EE 434 Lecture 21. MOS Amplifiers Bipolar Devices

Transcription

1 434 ecture MOS Amplifiers ipolar Devices

2 Quiz 3 The quiescent voltage across the 5K resistor in the circuit shown was measured to be 3. ) Determine the quiescent output voltage ) Determine the small signal voltage gain Assume M is manufactured in a process with μ OX =00uA/ and T =.

3 And the number is

4 And the number is

5 Quiz 3 The quiescent voltage across the 5K resistor in the circuit shown was measured to be 3. ) Determine the quiescent output voltage ) Determine the small signal voltage gain Assume M is manufactured in a process with μ OX =00uA/ and T =. Solution: OQ = DD IDQRD = 9 3 = 6

6 Quiz 3 The quiescent voltage across the 5K resistor in the circuit shown was measured to be 3. ) Determine the quiescent output voltage ) Determine the small signal voltage gain Assume M is manufactured in a process with μ OX =00uA/ and T =. Solution: A = g M R D = I DQ R

7 Quiz 3 The quiescent voltage across the 5K resistor in the circuit shown was measured to be 3. ) Determine the quiescent output voltage ) Determine the small signal voltage gain Assume M is manufactured in a process with μ OX =00uA/ and T =. Solution: A = g M R D I = DQ R 3 = 3 = 3

8 xample: Determine the small signal equivalent for the following device. Assume M operating in the saturation region G i =g + Q 0 gm i = = g 0 + g m g m

9 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region

10 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region

11 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region A = g g m m

12 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region m m g g A = DQ OX DQ OX I I A µ µ =

13 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region m m g g A = DQ OX DQ OX I I A µ µ = A =

14 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region A A = A = = µ µ g g m m OX OX I I DQ DQ Accurate gain control Nearly independent of process parameters an also show (but not from ss analysis) that this is quite very linear!

15 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region inearity of this common-source amplifier I D D μ I μ OX OX ( ) IN T ( - - ) DD OUT T I D = I D μ OX ( ) μ ( ) IN T OX DDD OUT T Taking the square root of both sides of this eqn, obtain OUT = IN + DD + T T

16 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region inearity of this common-source amplifier OUT = IN + DD + T T Appears to be perfectly linear Have neglected bulk effect for M which introduces small nonlinearity Have also neglected λ effects which introduce some more nonlinearity Dependent upon square-law model which may not be good enough Overall, good linearity and accurate control of gain but not perfect

17 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region How do these two amplifiers compare? I I A DQ OX DQ OX µ µ µ µ =

18 MOS Amplifier Determine the small signal voltage gain for the following circuit. Assume M and M operating in the saturation region How do these two amplifiers compare? g mb effects are removed for one on right! Gain can not be as accurately controlled

19 ipolar Junction Transistors Operation Modeling

20 arriers in Doped Semiconductors n-type p-type

21 arriers in Doped Semiconductors I urrent carriers are dominantly electrons Small number of holes are short-term carriers I urrent carriers are dominantly holes Small number of electrons are short-term carriers

22 arriers in Doped Semiconductors n-type p-type Majority arriers electrons holes Minority arriers holes electrons

23 arriers in MOS Transistors onsider n-channel MOSFT Saturation Region hannel Triode Region

24 arriers in MOS Transistors onsider n-channel MOSFT Saturation Region Triode Region arriers in electrically induced n-channel are electrons

25 arriers in MOS Transistors onsider p-channel MOSFT Saturation Region hannel Triode Region

26 arriers in MOS Transistors onsider p-channel MOSFT Saturation Region Triode Region arriers in electrically induced p-channel are holes

27 arriers in MOS Transistors arriers in channel of MOS transistors are Majority carriers

28 ipolar Transistors npn stack pnp stack npn transistor pnp transistor ith proper doping and device sizing these form ipolar Transistors

29 ipolar Transistors npn transistor pnp transistor n-channel MOSFT p-channel MOSFT In contrast to a MOSFT which has 4 terminals, a JT only has 3 terminals

30 ipolar Operation onsider npn transistor npn stack Under forward bias current flow into base and out of emitter urrent flow is governed by the diode equation arriers in emitter are electrons (majority carriers) hen electrons pass into the base they become minority carriers Quickly recombine with holes to create holes base region Dominant current flow in base is holes (majority carriers)

31 ipolar Operation onsider npn transistor npn stack Under forward bias and reverse bias current flows into base region arriers in emitter are electrons (majority carriers) hen electrons pass into the base they become minority carriers hen minority carriers are present in the base they can be attracted to collector

32 ipolar Operation onsider npn transistor npn stack 0 F If no force on electron is applied by collector, electron will contribute to base current

33 ipolar Operation onsider npn transistor npn stack 0 F If no force on electron is applied by collector, electron will contribute to base current lectron will recombine with a hole so dominant current flow in base will be by majority carriers

34 ipolar Operation onsider npn transistor npn stack 0 F F hen minority carriers are present in the base they can be attracted to collector with reverse-bias of junction and can move across junction

35 ipolar Operation onsider npn transistor npn stack 0 F F hen minority carriers are present in the base they can be attracted to collector with reverse-bias of junction and can move across junction ill contribute to collector current flow as majority carriers

36 ipolar Operation onsider npn transistor npn stack 0 F So, what will happen? F

37 ipolar Operation onsider npn transistor npn stack 0 F So, what will happen? F Some will recombine with holes and contribute to base current and some will be attracted across junction and contribute to collector Size and thickness of base region and relative doping levels will play key role in percent of minority carriers injected into base contributing to collector current

38 ipolar Operation onsider npn transistor npn stack Under forward bias and reverse bias current flows into base region arriers in emitter are electrons (majority carriers) hen electrons pass into the base they become minority carriers hen minority carriers are present in the base they can be attracted to collector Minority carriers either recombine with holes and contribute to base current or are attracted into collector region and contribute to collector current

39 ipolar Operation onsider npn transistor npn stack Under forward bias and reverse bias current flows into base region fficiency at which minority carriers injected into base region and contribute to collector current is termed α α is always less than but for a good transistor, it is very close to For good transistors.99 < α <.999 Making the base region very thin makes α large

40 ipolar Transistors npn stack pnp stack principle of operation of pnp and npn transistors are the same minority carriers in base of pnp are holes npn usually have modestly superior properties because mobility of electrons Is larger than mobility of holes

41 ipolar Operation onsider npn transistor npn stack In contrast to MOS devices where current flow in channel is by majority carriers, current flow in the critical base region of bipolar transistors is by minority carriers

42 ipolar Operation I I I I I β is typically very large + I = = α I I α I = I α β I α = α = β defn I often 50<β<999

43 I = β I ipolar Operation I I I = β I β is typically very large I ipolar transistor can be thought of a current amplifier with a large current gain In contrast, MOS transistor is inherently a tramsconductance amplifier urrent flow in base is governed by the diode equation ollector current thus varies exponentially with I I = I ~ S e = βi ~ S e t t

44 I = β I ipolar Operation I I I = β I I β is typically very large ollector current thus varies exponentially with I = βi ~ S e t This exponential relationship (in contrast to the square-law relationship for the MOSFT) provides a very large gain for the JT and this property is very useful for many applications!!