模拟集成电路（Baker）
9.Models for Analog Design
9.0 绪论
- 长沟器件（square-law）
- 短沟器件（are developed with graphs showing device characteristics）
- MOSFET的噪声模型
9.1 Long-Channel MOSFETs
9.1.1 The Square-Law Equations
-
常识:Often the controlling parameter in a semiconductor device is a voltage. The controlled parameter is then the device’s output current (and this is why current is on the y-axis and voltage is on the x-axis in an IV plot)
-
不同区域下的MOSFET等效模型:Clearly, in the triode region (also known as the linear or ohmic region), the MOSFET behaves like a resistor. In the saturation region, the MOSFET behaves as seen in Fig. 9.3b, like a current source in parallel with a resistor. The resistive component, whether in the triode or saturation regions, is often called the MOSFET’s output resistance.
-
\(V_{DS,sat}=V_{GS}-V_{THN}\) ,\(V_{DS,sat}\)is the voltage where the MOSFET moves from the triode region to the saturation region ,注意上述公式 is only valid for long-channel MOSFETs.对于短沟器件需要查表获得.
-
饱和区输出电阻公式:\(r_o=\frac{1}{\lambda I_{DS,sat}}\)
-
gate-drain connected MOSFET :要么工作在截止区要么工作在饱和区,等效小信号阻抗为\(\frac{1}{g_m}\).
-
直观认识:对于NMOS管在漏极注入电流会使 \(I_D,V_{DS}\) 增大,在漏极偷取电流会使 \(I_D,V_{DS}\) 减小,对于PMOS管则正好相反.
-
体效应:For an NMOS device, the threshold voltage increases when the source is at a higher potential than the NMOS body (the p-substrate, or ground, in this book). This change in threshold voltage is called the body effect.体效应会使MOS管的阈值电压增大.
-
The Triode Region:
\[I_D=KP_n \frac{W}{L}((V_{GS}-V_{THN})V_{DS}-\frac{V_{DS}^2}{2}) \\ 沟道电阻 R_{ch}=\frac{1}{KP_n\frac{W}{L}(V_{DS,sat}-V_{DS})}\approx\frac{1}{KP_n\frac{W}{L}(V_{GS}-V_{THN})} \] -
The Cutoff and Subthreshold Regions:电流电压方程与二极管类似.
9.1.2 Small Signal Models
-
小信号分析时的符号规范:A quick note concerning symbols: throughout the book we’ll represent a signal containing **both AC and DC **components by a lowercase letter with uppercase subscripts. AC signals are represented with both lowercase letters and subscripts, while DC signals are represented with both uppercase letters and subscripts. 一个简单的例子,交直流信号:\(v_{GS}\);交流信号:\(v_{gs}\);直流信号:\(V_{GS}\)
-
交流分析时直流量的处理:We often treat DC voltage sources as shorts when doing AC analysis (and current sources as opens).
-
The \(g_m\) of a device is an AC small-signal parameter that relates the AC gate voltage to the AC drain current, that is
\[i_d=g_m v_{gs} \] -
计算 \(g_m\) 时几个常用的公式
\[定义 \beta_n =KP_n\frac{W}{L}\\ g_m=\beta_n(V_{GS}-V_{THN})=\sqrt{2\beta_n I_D}=\frac{2I_D}{V_{GS}-V_{THN}} \]
- The key points are that gm goes up as the root of the drain current and linearly with \(V_{DS,sat}\)