistors Q1 and Q3. The base-emitter voltage of Q3 is converted into a current by Q6 and R6 and reconverted into a voltage across the base-emitter diode of Q7 and R7. The summed voltage across the base-emitter diodes of the output transistors Q3 and Q1 is proportional to the logarithm of the product of the push and pull currents IOP and ION, respectively. The combined voltages across diodes D1 and D2 are proportional to the logarithm of the square of the reference current IB1. When the diode characteristics and temperatures of the pairs Q1, D1 and Q3, Q2 are equal, the relation IOP×ION=IB1×IB1 is satisfied. Separating the functions of biasing and driving prevents the driving signals from becoming delayed by the biasing circuit. The output Darlington transistors are directly accessible for in-phase driving signals on the bases of Q5 and Q2. This is very important for simple high-frequency compensation. The output transistors can be high-frequency compensated by Miller capacitors CM1A and CM1B connected from the collectors to the bases of the output Darlington transistors. A general-purpose op amp of this type must have enough open-loop gain for applications when the output is driving a low resistance load. The NE5230 accomplishes this by inserting an intermediate common-emitter stage between the input and output stages. The three stages provide a very large gain, but the op amp now has three natural dominant poles — one at the output of each common-emitter stage. Frequency compensation is implemented with a simple scheme of nested, pole-splitting Miller integrators. The Miller capacitors CM1A and CM1B are the first part of the nested structure, and provide compensation for the output and intermediate stages. A second pair of Miller integrators provide pole-splitting compensation for the pole from the input stage and the pole resulting from the compensated combination of poles from the intermediate and output stages. The result is a stable, internally-compensated op amp with a phase margin of 70 degrees. Q5 Ib1 Ib2 Ib3 Ib5 Ib4 Q4 Q6 Q8 Q9 D1 Q7 Q3 D2 Q1 Q2 R7 R6 CM1B CM1A Vb2 Vb5 VCC VEE VOUT IOP ION SL00252 Figure 3. Output Stage |
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