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AD8042ARZ Datasheet(PDF) 12 Page - Analog Devices |
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AD8042ARZ Datasheet(HTML) 12 Page - Analog Devices |
12 / 16 page AD8042 Rev. E | Page 12 of 16 APPLICATIONS INFORMATION CIRCUIT DESCRIPTION The AD8042 is fabricated on the Analog Devices, Inc., proprietary eXtra-Fast Complementary Bipolar (XFCB) process, which enables the construction of PNP and NPN transistors with similar fts in the 2 GHz to 4 GHz region. The process is dielectrically isolated to eliminate the parasitic and latch-up problems caused by junction isolation. These features allow the construction of high frequency, low distortion amplifiers with low supply currents. This design uses a differential output input stage to maximize bandwidth and headroom (see Figure 35). The smaller signal swings required on the first stage outputs (nodes SIP, SIN) reduce the effect of nonlinear currents due to junction capacitances and improve the distortion performance. With this design, harmonic distortion of better than −77 dB @ 1 MHz into 100 Ω with VOUT = 2 V p-p (gain = +2) on a single 5 V supply is achieved. SIN R21 R3 VEE Q11 Q3 I10 R26 R39 Q5 Q4 Q40 I7 R2 R15 Q13 Q17 R5 C7 Q2 SIP Q22 Q7 Q21 Q24 R23 R27 I2 I3 I1 Q51 Q25 Q50 Q39 Q47 Q27 Q31 Q23 I9 I5 VEE VCC I8 Q36 Q8 VOUT C3 C9 VCC VINP VINN VEE Figure 35. Simplified Schematic The rail-to-rail output range of the AD8042 is provided by a complementary common-emitter output stage. High output drive capability is provided by injecting all output stage predriver currents directly into the bases of the output devices Q8 and Q36. Biasing of Q8 and Q36 is accomplished by I8 and I5, along with a common-mode feedback loop (not shown). This circuit topology allows the AD8042 to drive 40 mA of output current with the outputs within 0.5 V of the supply rails. On the input side, the device can handle voltages from 0.2 V below the negative rail to within 1.2 V of the positive rail. Exceeding these values does not cause phase reversal; however, the input ESD devices do begin to conduct if the input voltages exceed the rails by greater than 0.5 V. DRIVING CAPACITIVE LOADS The capacitive load drive of the AD8042 can be increased by adding a low valued resistor in series with the load. Figure 36 shows the effects of a series resistor on capacitive drive for varying voltage gains. As the closed-loop gain is increased, the larger phase margin allows for larger capacitive loads with less overshoot. Adding a series resistor with lower closed-loop gains accomplishes the same effect. For large capacitive loads, the frequency response of the amplifier is dominated by the roll-off of the series resistor and capacitive load. 1000 100 10 15 24 3 CLOSED-LOOP GAIN (V/V) CL RS VS = 5V 200mV STEP WITH 90% OVERSHOOT RS = 20Ω RS = 5Ω RS = 0Ω Figure 36. Capacitive Load Drive vs. Closed-Loop Gain OVERDRIVE RECOVERY Overdrive of an amplifier occurs when the output and/or input range are exceeded. The amplifier must recover from this overdrive condition. As shown in Figure 37, the AD8042 recovers within 30 ns from negative overdrive and within 25 ns from positive overdrive. 5.0V 2.5V 0V G = +2 VS = 5V VIN = 5V p-p RL = 1kΩ TO 2.5V 50ns 1V Figure 37. Overdrive Recovery |
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