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resistor [2012/07/01 18:07] mcmaster |
resistor [2013/10/20 10:59] (current) |
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| ====== Depletion load ====== | ====== Depletion load ====== | ||
| - | These are arranged with the transistor tied to be always on. A MOSFET is classified into 3 areas of operation: | + | FIXME: compare with analysis at [NMOS logic design], tried to do for enhancement mode and so analysis didn't work out |
| - | * NMOS | + | |
| - | * Cutoff (VGS < Vth): little conduction | + | |
| - | * Triode (VGS > Vth and VDS < (VGS - Vth)): resistive | + | |
| - | * Saturation (VGS > Vth and VDS > (VGS - Vth)): conductive | + | |
| - | * PMOS | + | |
| - | * Cutoff (VGS > Vth): little conduction | + | |
| - | * Triode (VGS < Vth and VDS > (VGS - Vth)): resistive | + | |
| - | * Saturation (VGS < Vth and VDS < (VGS - Vth)): conductive | + | |
| - | In essence a MOSFET is off if you don't apply any voltage between gate and source. Its fully on if you apply lots of voltage between drain and source and a little between the gate and source (ie switching a high voltage load). And then there are the cases in between which is what we want. | + | Most MOSFETs encountered are enhancement. In essence a its off if you don't apply any voltage between gate and source and fully on if you apply lots of voltage between drain and source and a little between the gate and source (ie switching a high voltage load). However, there are also depletion mode transistor which have essentially the opposite behavior: they are normally on with VGS = 0. |
| - | Pulldown PMOS resistor on Intel 4004 (IC images courtesy of Flylogic, mask from http://www.4004.com): | + | Depletion load PMOS resistor on Intel 4004 (IC images courtesy of Flylogic, mask from http://www.4004.com): |
| {{:flylogic:intel_400:pullup_metal_flylogic.png}} | {{:flylogic:intel_400:pullup_metal_flylogic.png}} | ||
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| {{:mcmaster:resistor:pmos_pulldown_sch.png}} | {{:mcmaster:resistor:pmos_pulldown_sch.png}} | ||
| - | (FIXME: compare with analysis at [NMOS logic design] The metal is VDD (-10V). For PMOS, cutoff occurs when VGS > Vth and assume Vth = -3V or something like that. If VGS is 0 the resistor does nothing as there is no potential difference. Now note that since VG = VD and so VGS = VDS. We are in the triode region if VDS > VGS - Vth and so if 0 > 0 - -3V or 0 > 3V) | + | So the natural question to ask is how do you tell a depletion from an enhancement load MOSFET? For a non-obfuscated digital design typically only depletion mode transistors have this configuration and are enhancement otherwise. Like this, once you get into analog all bets are off and you need to be careful. |