1. Analysis of Single Stage Amplifiers

2. Small Signal Models Small-signal models for the MOSFET:
(a) neglecting the dependence of iD on vDS in saturation (channel-length modulation effect); and
(b) including the effect of channel-length modulation modeled by output resistance ro = |VA|/ID.

3. Small Signal T Models
The T model of the MOSFET augmented with the drain-to-source resistance ro

4. Single Stage Common Source Amplifier

5. Small Signal equivalent circuit
Small Signal Model
Small Signal equivalent circuit

6. Single Stage Cs Amplifier
Input resistance (Very High input resistance )
Voltage gain (Moderately High Voltage Gain)
Output resistance (Relatively High output Resistance)
Voltage Gain for small signal
The magnitude of the gain Av can be increased by increasing W/L Ratio if the other parameter are constant.

7. Common Gate Amplifier

8. Common Gate Amplifier

9. Common Gate Amplifier Input resistance Voltage gain Output resistance
Non inverting amplifier
Low input resistance
Has nearly identical voltage gain of CS amplifier
Relatively high output resistance
Current follower
Superior high-frequency performance

10. Source Follower Or Common Drain Amplifier

11. Small Signal Model

12. Source Follower Input resistance Voltage gain (Voltage Divider)
Output resistance
Very high input resistance
Voltage gain is less than but close to unity
Relatively low output resistance
Voltage buffer amplifier
Power amplifier

13. Common Source with Source Resistance

14. Common Source with Source Resistance

15. Common Source with Source Resistance
Input resistance
Voltage gain
Output resistance
Including RS results in a gain reduction by the factor (1+gmRS)
RS takes the effect of negative feedback.

16. MOS Cascode
In response to the input signal voltage vi the transistor will conduct a current gm1vgs1 and feed it to the source terminal of the common gate transistor Q2 called as cascode transistor.
This configuration is called as cascode configuration.
The basic idea behind the cascode amplifier is to combine the high input resistance and large Transconductance achieved in a common-source amplifier with the current-buffering property and the superior high-frequency response of the common-gate circuit.

17. MOS Cascode

18. MOS Cascode

19. MOS Cascode

20. MOS Cascode (to raise the output resistance)
Numerical :
It is required to design the Cascode current source to provide a current of 100µA and an output resistance of 500 KΩ. Assume the availability of a 0.18µm Technology for which
and calculate L and W/L for each transistor. And the bias voltages and
Cascoding can also be employed to raise the output resistance of the current source load.
Q4 is the current source transistor and Q3 is the common gate (CG) Cascode transistor.
The Cascode transistor Q3 multiplies the output resistance of of Q4 to provide an output resistance for the Cascode current source.

21. Example

22. Example