Bias circuit based on MOS transistor threshold voltage VT

The third section of this chapter introduces various bias circuits, which are often used in MOS analog integrated circuits and become an important part of the circuit. However, these bias circuits have common shortcomings. The current it provides varies with temperature and power supply voltage. Therefore, in high-demand occasions, the following high stability that has little relationship with temperature and power supply voltage can be used. Bias circuit.

1. Bias circuit based on MOS transistor threshold voltage VT

A typical bias circuit based on the threshold potential VT of the MOS tube is shown in Figure 2.7-1. M1~M4 form a positive feedback loop, and the width-to-length ratio W/L of M3 and M4 are the same, so that the current flowing through M1 and M2 is the same. Available from the picture

The channel length modulation effect and body effect are ignored here. If you choose a smaller working current I, increase the width-to-length ratio W1/L1 of the M1 tube to satisfy

Then (2.7-1) can be written as

The equation (2.7-2) shows that the bias current I of the circuit in Figure 2.7-1 has nothing to do with the power supply voltage, but only with the threshold voltage VT and resistance R. The threshold voltage VT is a function of temperature, about -2 mV/℃, and The diffusion resistance R has a large positive temperature coefficient, so the bias current I has a large negative temperature coefficient.

We can use the difference between the threshold voltages of the two tubes to obtain the reference voltage with low temperature coefficient. The method is to use an enhanced tube for one tube and a depletion tube for the other. The circuit form is shown in Figure 2.7-2. Therefore, the reference voltage Vo at the output terminal is

Where VTS is the threshold voltage of the enhanced MOS tube, and VTD is the threshold voltage of the depletion mode MOS tube.

Because VTS is a positive value, VTD is a negative value, and VTS, VTD have a negative temperature coefficient, so the threshold voltage of the enhanced tube

The variation of the voltage VTS and the threshold voltage VTD of the depletion tube can be offset, so that the temperature coefficient of the Vo voltage is very low, which can achieve 20 parts per million/℃. It is an ideal reference voltage source. Use this reference voltage The source can obtain a stable bias current.

Relevant information

Bias circuit based on MOS transistor threshold voltage VT

Bias circuit based on transistor VBE voltage

Bias circuit based on transistor thermal voltage (T/q)

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