CMOS analog IC Design presentation.pptx

CMOS analog IC design seminar presentation

1. CMOS ANALOG IC DESIGN SEMINAR TOPIC- CS Stage with Source Degeneration PRESENTED BY- Satyam Kumar Mishra Roll No -19134502023 Branch –ECE(6 th sem )

2. CS Stage with Source Degeneration Degeneration resistor R S in series with source terminal makes input device more linear As V in increases, so do I D and the voltage drop across R S Part of the change in V in appears across R S rather than gate-source overdrive, making variation in I D smoother Gain is now a weaker function of g m 2

3. CS Stage with Source Degeneration 3 Nonlinearity of circuit is due to nonlinear dependence of I D upon V in Equivalent transconductance G m of the circuit can be defied as

4. CS Stage with Source Degeneration 4 g m is the transconductance of M 1 Small-signal voltage gain A v is then given by

5. CS Stage with Source Degeneration 5 Same result for G m is obtained from small-signal equivalent circuit, by noting that As R S increases, G m becomes a weaker function of g m and hence I D For , , i.e., Most of the change in V in across R S and drain current becomes a “linearized” function of input voltage

6. CS Stage with Source Degeneration 6 Including body-effect and channel-length modulation, G m is found from modified small-signal equivalent circuit

7. CS Stage with Source Degeneration 7 Small-signal derived previously can be written as Denominator = Series combination of inverse transconductance + explicit resistance seen from source to ground Called “resistance seen in the source path” Magnitude of gain = Resistance seen at the drain/ Total resistance seen in the source path

8. CS Stage with Source Degeneration 8 Degeneration causes increase in output resistance Ignoring R D and including body effect in small-signal equivalent model, r o is boosted by a factor of {1 + ( g m + g mb ) R S } and then added to R S Alternatively, R S is boosted by a factor of {1 + ( g m + g mb ) r o } and then added to r o

9. CS Stage with Source Degeneration 9 Compare R S = with R S > If R S = 0, and If R S > 0, and , obtaining negative g m V 1 and g mb V bs Thus, current supplied by V X is less than V X / r o and hence output impedance is greater than r o

10. CS Stage with Source Degeneration 10 Intuitive understanding of increased output impedance Apply voltage change ΔV at output and measure resulting change ΔI in output current, which is also the change in current through R S Resistance seen looking into the source of M 1 is ( g m + g mb ) Voltage change across R S is

11. CS Stage with Source Degeneration 11 Intuitive understanding of increased output impedance Change in current across R S is Output resistance is thus

12. CS Stage with Source Degeneration 12 To compute gain in the general case including body effect and channel-length modulation, consider above small- signal model From KVL at input, KCL at output gives

13. CS Stage with Source Degeneration 13 Since voltage drops across r O and R S must add up to V out , Voltage gain is therefore

14. THANK YOU!