Lecture 16

Ming c wu 8 linear resistance ron 1 ncox w vgs

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Unformatted text preview: ?mes the gate voltage in excess of the threshold voltage. EE105, Fall 2013 Lecture 16 – Slide 13 Prof. Ming C. Wu Charge Density at a Point Q( x) = WCox [VGS − V ( x) − VTH ] §༊  Let x be a point along the channel from source to drain, and V(x) its poten?al; the expression above gives the charge density (per unit length). EE105, Fall 2013 Lecture 16 – Slide 14 Prof. Ming C. Wu 7 Parabolic ID- VDS Rela?onship ID = Q ⋅ v Q ( x ) = WCox #VGS − V ( x ) − VTH % $ & v = +µ n dV dx dV ( x ) I D = WCox #VGS − V ( x ) − VTH % µ n $ & dx 1 W# 2 I D = µ nCox $2(VGS − VTH )VDS − VDS % & 2 L EE105, Fall 2013 §༊  By keeping VG constant and varying VDS, we obtain a parabolic rela?onship. §༊  The maximum current occurs when VDS equals to VGS- VTH. Lecture 16 – Slide 15 Prof. Ming C. Wu ID- VDS for Different Values of VGS 2 I D ,max ∝ (VGS − VTH ) EE105, Fall 2013 Lecture 16 – Slide 16 Prof. Ming C. Wu 8 Linear Resistance Ron = 1 µ nCox W (VGS − VTH ) L §༊  At small VDS, the transistor can be viewed as a resistor, with the resistance depending on the gate voltage. §༊  It finds applica?on as an electronic switch. EE105, Fall 2013 Lecture 16 – Slide 17 Prof. Ming C. Wu Applica?on of Electronic Switches §༊  In a cordless telephone system in which a single antenna is used for both transmission and recep?on, a switch is used to connect either the receiver or transmiJer to the antenna. EE105, Fall 2013 Lecture 16 – Slide 18 Prof. Ming C. Wu 9 Effects of On- Resistance §༊  To minimize signal aJenua?on, Ron of the switch has to be as small as possible. This means larger W/L aspect ra?o and greater VGS...
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