ByChris Francis
双极运算放大器具有必要的输入偏置电流要求。这种偏置电流必须来自某处，并且可以在某些类型的高阻抗电路（例如电荷放大器和跨阻抗放大器）中滋扰，并且可以导致输入电阻不匹配的偏移电压。下面显示的示例具有2.8NA的输入偏置电流。这是937NA的长尾双尾电流的结果，其在长尾对（Q1 / Q2）的两个晶体管之间分开，然后通过晶体管电流增益分开。
While a darlington input stage is one possible solution, that tends to result in a lower bandwidth, as does simply reducing the long-tailed pair bias current. It can also make the circuit more susceptible to temperature related leakage and matching issues.
One possible solution is to use input bias current compensation circuitry. This involves generating a current which is equal to the input bias current and feeding that into the opamp inputs so all of the input bias current doesn’t need to come from the external circuitry. An example is shown below:
Here an extra five transistors have been added. Q13 generates a current equal to half the long tailed pair current (it has one emitter whereas Q3 has two emitters). This is fed into the emitter of Q14 to generate a base current which is Q13 collector current divided by the NPN current gain. This is then mirrored with Q6/Q7 and Q8 and fed to the bases of the two input transistors Q1 & Q2. Note that all PNP transistors are dual collector and in the case of Q5 the transistor forms a current mirror without the need for a second PNP transistor.
因此，如果您遵循电流，您将看到我们已生成电流以进入输入晶体管基座，这与所需的输入电流相同。我们具有2.77NA的输入基准电流和2.77NA补偿电路的电流（2×1.3886NA，如Q6的两个收集器所示）。因此，需要由外部电路提供的所得到的电流几乎为零。事实上，在模拟中，仅剩余2.77na的残留1.3pa。实际上，电路通常不会是良好的 - 偏置电流补偿将随输入引脚上的电压而变化，并且也将取决于晶体管匹配。
You can add more transistors to improve the current compensation and to try to make it track with input voltage variation. This can be achieved by making the input current compensation circuit collector-base voltage track the collector-base voltage of the input transistors. This helps to prevent the current error due to the Early effect from affecting performance. However, the circuit shown gives a practical improvement in the input bias current of a factor of more than 10 with no loss of bandwidth. A more sophisticated circuit could improve on that. The circuit could actually be made even simpler by using a dual emitter transistor for Q13 and then using a single PNP for Q6/Q7, relying on the dual collector to split the current to the two input transistors.
帖子双极opamps中的输入偏置电流取消首先出现了Analog IC Tips.
Filed Under:Analog IC Tips,TECHNOLOGIES + PRODUCTS,TIPS SITES (Design World + Fluid Power World)
