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Above Threshold » Single Stage Amplifiers

Member of: Circuits

This is collected information on basic single stage amplifiers

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1. Common Source
1. 1.1 Basic Specs
2. 1.2 Frequency Response
2. Common Gate
3. Common Drain or Source Follower
1. 3.1 Frequency Response

1. Common Source

1.1 Basic Specs

Gain
- With resistive load: $A_v = -g_m (r_o \parallel R)$
- Current source load: $A_v = -g_m (r_{on} \parallel r_{op})$
Output Impedance
- Current source load: $R_{out} = (r_{on} \parallel r_{op})$
- With source degeneration: $R_{out} = \left( [1+(g_m + g_{mb}R_s)]r_{on} \right) \parallel r_{op}$
Input Impedance
Note
- Equiv. Resistance of diode connected transistor: $R_{in} = \frac1{g_m + g_{mb} }$

1.2 Frequency Response

Transfer function:
- R_s - series resistance of stimulus
- r_ob - Load resistance
Poles
- Pole at the input node: $\omega_{in} = \frac1{ R_s [ (1+g_m r_{ob}) C_{gd} + C_{gs} ] + r_{ob}(C_{gd} + C_{db} ) }$
- Pole at the output node: $\omega_{out} \approx \frac1{ r_{ob}( C_{gd} + C_{db} ) }$
- Expect ω_in to be the dominant pole.

2. Common Gate

3. Common Drain or Source Follower

Gain
- $A_v = \frac{ 1/g_{mb} \parallel r_{of} \parallel r_{ob} }{ 1/g_{mb} \parallel r_{of} \parallel r_{ob} + 1/g_m }$
- f - Follower transistor
- b - Bias transistor
Output Impedance
- In full: $R_{out} = \frac1{g_{mb}} \parallel \frac1{g_{m}} \parallel \frac1{r_{of}} \parallel \frac1{r_{ob}}$
- Because 1/g_m < r_o we can simplify to $R_{out} \approx \frac1{g_{mb}} \parallel \frac1{g_{m}}$

3.1 Frequency Response

Transfer function:
- R_s - series resistance of stimulus
Poles
- Dominant Pole: $\omega_{p} = \frac1{ R_s C_{gd} + \frac{ C_L + C_{gs} }{ g_m } }$

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Last edited by DrLock. Originally by DrLock.