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AD825AR-REEL Arkusz danych(PDF) 9 Page - Analog Devices |
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AD825AR-REEL Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 11 page AD825 –9– REV. C +VS TEKTRONIX P6204 FET PROBE HP PULSE GENERATOR 50 CL VOUT 0.01 F 10 F –VS VIN TEKTRONIX 7A24 PREAMP 0.01 F 10 F AD825 RIN 1k 1k Figure 26a. Inverting Amplifier Driving a Capacitive Load Figure 26b. Inverting Amplifier Pulse Response While Driving a 400 pF Capacitive Loads CF VNEG VOUT VPOS POS NEG Figure 27. Simplified Schematic DRIVING CAPACITIVE LOADS The internal compensation of the AD825, together with its high output current drive, permits excellent large signal performance while driving extremely high capacitive loads. THEORY OF OPERATION The AD825 is a low cost, wide band, high performance FET input operational amplifier. With its unique input stage design, the AD825 assures no phase reversal even for inputs that exceed the power supply voltages, and its output stage is designed to drive heavy capacitive or resistive load with small changes rela- tive to no load condition. The AD825 (Figure 27) consists of common-drain common- base FET input stage driving a cascoded, common base matched NPN gain stage. The output buffer stage uses emitter followers in a class AB amplifier that can deliver large current to the load while maintaining low levels of distortion. The capacitor, CF, in the output stage, enables the AD825 to drive heavy capacitive load. For light load, the gain of the out- put buffer is close to unity, CF is bootstrapped and not much happens. As the capacitive load is increased, the gain of the output buffer is decreased and the bandwidth of the amplifier is reduced through a portion of CF adding to the dominant pole. As the capacitive load is further increased, the amplifier’s band- width continues to drop, maintaining the stability of the AD825. Input Consideration The AD825 with its unique input stage assures no phase rever- sal for signals as large or even larger than the supply voltages. Also, layout considerations of the input transistors assure func- tionality even with a large differential signal. The need for a low noise input stage calls for a larger FET transis- tor. One should consider the additional capacitance that is added to assure stability. When filters are designed with the AD825, one needs to consider the input capacitance (5 pF–6 pF) of the AD825 as part of the passive network. Grounding and Bypassing The AD825 is a low input bias current FET amplifier. Its high frequency response makes it useful in applications such as photo diode interfaces, filters and audio circuits. When designing high frequency circuits, some special precautions are in order. Cir- cuits must be built with short interconnects, and resistances should have low inductive paths to ground. Power supply leads should be bypassed to common as close as possible to the ampli- fier pins. Ceramic capacitors of 0.1 µF are recommended. INPUT OUTPUT |
Podobny numer części - AD825AR-REEL |
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Podobny opis - AD825AR-REEL |
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