Structure and parameters of op amp

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Structure and parameters of op amp

1. Single-supply op amp

The first problem with single-supply op amps is that the input and output signal ranges are reduced, causing the op amp to be more sensitive to internal and external noise.

1. Input stage of op amp

1) The bipolar input stage is composed of a pair of long tails based on bipolar transistors. It has a simple structure and has very low offset voltage and small zero drift.

2) The bipolar input stage with bias current compensation provides a certain bias current through an internal current source, and has the advantages of low voltage noise, low offset voltage, and low drift.

3) With a super bipolar input stage with bias current compensation, the op amp's input common-mode voltage is safely within the supply voltage range.

4) The field effect transistor crystal input stage has a higher input impedance, but the field effect transistor has a relatively high shutdown voltage, so it cannot work at a lower power supply voltage.

5) Rail-to-rail input stage, transistors or field effect transistors can be used. This type of op amp is required for high-voltage side current detection. The disadvantage is that the common-mode rejection ratio is relatively low and the input impedance changes with the input voltage.

2. Output stage of the op amp

1) The earliest output stage was an NPN emitter follower circuit with an NPN current source or a pull-down resistor.

2) Modern technology has produced complementary common-emitter/common-source output stage circuits, which have an output voltage very close to the power supply voltage, but the output impedance is also relatively large, and it is very sensitive to the capacitance of the load, and exceeding the range can cause overshoot.

2. Parameters of op amp

1) Input offset voltage. Ideally, when the voltages at the two input terminals of the regulator are equal, the output voltage is 0. However, in reality, a small voltage must be applied to the two input terminals to make the output equal to 0. This small voltage is called the input offset voltage.

2) Input bias current. Rationally, the current input into the op amp is 0. In fact, current flows into both input terminals of the op amp, and the bias current of the op amp is likely to be bidirectional. The bias current will cause voltage to be generated when flowing through external impedance, which will greatly affect the detection error of the signal.

3) Input impedance: Common-mode input impedance refers to the impedance from any input pin to ground, and differential input impedance refers to the impedance between two pins.