Trong khi các giá trị tuyệt đối của các thông số thiết bị là khó khăn để duy trì, hai thiết bị có thể được chính xác phù hợp trong một mạch. Mạch này cho phép thiết kế kỹ thuật được sử dụng mà kết quả trong các chức năng chính xác. Trong phần này, chip bố trí cho phù hợp với chính xác của các thành phần thảo luận. | now to Figure . Here we have placed a second transistor such that Vbe Q2 Vbe Q1 . If we assume that Q1 and Q2 are identical in all respects then Is Q1 Is Q2 and ultimately Ic Q2 Ic Q1 . This is the basic principle of operation for a current mirror. Example Using the circuit from Figure find the collector current in transistor Q2 if R1 10 kQ. Use VCC 5 V Is 200E - 18 A and VT 26 mV. Assume ideal NPN transistors with Ị3 TO. Using the approximation Vbe solve for Ic Q1 Ic Q1 5- 430 aA c J 10 000 Find the real Vbe value Vbe 26mVln - mV 200E - 18A Recalculate the current Ic Q1 5 - 10 000 aA Recalculate Vbe Vbe 26mVln - 6A 200E - 18A mV Another iteration may be made but the change in current between iterations was only 1 . This level of refinement is usually good enough for first-pass design. Based on our assumption that both transistors are ideal we can conclude that the collector current in Q2 is equal to that in Q1 and so Ic Q2 Ic Q1 aA. We can expand this analysis to multiple transistors. Consider the circuit in Figure . This circuit has two mirror transistors. Using the same assumptions of ideality and identical transistors we come to the conclusion that each mirror transistor is sinking current equal to the reference current. Furthermore if we tie the collectors of both mirror transistors together the output current of the mirror is equal to twice the reference current as shown in Figure . This leads to an interesting point. What happens if transistors Q2 and Q3 are merged into a single device Making the emitter size twice as big as in Q1 can do this. The correct answer is that the output current of the 2X mirror will be twice the reference current. Accuracy of the Figure Multiple transistor current mirror. Figure NPN current mirror layout. Blue indicates shallow-n doping for emitter and collector ohmic contact. Green indicates shallow-p base. White indicates contact openings. Light yellow .
