Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege

1. Engineering 43 FETs-2(Field Effect Transistors) Bruce Mayer, PE Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

2. Learning Goals • Understand the Basic Physics of MOSFET Operation • Describe the Regions of Operation of a MOSFET • Use the Graphical LOAD-LINE method to analyze the operation of basic MOSFET Amplifiers • Determine the Bias-Point (Q-Point) for MOSFET circuits

3. Load Line: Common Source Amp • Shown below is typical “Common-Source” Amplifier circuit • THE DC sources, VDD & VGGbias the MOSFET for Amp operation • That is, the two DC sources set The Operating, or Q, Pt • Now apply KVL to left loop

4. Load Line: Common Source Amp • Using the values given the Schematic • Now KVL on Right Loop • Rearranging • Of form: y = mx + b • Using given values

5. Load Line: Common Source Amp • Thus the LoadLineEquation • Plot this on the FET vi Curve to determine the Operating Point • Since this is a LINE need only 2-points • Intercepts are easy • Making a T-Table

6. Draw LoadLine on FET vi Curve VGG = VGS sets Q-Pt

7. Max and Min Opp-Points • The common source Amp is designed to Operate in the SATURATION Region. Recall the vGSEqn • By sin behavior • Reading the vi-LL graph find (vDS,iD) co-ords • (vDS,iD)max = (4V,16mA) • vGS = 5V • (vDS,iD)min = (16V,4mA) • vGS = 3V

8. Voltage Swing • The common source Amp is must stay in Saturation. For this nFET that means max & min vGS values of 5V & Vto given the 1V amplitude of the sin • From Last Slide We calculated corresponding Xmax/min values for vDS • vDS,min = 4V (vGS = 5) • vDS,max = 16V (vGS = 3) • Note that the output direction is Opposite the Input direct • The ckt produces a SATURATED output Voltage Swing of 4V-16V = −12V

9. Input and OutPut Compared • Note that OUTput peaks occur at INput Valleys → Inversion • The ratio of the V-swings • This NOT the Gain, Av = vin/vout Gate Excitation vDS Response

10. LoadLine Gain • Notice • vGS: 4→3 • VDS: 11→15 (∆ = 4) • vGS: 4→5 • VDS: 11→4 (∆ = 7) • This is due to the NONlinear nature of MOSFETS; they are “Square-Law” devices • The iD lines in SAT are not evenly Spaced

11. LoadLine Gain • Since the FET is NOT linear, vout is NOT directly proportional to vin, so we can NOT Define a true Gain • “Small Signal” methods WILL allow us to define a true grain for the AC part of the voltage input • Requires Calculus • To “Linearize” ckt

12. Common-Source Amp Analysis • To analyze this amplifier we will do the following: • Perform DC analysis (find bias, or Q, point; i.e., find the DC drain current and check that the transistor is in the saturation region) • Find circuit small-signal AC model (based on the bias point obtained) • Perform AC (small signal) analysis

13. Saturation Slippery-Slope • Must also take care that the small-signal input does NOT push the FET Out of Saturation at ANY Time. • The vin-Amplitude and Bias-Pt Selection could • Drive the FET out of SAT and into TRIODE Operation • Drive the FET into CutOff(vGS < Vto)

14. One-Supply Bias Circuit • Usually only ONE supply voltage is available. In this case set VG with a voltage divider • By Thevenin • Since the Gate on a IGFET draws NO current, we have a simple voltage divider on the Left thru R1 & R2. • Thus VG to GND

15. One-Supply Bias Circuit • Replacingthe left side of the cktwith its V-Divider equivalent • Then theKVL Eqn for the Gate Loop • The is NO V-Drop across RG as iG = 0 • Thus • Recall the CE amp is designed to operate in Saturation which produces iDat this level

16. One-Supply Bias Circuit • If the Circuit has been properly Biased the FET is in SATURATION • In Saturation iD is independent of vDS and equals, at the operating , or Q, point • Recall theKVL eqnon theGATELoop (nowAssumed at the “Q” point): • Sub into SAT eqn

17. One-Supply Bias Circuit • Solve for vGSQ: • Or: • Introduce new Constant: • Yields quadratic Eqn invGSQ: • Now Solve by MATLAB’s MuPAD

18. vGSQ by MuPAD

19. One-Supply Bias Circuit • Discarding the Negative Root find • Then Find iDQ by subbing vGSQ from above into the gate KVL Eqn:

