tailieunhanh - Introduction to Electronics - Part 6

Bipolar IC Bias Circuits Mạch tích hợp trình bày vấn đề đặc biệt phải được xem xét trước khi thiết kế mạch được thực hiện. Đối với mục đích của chúng tôi ở đây, việc xem xét quan trọng nhất là bất động sản. Không gian trên một wafer IC là một phí bảo hiểm. Bất cứ điều gì mà chiếm quá nhiều không gian là một trách nhiệm. Hãy xem xét những điều sau đây: G điện trở là rất không hiệu quả khi nói đến bất động sản. Khu vực yêu cầu là tỷ lệ thuận với giá trị. | Bipolar IC Bias Circuits Introduction to Electronics 129 Bipolar IC Bias Circuits Introduction Integrated circuits present special problems that must be considered before circuit designs are undertaken. For our purposes here the most important consideration is real estate. Space on an IC wafer is at a premium. Anything that takes up too much space is a liability. Consider the following Resistors are very inefficient when it comes to real estate. The area required is directly proportional to the value of resistance remember R pL A . As a result use of resistances in ICs is avoided if possible. And resistances greater than 100 kQ are extremely rare. When used it is quite difficult to control resistance values with accuracy unless each resistor is laser-trimmed. Tolerances are as large as 50 are not unusual. Because all resistors are fabricated at the same time all resistors are off by the same amount. This means that resistors that are intended to be equal will essentially be equal. Capacitors are also liabilities. Capacitance values greater than 100 pF are virtually unheard of. Inductors only recently became integrable. Their use is quite limited. BJTs are very efficient. And while p values suffer the same 3 1 to 5 1 variation found in discrete transistors allBJTs on an IC wafer are essentially identical if intended to be . This latter point is most important and drives all IC circuit design. We begin to examine this on the following pages. Bipolar IC Bias Circuits Introduction to Electronics 130 The Diode-Biased Current Mirror Current Ratio Fig. 190. Diode-biased current mirror. This is the most simple of all IC bias circuit techniques. The key here is that the BJTs are identical Because VBE1 VBE2 this means that IB1 IB2 IB . Note that VCB1 0 thus Q1 is active at the edge of saturation . If we assume Q2 is also active we have IC1 IC2 IC . From this point the analysis proceeds straightforwardly . . . IO IC 2 IC1 IC PIB 175 And from a KCL equation at the collector of