Difference between revisions of "Chapter 5"
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(→Transconductance & Drain Resistance) 
(→Transconductance & Drain Resistance) 

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*<math>g_m=\frac{\partial i_D}{\partial v_{GS}}\bigg_{Qpoint}</math> 
*<math>g_m=\frac{\partial i_D}{\partial v_{GS}}\bigg_{Qpoint}</math> 

*<math>\frac{1}{r_d}=\frac{\partial i_D}{\partial v_{DS}}\bigg_{Qpoint}</math> 
*<math>\frac{1}{r_d}=\frac{\partial i_D}{\partial v_{DS}}\bigg_{Qpoint}</math> 

−  :*Looking at the FET smallsignal equivalent circuit, we can write <math>i_d=g_m v_{gs}+\frac{v_{ds}}{r_d}</math>, thus <math>g_m=\frac{i_d}{v_{gs}} \bigg_{v_{ds}=0}</math>. Since these are small changes from the Qpoint, we can write <math>g_m=\frac{\partial i_D}{\partial v_{GS}}\bigg_{Qpoint}</math>. Similarly, we can write <math>\frac{1}{r_d}=\frac{\partial i_D}{\partial v_{DS}}\bigg_{Qpoint} 
+  :*Looking at the FET smallsignal equivalent circuit, we can write <math>i_d=g_m v_{gs}+\frac{v_{ds}}{r_d}</math>, thus <math>g_m=\frac{i_d}{v_{gs}} \bigg_{v_{ds}=0}</math>. Since these are small changes from the Qpoint, we can write <math>g_m=\frac{\partial i_D}{\partial v_{GS}}\bigg_{Qpoint}</math>. Similarly, we can write <math>\frac{1}{r_d}=\frac{\partial i_D}{\partial v_{DS}}\bigg_{Qpoint}</math> 
===Smallsignal analysis=== 
===Smallsignal analysis=== 
Revision as of 15:10, 21 March 2010
Contents
Metaloxide semiconductor field effect transistor (MOSFET)
"The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals (For ease of discussion, this assumes body and source are connected). This conductive channel is the "stream" through which electrons flow from source to drain."<ref>Wikipedia: Fieldeffect transistor http://en.wikipedia.org/wiki/Fieldeffect_transistor</ref>
 Enhancement: The electric field from the gate voltage forms an induced channel allowing current to flow.
 Depletion: The channel is physically implanted rather than induced.
 JFET: Charge flows through a semiconducting channel (between the source and drain). Applying a bias voltage to the gate terminal impedes the current flow (or pinches it off completely).
Threshold Voltage
 The threshold voltage, , is the minimum needed to move the transistor from the Cutoff to Triode region.
 is usually on the order of a couple of volts


Modes of operation
 Cutoff
 The channel has not been created (Enhancement) or is pinched off (Depletion & JFET). No current flows.
 Triode:
 When is reached, a channel forms beneath the gate (Enhancement) or is no longer pinched off (Depletion & JFET), allowing current to flow.
 As increases, the voltage between the gate and channel becomes smaller as you progress towards the drain. This results in the channel tapering off as you get closer to the drain.
 " Because of the tapering of the channel, its resistance becomes larger with increasing , resuling in a lower rate of increase of ." <ref>Electronics p. 291</ref>
 Why doesn't it just cut the current off completely when v_DS gets high enough? If it is pinched off, how does the current flow still?
 Saturation:
 When is reached, the channel thickness at the drain end becomes zero (Enhancement, Depletion & JFET).
Device equations




 Device Parameters:
 Surface Mobility: , the electrons in the channel
 Capacitance of the gate per unit area:
Transconductance & Drain Resistance
 "Transconductance, , is an important parameter in the design of amplifier circuits. In general, better performance is obtained with higher values of ." It is obtained at the cost of chip area.<ref>Electroincs p. 310</ref>
 Looking at the FET smallsignal equivalent circuit, we can write , thus . Since these are small changes from the Qpoint, we can write . Similarly, we can write
Smallsignal analysis
 Analyze the DC circuit to find the Qpoint (using nonlinear device equations or characteristic curves)
 Use the smallsignal equivalent circuit to find the impedance and gains
Type  Voltage Gain  Current Gain  Power Gain  Input Impedance  Output Impedance  Frequency Response 

CommonSource  
CommonDrain Source Follower 
High  Low  
CommonGate 
MOSFET vs JFET vs BJT
Transistor  Pros  Cons 

MOSFET  *Draws no gate current *Infinite input resistance *Voltagecontrolled current source 
Gate protection needed to prevent static electricity from breaking down the insulation 
JFET  
BJT  Currentcontrolled current source 
Questions
 How do you find r_{d}?
 Roughly what are the breakdown voltages for JFETs?
 CMOS nand/nor gates
 JFET only goes to I_{DSS}?
References
<references/>