The AD847 represents a breakthrough in high speed amplifiers offering superior ac & dc performance and low power, all at low cost. The excellent dc performance is demons trated by its ±5 V
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.specifications which include an open-loop gain of 3500 V/V(500 Ω load) and low input offset voltage of 0.5 mV. Common-mode rejection is a minimum of 78 dB. Output voltage swing is ±3 V into loads as low as 150 Ω. Analog Devices also offers over 30 other high speed amplifiers from the low noise AD829(1.7 nV/√Hz) to the ultimate video amplifier, the AD811, which features 0.01% differential gain and 0.01° differential phase.
The input offset voltage of the AD847 is very low for a high speed op amp, but if additional nulling is required, the circuit shown in Figure 21 can be used.
An input resistor (RIN in Figure 20) is required in circuits where the input to the AD847 will be subjected to transient or con-tinuous overload voltages exceeding the ±6 V maximum differ-ential limit. This resistor provides protection for the input transistors by limiting the maximum current that can be forced into their bases.
For high performance circuits it is recommended that a resistor(RB in Figures 19 and 20) be used to reduce bias current errors by matching the impedance at each input. The offset voltage er-ror will be reduced by more than an order of magnitude.
AD847AR操作理论
The AD847 is fabricated on Analog Devices’ proprietary complementary bipolar (CB) process which enables the con-struction of pnp and npn transistors with similar fTs in the 600 MHz to 800 MHz region. The AD847 circuit (Figure 22)includes an npn input stage followed by fast pnps in the folded cascode intermediate gain stage. The CB pnps are also used in the current amplifying output stage. The internal compensation capacitance that makes the AD847 unity gain stable is provided by the junction capacitances of transistors in the gain stage.
The capacitor, CF, in the output stage mitigates the effect of ca-pacitive loads. At low frequencies and with low capacitive loads, the gain from the compensation node to the output is very close to unity. In this case CF is bootstrapped and does not contribute to the compensation capacitance of the part. As the capacitive load is increased, a pole is formed with the output impedance of the output stage. This reduces the gain, and therefore, CF is incompletely bootstrapped. Some fraction of CF contributes to the compensation capacitance, and the unity gain bandwidth falls. As the load capacitance is increased, the band-width continues to fall, and the amplifier remains stable.
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