Bias circuit based on transistor thermal voltage

Transistor thermal voltage

Usually define thermal voltage =,wher  is Boltzmann's constant, T is the absolute temperature, and q is the electronic charge. The typical circuit form of the bias circuit based on thermal voltage is shown in Figure 2.7-4.

The area of the emitter area of the transistor Q2 in the figure is n times the area of the emitter area of the transistor Q1. The M1~M4 tubes form a feedback loop to make the currents flowing through Q1 and Q2 equal. Available from the picture

and

Substituting the VBE relational expression into (2.7-6), we get

 and the resistance R are both positive temperature coefficients. As long as the appropriate coefficient n is selected, the bias current I can have a zero temperature coefficient. Therefore, the bias current of the circuit in Figure 2.7-4 is independent of temperature and can be used as a reference current source.

Due to the channel modulation effect, the currents flowing through the M1 and M4 tubes are slightly different, and the size of the output current image.png is also related to the size of the output terminal voltage. To overcome this shortcoming, a cascade current source can be used, the circuit form of which is shown in Figure 2.7-5.

With the circuit structure shown in Figure 2.7-5, a reference voltage source and a reference current source can be obtained. The circuit form is shown in Figure 2.7-6. From the figure and (2.7-7) formula, the Vo voltage can be written as

Choose appropriate n and resistivity X so that the temperature coefficients of the first term and the second term of the above formula cancel each other, then Vo can be used as the reference voltage. It can be seen from the figure that using a negative feedback circuit, this reference voltage Vo can be converted into a stable current。

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