20. One-Supply Bias Circuit • Solving the last two eqns yields iDQ and vGSQ • Beware that the parabolic iDeqn will produce an extraneous root • Discard the SMALLER root as SAT requires:vGS−Vto ≥ 0 • The KVL eqn on cktRight-Side • ReArranging • Then with iDQ from before (MuPAD)

21. Small Signal FET Model • At the Operating Point the quantities are DC, and should be Noted in Upper case letters: • If a small-amplitude ac signal is injected into the circuit the instantaneous quantity Eqns: • Where id and vgs are the small signal quantities • A conceptual Diagram

22. Small Signal FET Model • Recall iD in SAT • From last slide • Subbing for iD & vGS • Now the Q-Pt is also in Saturation so, • Using this reln and expanding the IDQ+ideqn yields • Where gm is called the “small signal transconduce” for an nMOSFET • Again for IGFET

23. Small Signal FET Model • These eqns describe the linear small signal operation in the Saturation region • A graphical representation of the model

24. gm(Q) • Usually a greater value of gm is better than a smaller one • gm is a function of the Device “K” Parameter and the Q-Pt values • Recall • Also recall at Q-Pt • Or: • Thus • Simplifying: • Recall from our MOSFET Construction Discussion • “KP” is singlequantity

25. gm(Q) • Using • And • Find • Simplifying • Thus to increase the transconductance of a KP-fixed MOSFET • Increase IDQ • Remember, Vswing must be entirely in the SATURATION region • Increase the W/L ratio • But this makes the transistor BIGGER; usually NOT desired

26. Refined Small Signal Model • The previous model assumed CONSTANT iD in Saturation • Real MOSFETs exhibit an upward Slope in SAT: • Recall that a SLOPE on a vi Curve is effectively • A Conductance;G or g • An inverse Resistance 1/R or 1/r • On a MOSFET this slope is called the “Drain Resistance, rd

27. Refined Small Signal Model • The KCL Equation for the model that accounts for the upward iD Slope in SAT • The Graphical Representation

28. gm & rd by Calculus • Start with Refined Model Equation • ReCall • G & g in Siemens (amps per volt) • Now let the ∆’s approach ZERO to make derivatives • Specifically find the slope of model eqn when vds = 0 • Thus gm: • Note that MOSFET operates at the Q-Pt

29. gm & rd by Calculus • Again use approx: • In the this case vds = 0 implies vDS = VDSQ so can approximate: • This Eqn is an approximation of a derivation amongst iD, vGS and vDS • G & g in Siemens (amps per volt) • Now let the ∆’s approach ZERO to make derivatives • Again letting the ∆’s go to zero • Recall • Now in the small-signal eqn let vgs = 0

30. gm & rd by Calculus • Solving this Eqnfor rd • In this Case • Again use approx: • Thus similar to before • Thus we can write a partial derivative for rd • Or, as stated in the Text Book

31. Example 12.3: Find gm & rd Q-Pt → (VDSQ, IDQ) = (10 V, 7.4 mA)Also VGSQ = 3.5 V

32. Example 12.3: Find gm & rd • Recall approx. • In This Example VDSQ = 10 V • Make a t-Table when vDS = 10V • See vi Graph • Thus • ∆vGS = (4 − 3) V = 1V • ∆iD = (10.7 − 4.7) mA = 6mA • Then gm:

33. Example 12.3: Find gm & rd • Recall approx. • In This Example VGSQ = 3.5 V • Make a t-Table when vGS = 3.5V • See vi Graph • Thus • ∆vDS = (14 − 4) V = 10V • ∆iD = (8 − 6.7) mA = 1.3mA • Then rd:

34. Example 12.3: Find gm & rd Q-Pt → (VDSQ, IDQ) = (10 V, 7.4 mA)Also VGSQ = 3.5 V ΔiD ΔvDS

35. All Done for Today Large ScaleResistanceChallenge

36. All Done for Today 3 & 4ConnectionnFET

37. Engineering 43 Appendix Bruce Mayer, PE Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

38. DC Srcs SHORTS in Small-Signal • In the small-signal equivalent circuit DC voltage-sources are represented by SHORT CIRUITS; since their voltage is CONSTANT, the exhibit ZERO INCREMENTAL, or SIGNAL, voltage • Alternative Statement: Since a DC Voltage source has an ac component of current, but NO ac VOLTAGE, the DC Voltage Source is equivalent to a SHORT circuit for ac signals