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Solid-State Devices & Circuits
ECE 342 Solid-State Devices & Circuits 10. MOS Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois
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Biasing of MOS Transistors
Bias Characteristics Operation in saturation region Stable and predictable drain current
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Two-Supply MOS Bias RG provides DC ground at gate and high input resistance to signal source.
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Single-Supply MOS Bias
Choose R1 and R2 to fix VG Choose RS and R2 to fix VS VGS determines ID Choose RD to fix VD
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Bias with Feedback Resistor
Negative feedback (degeneration) provided by RG
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Common Source MOSFET Amplifier
Bias is to keep MOS in saturation region
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Common Source MOSFET Amplifier
Small-Signal Equivalent Circuit for MOS (device only) Which leads to
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MOSFET Output Impedance
To calculate rds, account for l rds, accounts for channel width modulation resistance.
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Midband Frequency Gain
Incremental model for complete amplifier
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Example In the circuit shown, VT=1 V, l=0, mCoxW/2L=0.1 mA/V2. Select RD and R1 to result in midband voltage gain of –4 and VDSQ=7 V.
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Example (Cont’)
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Example For the circuit shown, k=75 mA/V2, VT=1 V, l=0 Find VDQ, VSQ
Find the midband gain
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Example (Cont’) reject since voltage drop across RD will be too large
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Example (Cont’)
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MOS Body Effect The threshold voltage VT The body effect
Depends on equilibrium potential Controlled by inversion in channel The body effect VT varies with bias between source and body Leads to modulation of VT
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Body Effect Fermi potential of material Body bias coefficient
Potential on substrate affects threshold voltage Fermi potential of material Body bias coefficient
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Body Effect – (Con’t) Define gmb as the body transconductance
Can show that
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Source Follower Configuration
Since source is not tied to the substrate, we need to model the body effect. Note: substrate is always tied to ground.
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Source Follower
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Source Follower
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Source Follower Neglecting GL and gds (since they are small)
This value is close to 1 Output impedance of source follower Internal output impedance
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Common Gate Amplifier Circuit Small-Signal Model
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Common Gate Amplifier Common Gate (CG) CG amplifier is non-inverting
CG amplifier has low input impedance CG is unity current-gain amplifier
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MOS Topologies - Ideal CS CG SF 1 Avo Rin Rout
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