ECE 342 – Jose Schutt-Aine 1 ECE 342 Solid-State Devices & Circuits 16. Active Loads Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois

ECE 342 – Jose Schutt-Aine 2 Ideal MOS Common Source CKT

ECE 342 – Jose Schutt-Aine 3 PMOS Implementation of Active Load

ECE 342 – Jose Schutt-Aine 4 Example Assume V DD =3 V, V tn = |V tp | = 0.6 V,  n C ox = 200  A/V 2,  p C ox =65  A/V 2, L = 0.4  m, W = 4  m, V An = 20 V, |V AP | = 10 V, I REF = 100  A. Find small-signal gain.

ECE 342 – Jose Schutt-Aine 5 IC BJT CE

ECE 342 – Jose Schutt-Aine 6 C s1 & C s2 are the collector-to- substrate capacitances of Q 1 and Q 2 respectively IC BJT Common Emitter

ECE 342 – Jose Schutt-Aine 7 IC Common Emitter – High Frequency Model High Frequency Calculations 1.Upper corner frequencies more difficult to evaluate than for discrete amp 2.Miller effect will be larger  corner frequency lower

ECE 342 – Jose Schutt-Aine 8 Where C M1 is the Miller capacitance associated with C  1 IC-CE: High-Frequency Analysis The total input capacitance in parallel with r  1 is The input and output corner frequencies are

ECE 342 – Jose Schutt-Aine 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:  =160, r x1 = 10 , r ce1 = 68 k , C  1 = 20 pF, and C  1 = 2.1 pF. For device Q2, the parameters are r ce2 = 21 k  and C  2 = 3.1 pF. Each device has a value of C cs1 = C cs2 = 2.5 pF. In this circuit, the power supply is 10 V and R 1 = 10 k . Find the midband gain and the upper corner frequency. Evaluate r  1 and R out

ECE 342 – Jose Schutt-Aine 10 The midband gain is: The corner frequency of the input circuit is Example (cont’)

ECE 342 – Jose Schutt-Aine 11 Example (cont’) The corner frequency of the output circuit is For overall corner frequency, use SPICE

ECE 342 – Jose Schutt-Aine 12 Source Follower

ECE 342 – Jose Schutt-Aine 14 Source Follower – Output Resistance

ECE 342 – Jose Schutt-Aine 15 Source Follower with Active Load Characteristics –Provides a buffer stage –M1 is amplifying stage –M2 is active load

ECE 342 – Jose Schutt-Aine 16 Source Follower – Incremental Model

ECE 342 – Jose Schutt-Aine 17 (current mirror) Emitter Follower with Active Load Emitter follower can be used to drive a low-impedance load

ECE 342 – Jose Schutt-Aine 18 Midband gain: Emitter Follower with Active Load

ECE 342 – Jose Schutt-Aine 19 Emitter Follower with Active Load AC Properties 1.Gain is less than 1 and near 1 for typical element values 2.Frequency response has one zero and two poles 3.Exact frequency response is difficult  Use SPICE 4.Output stage of NPN current mirror serves as high impedance load at emitter of Q1

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

ECE 342 – Jose Schutt-Aine 21 IC - Common Gate Amplifier

ECE 342 – Jose Schutt-Aine 22 IC - Common Gate Amplifier

ECE 342 – Jose Schutt-Aine 23 Taking r o into account adds a component ( R L /A o ) to the input resistance. The open-circuit voltage gain is: The voltage gain of the loaded CG amplifier is: IC - Common Gate Amplifier

ECE 342 – Jose Schutt-Aine 24 CG Output Resistance

CB Amplifier

ECE 342 – Jose Schutt-Aine 26 Common source amplifier, followed by common gate stage – G 2 is an incremental ground MOS Cascode Amplifier

ECE 342 – Jose Schutt-Aine 27 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 MOS Cascode Amplifier

ECE 342 – Jose Schutt-Aine 28 MOS Cascode Incremental Model

ECE 342 – Jose Schutt-Aine 29 KCL at v s2 MOS Cascode Analysis

ECE 342 – Jose Schutt-Aine 30  Two cases MOS Cascode Analysis

ECE 342 – Jose Schutt-Aine 31 CASE 1 The voltage gain becomes MOS Cascode Analysis

ECE 342 – Jose Schutt-Aine 32 CASE 2 The voltage gain becomes MOS Cascode Analysis

ECE 342 – Jose Schutt-Aine 33 Cascode Example

ECE 342 – Jose Schutt-Aine 34 Cascode Example The cascode circuit has a dc drain current of 50  A for all transistors supplied by current mirror M3. Parameters are g m1 =181  A/V, g m2 =195  A/V, g ds1 = 5.87  A/V, g mb2 =57.1  A/V, g ds2 =  A, g ds3 = 3.76  A/V, C db2 = 9.8 fF, C gd2 = 1.5 fF, C db3 =40.9 fF, C gd3 = 4.5 fF. Find midband gain and approximate upper corner frequency Therefore, we use Case 2 to compute the gain The internal conductance of the current source is:

ECE 342 – Jose Schutt-Aine 35 Cascode Example Gain can be approximated by Upper corner frequency is approximated by

ECE 342 – Jose Schutt-Aine 36 Common emitter amplifier, followed by common base stage – Base of Q 2 is an incremental ground BJT Cascode Amplifier

ECE 342 – Jose Schutt-Aine 37 BJT Cascode Incremental Model

ECE 342 – Jose Schutt-Aine 38 BJT Cascode Analysis Ignoring r x2

ECE 342 – Jose Schutt-Aine 39 BJT Cascode Analysis

ECE 342 – Jose Schutt-Aine 40 BJT Cascode Analysis If R s << r x1 +r  1, the voltage gain can be approximated by