MOSFET Common Source Amplifier

Transcription

1 Microelectronic Circuits MOSFET Common Source Amplifier Slide 1

2 Small nal Model The definition of Transconductance g m i D S S S k n W L O The definition of output resistance r o DS I The definition of oltage gain A i o i D i D g m D D I A D Slide 2

3 Small nal model (a) (b) neglecting the channel-length modulation effect including the effect of channel-length modulation, modeled by output resistance r o = A /I D. Slide 3

4 Small nal model (a) The T model of the MOSFET augmented with the drain-to-source resistance r o. (b) An alternatie representation of the T model. Slide 4

5 Single Stage Cs Amplifier Input resistance with no load i L Input resistance Open-circuit oltage gain A o oltage gain A i i i i o i i i i L o i Open-circuit oerall oltage gain 0 o oerall oltage gain 0 L Slide 5

6 Single Stage Cs Amplifier Slide 6

7 Single Stage Cs Amplifier Small Signal equialent circuit Slide 7

8 Single Stage Cs Amplifier Input resistance in oltage gain A g m Oerall oltage gain Output resistance ( ro // D // L ) g m r // out o ( D // L // ro ) D Slide 8

9 Common Source Amplifier ery high input resistance Moderately high oltage gain elatiely high output resistance Slide 9

10 CS Amplifier with Source resistance Slide 10

11 CS Amplifier with Source resistance Slide 11

12 CS Amplifier with Source resistance Input resistance oltage gain Oerall oltage gain Output resistance in A gm( D // L ) 1 g out D m S gm( D // 1 g m S L ) Slide 12

13 CS Amplifier with Source resistance Including S results in a gain reduction by the factor (1+g m S ) S takes the effect of negatie feedback. Slide 13

14 Common ate Amplifier Slide 14

15 Common ate Amplifier Slide 15

16 Common ate Amplifier Input resistance in 1/ g m oltage gain A g ( // L ) m D Oerall oltage gain Output resistance gm ( D // L ) 1 out D g m Slide 16

17 Common ate Amplifier Noninerting amplifier Low input resistance Has nearly identical oltage gain of CS amplifier, but the oerall oltage gain is smaller by the factor (1+g m ) elatiely high output resistance Current follower Superior high-frequency performance Slide 17

18 Common Drain or oltage follower Slide 18

19 Common Drain or oltage follower Slide 19

20 Common Drain or oltage follower Input resistance in oltage gain Oerall oltage gain A Output resistance out L g 1 L m 1 g m L L 1 g m Slide 20

21 Common Drain or oltage follower ery high input resistance oltage gain is less than but close to unity elatiely low output resistance oltage buffer amplifier Power amplifier Slide 21

22 MOSFET Internal Capacitances Internal capacitances The gate capacitie effect Triode region Saturation region The MOSFET Internal Capacitance and High-Frequency Model Cutoff region Oerlap capacitance The junction capacitances Source-body depletion-layer capacitance Drain-body depletion-layer capacitance Slide 22

23 MOSFET Internal Capacitances MOSFET operates at triode region C gs C gd 1 2 WLC ox MOSFET operates at saturation region C C MOSFET operates at cutoff region 2 The MOSFET gs 3Internal Capacitance ox and High-Frequency Model gd 0 WLC Cgs Cgd 0 Cgb WLCox Slide 23

24 MOSFET Internal Capacitances Oerlap capacitance results from the fact that the source and drain diffusions extend slightly under the gate oxide. The expression for oerlap capacitance C WL o o C ox The MOSFET Internal Capacitance and High-Frequency Model Typical alue L o L This additional component should be added to C gs and C gd in all preceding formulas. Slide 24

25 MOSFET Internal Capacitances Source-body depletion-layer capacitance C Built in oltage (0.6 to 0.8) sb C sb0 1+ SB The MOSFET Internal Capacitance o and High-Frequency Model C sb at zero bodysource bias Magnitude of reerse bias oltage Drain-body depletion-layer capacitance C db C db0 1+ DB o Slide 25

26 High Frequency esponse + C gd The MOSFET Internal Capacitance and High-Frequency Model gs C gs D g m gs r L L o D + o L - S - Slide 26

27 High Frequency esponse The MOSFET Internal Capacitance and High-Frequency Model Theenin s Equialent Parallel Combination This circuit can be further simplified Slide 27

28 High Frequency esponse C gd is a bridging capacitor, which connects the output node to the input node. + gs - C gs X C gd I gd X S D g m gs L + o - Slide 28

29 High Frequency esponse The load current = g I ) ( m gs gd X + gs C gs X C gd I gd D g m gs L + o - - o S ( g m gs ) L g m L gs Slide 29

30 High Frequency esponse + gs C gs X X C gd I gd I gd sc sc sc gd gd gd D g m gs gs gs 1 g m o g m L L gs L gs + o - - S Slide 30

31 High Frequency esponse At XX the existence of C gd is known through the current Igd, so C gd can be replaced by C eq X + gs C gs X C gd I gd D g m gs L + o - - S Slide 31

32 High Frequency esponse Look at the input part of the circuit, which is a single time constant circuit of low pass type. Slide 32

33 High Frequency esponse wherew w 0 in 1 gs s 1 w0 is corneror break frequencyof 1, where Cin Cgs Ceq Cgs Cgd ( 1 C 0 STCcircuit g m L ) Slide 33

34 Slide 34 High Frequency esponse gs L m L gs m o g ) g ( w s gs w s ) g ( L m o Combining We get,

35 High Frequency esponse Which can be expressed in the form Mid band gain o AM s 1 w H w H and w f H 0 C in 1 w 1 H 2 2C in Slide 35

36 High Frequency esponse Thus we can see that, high frequency response will be that of a low pass filter. With 3dB cut off frequency f H o db 3dB 20 log A M f H f (Hz) (log scale) Slide 36

37 High Frequency esponse Find the midband gain A M and the upper 3-dB frequency f H of a CS amplifier fed with a nal source haing an internal resistance = 100kΩ. The amplifier has = 4.7MΩ, D, L =15kΩ, g m = 1mA/, r 0 = 150kΩ, C gs = 1pF and C gd = 0.4pF. For the CS amplifier for aboe example, find the alue of A M and the upper 3-dB frequency f H if the source resistance reduced to = 10kΩ. Ans: /, 3.7MHz. Slide 37