Bipolar current source maintains high output impedance at high frequencies

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Bipolar current source maintains high output impedance at high frequencies [Copy link]

基于仪器和运算放大器的传统电流源和电压/电流转换器在低频下提供很高的输出阻抗，这是因为放大器具有良好的低频 CMRR（共模抑制比）。在较高频率下，降低的 CMRR、固有的输出电容、转换率的局限性阻止了高质量电流源的实现。Analog Devices 公司的两款 200 MHz线路接收/放大 IC——AD8129 和 AD8130——提供了差分输入和显著的 CMRR，使它们成为构建高频恒流源的有力候选者。图 1 中的电路提供了良好的起点，但 AD8130 较高的输入偏置电流可能会在电流电平较低时影响输出电流精度。

　　To overcome this problem, a unity-gain buffer IC ₂ can be added to isolate the current-sense resistor (Figure 2). Alternatively, a buffer amplifier can be used to measure the load voltage and bootstrap the output cable capacitance. This circuit presents an output impedance of approximately 500k at 1MHz and provides a current range of 0V to ±3V using ±5V supplies.

　　Current sources with capacitively coupled loads benefit from a dc servo loop to stabilize the circuit's operating point (Figure 3). The value of the output coupling capacitor, C _O, depends on the desired low-frequency attenuation characteristics. Further improvements to the basic circuit achieve compensation for the output capacitance and increase the circuit's output impedance. A small adjustable feedback capacitor, C _COMP, approximately half the stray capacitance at the output, provides feed-forward compensation and further reduces the effects of stray capacitance at the output (Figure 4). To prevent oscillation, the gain of the cable's shield-driving circuit should be slightly less than unity. Note: If the output current-sense resistor, R _9, is reduced to 100 Ω, the input attenuator formed by R ₁ and R ₂ can be compensated and the 1 mA/V characteristic maintained. The frequency range of this voltage/current source is 20 Hz to 10 MHz. For best results, use high-frequency circuit layout and power-supply bypassing techniques.