ECE 342 Solid-State Devices & Circuits 16. Active Loads. Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected] Ideal MOS Common Source CKT. PMOS Implementation of Active Load. Example.
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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.
Cs1 & Cs2are the collector-to-substrate capacitances of Q1 and Q2 respectively
The total input capacitance in parallel with rp1 is
Where CM1is the Miller capacitance associated with Cm1
The input and output corner frequencies are
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
The midband gain is:
The corner frequency of the input circuit is
The corner frequency of the output circuit is
For overall corner frequency, use SPICE
Emitter follower can be used to drive a low-impedance load
Substrate is not connected to the source must account for body effect
Drain signal current becomes
Body effect is fully accounted for by using
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:
Common source amplifier, followed by common gate stage – G2 is an incremental ground
KCL at vs2
The voltage gain becomes
The voltage gain becomes
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
Gain can be approximated by
Upper corner frequency is approximated by
Common emitter amplifier, followed by common base stage – Base of Q2 is an incremental ground
If Rs << rx1+rp1, the voltage gain can be approximated by