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Transcript
4/25/2011 The MOSFET Current Mirror 1/6 The MOSFET Current Mirror Consider the following MOSFET circuit: VDD Note VD = VG , therefore: ID VDS = VGS R and thus: VDS > VGS −Vt + So, if VGS > Vt , then the - MOSFET is in saturation! VGS We know that for a MOSFET in saturation, the drain current is equal to: ID = K (VGS −Vt ) 2 Say we want this current ID to be a specific value—call it Iref . Since Vs = 0 , we find that from the above equation, the drain voltage must be: VD = Iref +Vt K 4/25/2011 The MOSFET Current Mirror 2/6 Likewise, from KVL we find that: Iref = VDD −VD R And thus the resistor value to achieve the desired drain current Iref is: V −V R = DD D Iref where: VD = Iref +Vt K Q: Why are we doing this? A: Say we now add another component to the circuit, with a second MOSFET that is identical to the first : VDD VDD Iref IL R + - VGS1 + VGS2 - RL 4/25/2011 The MOSFET Current Mirror 3/6 Q: So what is current IL ? A: Note that the gate voltage of each MOSFET is the same (i.e., VGS 1 = VGS 2 ), and if the MOSFETS are the same (i.e., K1 = K2 , Vt 1 = Vt 2 ), and if the second MOSFET is likewise in saturation, its drain current I L is: I L = K2 (VGS 2 −Vt 2 ) 2 = K1 (VGS 1 −Vt 1 ) = Iref 2 Therefore, the drain current of the second MOSFET is equal to the current of the first! Iref = I L Q: Wait a minute! You mean to say that the current through the resistor RL is independent of the value of resistor RL ? A: Absolutely! As long as the second MOSFET is in saturation, the current through RL is equal to Iref—period. The current through RL is independent on the value of RL (provided that the MOSFET remains in saturation). Think about what this means—this device is a current source ! 4/25/2011 The MOSFET Current Mirror VDD VDD VDD R 4/6 RL Iref RL Iref Current Source Remember, the second MOSFET must be in saturation for the current through RL to be a constant value Iref . As a result, we find that: VDS 2 > VGS 2 −Vt 2 or for this example, since Vs = 0 : VD 2 > VG 2 −Vt 2 Since VD 2 = VDD − RLIref , we find that in order for the MOSFET to be in saturation: VDD − RLI ref > VG 2 −Vt 2 = VG 1 −Vt 1 Or, sated another way, we find that the load resistor RL can be no larger than: 4/25/2011 The MOSFET Current Mirror RL < 5/6 VDD −VG 1 +Vt 1 I ref Where we know that: VG 1 = VDD − R Iref and thus we can alternatively write the above equation as: RL < R + Vt 1 Iref If the load resistor becomes larger than R +Vt 1 Iref , the voltage VDS 2 will drop below the excess gate voltage VGS 2 −Vt 2 , and thus the second MOSFET will enter the triode region. As a result, the drain current will not equal Iref —the current source will stop working! Although the circuit presented here is sometimes referred to as a current sink, understand that the circuit is clearly a way of designing a current source. 4/25/2011 The MOSFET Current Mirror 6/6 We can also use PMOS devices to construct a current mirror! VSS + + VGS1 - - VSS VGS2 This better be in saturation! I L = Iref , Iref R RL IL regardless of the value of RL!!!