课程名称︰电子学(二)
课程性质︰电机工程学系大二必修
课程教师︰林宗贤
开课学院：电机资讯学院
开课系所︰电机工程学系
考试日期（年月日）︰2020/4/21
考试时限（分钟）：50分钟
试题 :
1. A CS amplifier that can be represented by the equivalent circuit of Fig.1
has Cgs = 1pF, Cgd = 0.05pF, CL = 1pF, gm = 4mA/V, and R'sig = R'L = 20kΩ
(a) Find the midband gain AM. (10%)
(b) Find tje input capacitance Cin using the Miller approximation. (20%)
(c) Obtain the estimate of fH using open-circuit time constants. (20%)
┌─R'sig┬─Cgd───┬──┬──┐ +
+ │ + │ │ │ │
V'sig○ Vgs Cgs gmVgs↓ ◇ R'L CL Vo
- │ - │ │ │ │ -
≡ ≡ ≡ ≡ ≡
Fig.1 Generalized high-frequency equivalent circuit for the CS amplifier
2. Consider a feedback amplifier for which the open-loop gain A(s) is given by
10000
A(s)= ────────────────
(1 + s/10000) (1 + s/100000)^2
If the feedback factor β is independent of frequency,
(a) Find the frequency at which the phase shift is 180° . (25%)
(b) Find the critical value of β at which oscillation will commence. (25%)