电压和电流反馈放大器应用电路

       电压和电流反馈放大器应用电路的结构一般相同，除了几个关键点。

　　电流反馈比电压反馈放大器有更高的转换速率。像这样，电流反馈比电压反馈放大器能更好的解决高速问题。“电流反馈放大器”的名字带有一些神秘色彩，但一般电流反馈和电压反馈放大器应用电路的结构是一致的，除了几个关键点。

　　首先，电流反馈放大器电路的反馈电阻值必须保持在小范围内。电阻值越低，电流反馈放大器的稳定性越差。可以在电流反馈放大器产品手册中找到规定的电阻值。电压反馈放大器的反馈电阻值更宽。放大器的驱动能力限制了电阻的最小值，全电路噪声限制电阻最大值。

　　图1显示了适于电流或电压反馈放大器的电路。如果反馈电阻RF等于2RIN，其中RIN为输入电阻，各通道的闭环增益为–2V/V。乍看，容易假设闭环带宽与增益带宽积除以各通道的增益相等，或|–2V/V|。但千万别做这个假设！

　　如果使用如图1电路中电压或电流反馈放大器，噪声增益为：

　　其中，N为输入通道数。带电压反馈放大器的电路带宽等于增益带宽积除以噪声增益。例如，如果使用180MHz增益带宽积的电压反馈放大器，三个输入通道（N=3），增益–2V/V，电路闭环带宽为25.7 MHz。额外的通道减少闭环带宽，即使输入信号持续达到–2V/V的增益。

　　如果使用如图1电路的电流反馈放大器，放大器闭环带宽较少的依靠闭环增益和输入通道数。如果用这样的放大器设计电路，首先应该挑选合适的反馈电阻，各厂商的规格和电路噪声增益。然后选择合适的RIN值。从这一点，如果电路增加通道，或许将发生信号带宽和增益剧增的微小变化。如果那个情形出现，退回并确定反馈电阻的选择。对电流和电压反馈放大器，噪声增益通常与公式1的结果相等，但减少电流反馈放大器电路的反馈电阻值，使电路带宽增加。

英文原文：

　　Voltage- and current-feedback amps are almost the same

　　The application-circuit configurations for voltage- and current-feedback amps are generally the same, except for a few key points.

　　By Bonnie Baker -- EDN, 10/25/2007

　　Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. As such, current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. The name “current-feedback amp” carries some mystique, but, generally, the application-circuit configurations for voltage- and current-feedback amps are the same, except for a few key points.

　　First, the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. Lower value resistors reduce the current-feedback amp’s stability. The feedback resistor’s higher values reduce the current-feedback amp’s bandwidth. You can find the prescribed feedback-resistor value in the current-feedback amp’s product data sheet. The voltage-feedback-amp’s feedback-resistance value is more forgiving. This amplifier’s drive capability limits the resistor’s minimum value, and the overall circuit noise limits the maximum value.

　　Figure 1 sho ws a circuit that is appropriate for either a current- or a voltage-feedback amp. If the feedback resistance, RF, equals 2RIN, where RIN is the input resistance, the closed-loop gain of each channel is –2V/V. At first glance, it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel’s gain, or |–2V/V|. Don’t make this assumption!

　　If you use a voltage- or current-feedback amp with the circuit in Figure 1, the noise gain is:

　　where N is the number of input channels. This circuit’s bandwidth, with a voltage-feedback amp, equals the gain-bandwidth product divided by the noise gain. For instance, if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V, the circuit’s closed-loop bandwidth is 25.7 MHz. Additional channels reduce the closed-loop bandwidth, even though the input signals continue to see a gain of –2V/V.