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ECE 342 Solid-State Devices & Circuits 16. Active Loads

ECE 342 Solid-State Devices & Circuits 16. Active Loads. Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu. Ideal MOS Common Source CKT. PMOS Implementation of Active Load. Example.

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ECE 342 Solid-State Devices & Circuits 16. Active Loads

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  1. ECE 342 Solid-State Devices & Circuits 16. Active Loads Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu

  2. Ideal MOS Common Source CKT

  3. PMOS Implementation of Active Load

  4. Example Assume VDD=3 V, Vtn = |Vtp| = 0.6 V, mnCox = 200 mA/V2, mpCox=65 mA/V2, L = 0.4 mm, W = 4 mm, VAn = 20 V, |VAP| = 10 V, IREF= 100 mA. Find small-signal gain.

  5. IC BJT CE

  6. IC BJT Common Emitter Cs1 & Cs2are the collector-to-substrate capacitances of Q1 and Q2 respectively

  7. IC Common Emitter – High Frequency Model • High Frequency Calculations • Upper corner frequencies more difficult to evaluate than for discrete amp • Miller effect will be larger corner frequency lower

  8. IC-CE: High-Frequency Analysis The total input capacitance in parallel with rp1 is Where CM1is the Miller capacitance associated with Cm1 The input and output corner frequencies are

  9. Example The CE circuit (see next page) is biased so that the collector currents of Q1 and Q2 are 1.14 mA. The parameters for Q1 are: b=160, rx1= 10 W, rce1= 68 kW, Cp1 = 20 pF, and Cm1= 2.1 pF. For device Q2, the parameters are rce2= 21 kW and Cm2= 3.1 pF. Each device has a value of Ccs1 = Ccs2 = 2.5 pF. In this circuit, the power supply is 10 V and R1 = 10 kW. Find the midband gain and the upper corner frequency. Evaluate rp1and Rout

  10. Example (cont’) The midband gain is: The corner frequency of the input circuit is

  11. Example (cont’) The corner frequency of the output circuit is For overall corner frequency, use SPICE

  12. Source Follower

  13. Source Follower

  14. Source Follower – Output Resistance

  15. Source Follower with Active Load • Characteristics • Provides a buffer stage • M1 is amplifying stage • M2 is active load

  16. Source Follower – Incremental Model

  17. Emitter Follower with Active Load (current mirror) Emitter follower can be used to drive a low-impedance load

  18. Emitter Follower with Active Load Midband gain:

  19. Emitter Follower with Active Load • AC Properties • Gain is less than 1 and near 1 for typical element values • Frequency response has one zero and two poles • Exact frequency response is difficult  Use SPICE • Output stage of NPN current mirror serves as high impedance load at emitter of Q1

  20. IC - Common Gate Amplifier Substrate is not connected to the source must account for body effect Drain signal current becomes And since Body effect is fully accounted for by using

  21. IC - Common Gate Amplifier

  22. IC - Common Gate Amplifier

  23. IC - Common Gate Amplifier Taking ro into account adds a component (RL/Ao) to the input resistance. The open-circuit voltage gain is: The voltage gain of the loaded CG amplifier is:

  24. CG Output Resistance

  25. CB Amplifier

  26. MOS Cascode Amplifier Common source amplifier, followed by common gate stage – G2 is an incremental ground

  27. MOS Cascode Amplifier • CS cacaded with CG  Cascode • Very popular configuration • Often considered as a single stage amplifier • Combine high input impedance and large transconductance in CS with current buffering and superior high frequency response of CG • Can be used to achieve equal gain but wider bandwidth than CS • Can be used to achieve higher gain with same GBW as CS

  28. MOS Cascode Incremental Model

  29. MOS Cascode Analysis KCL at vs2

  30. MOS Cascode Analysis Two cases

  31. MOS Cascode Analysis CASE 1 The voltage gain becomes

  32. MOS Cascode Analysis CASE 2 The voltage gain becomes

  33. Cascode Example

  34. Cascode Example The cascode circuit has a dc drain current of 50 mA for all transistors supplied by current mirror M3. Parameters are gm1=181 mA/V, gm2=195 mA/V, gds1= 5.87 mA/V, gmb2=57.1 mA/V, gds2= 0.939 mA, gds3= 3.76 mA/V, Cdb2 = 9.8 fF, Cgd2 = 1.5 fF, Cdb3=40.9 fF, Cgd3= 4.5 fF. Find midband gain and approximate upper corner frequency The internal conductance of the current source is: Therefore, we use Case 2 to compute the gain

  35. Cascode Example Gain can be approximated by Upper corner frequency is approximated by

  36. BJT Cascode Amplifier Common emitter amplifier, followed by common base stage – Base of Q2 is an incremental ground

  37. BJT Cascode Incremental Model

  38. BJT Cascode Analysis Ignoring rx2

  39. BJT Cascode Analysis

  40. BJT Cascode Analysis If Rs << rx1+rp1, the voltage gain can be approximated by

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