"Transport Amplifier Parameter Analysis and LTspice Application Simulation" III. Bias Current

皓月光兮非自明

"Transport Amplifier Parameter Analysis and LTspice Application Simulation" III. Bias Current [Copy link]

3.1 Bias Current and Offset Voltage

Definition of bias current and offset current:

Real amplifiers draw a small amount of current at their input pins. In some applications, this current can cause errors that affect the accuracy of the circuit or cause the circuit to operate erratically.

The input bias current (I _b ) of an amplifier is defined as the average of the currents flowing through the two input terminals:

The input offset current (I _os ) of an amplifier is defined as the difference between the currents flowing through the two input terminals:

Circuit DC noise caused by offset current and bias current: It is the voltage difference formed by the current at the two input ends passing through their respective resistor networks.

In the design of the amplifier, it is necessary to ensure that the bias current has a complete DC loop.

3.2 Causes of Bias Current

The differential input stage transistors of the voltage feedback amplifier include bipolar junction transistors (BJT) and field effect transistors (FET).

1. In order to ensure that the internal transistors operate in the linear region, a base bias voltage or base current must be provided. In addition, it is difficult to achieve complete matching of the two transistors in the input stage in the semiconductor manufacturing process, resulting in differences in the current of the input stage transistors.
2. Since the field effect tube is a voltage-controlled current device, its gate current is very small. However, each input end of the FET input amplifier is designed with an ESD protection diode, and the leakage current of the two diodes is generally much larger than the gate current.

The bias current flows unidirectionally in an amplifier with a simple input structure, and its direction of flow is related to the type of input stage transistors.

In complex input structures (such as bias compensation and current feedback op amps), the bias current may be the differential current between two or more internal current sources and may flow in both directions.

3.2 Bias Current Processing Method

1. To handle the bias current, we must first ensure that the DC loop of the bias current is complete.
2. The amplifier input resistance must be matched.
3. Control the resistance value of the loop through which I _b flows, or choose an amplifier with low bias current.

chejm

Thanks for the technical sharing provided by the host. Learn first and then participate in the discussion. Thumbs up