Analysis of Transistor Amplifiers

Frequency Response Graph

Notes

• At low frequencies, parasitic, bypass and coupling capacitors are open circuited
• At the midband, bypass and coupling capacitors are short circuited, parasitic capacitors are open circuited
• At high frequencies, parasitic, bypass and coupling capacitors are short circuited

DC Analysis

• Bypass, coupling and parasitic capacitors are open circuited
• AC voltage sources are short circuited
• AC current sources are open circuited

Small Signal Analysis

• Bypass and coupling capacitors are short circuited, parasitic capacitors are open circuited
• DC voltage sources are short circuited
• DC current sources are open circuited

Low Frequency Analysis

Method of Open Circuit Time Constants

1. Set all parasitic capacitors to open circuit
2. Set input voltage $v_{sig}=0$$\Large v_{sig} = 0$.
3. Consider each capacitor individually in the small signal model; i.e. short circuit all other capacitors
4. For each capacitor $C_i$$\Large C_i$, find its input resistance $R_i$$\Large R_i$. This can be done by setting test source $V_x$$\Large V_x$ and evaluating current $I_x$$\Large I_x$, so that $R_i=\frac{V_x}{I_x}$$\Large R_i = \frac{V_x}{I_x}$.
5. $f_L=\sum _{i=1}^n\frac{1}{2\pi C_i{R}_i}$$\Large f_L = \sum\limits_{i = 1}^{n}\frac{1}{2\pi C_iR_i}$.

High Frequency Analysis

Method of Open Circuit Time Constants

1. Set all bypass and coupling capacitors to capacitors to short circuit
2. Set input voltage $v_{sig}=0$$\Large v_{sig} = 0$.
3. Consider each capacitor individually in the small signal model; i.e. short circuit all other capacitors
4. For each capacitor $C_i$$\Large C_i$, find its input resistance $R_i$$\Large R_i$. This can be done by setting test source $V_x$$\Large V_x$ and evaluating current $I_x$$\Large I_x$, so that $R_i=\frac{V_x}{I_x}$$\Large R_i = \frac{V_x}{I_x}$.
5. $f_L=\sum _{i=1}^n\frac{1}{2\pi C_i{R}_i}$$\Large f_L = \sum\limits_{i = 1}^{n}\frac{1}{2\pi C_iR_i}$.

Miller Theorem