8.2.1 The Basic MOSFET Current SourceFigure 8.1 shows the circuit of a simple MOS constant-current source. The heart of the circuitis transistor Q1 , the drain of which is shorted to its gate, 1 thereby forcing it to operate in thesaturation mode withID1 = 12 k′n( WL)1(V GS − Vtn)2 (8.1)where we have neglected channel-length modulation. The drain current of Q1 is supplied byVDD through resistor R, which in most cases would be outside the IC chip. Since the gatecurrents are zero,I D1 = IREF = VDD − VGSR (8.2)where the current through R is considered to be the reference current of the current source andis denoted IREF . Equations (8.1) and (8.2) can be used to determine the value required for R.Now consider transistor Q2 : It has the same VGS as Q1 ; thus, if we assume that it is operatingin saturation, its drain current, which is the output current IO of the current source, will beIO = ID2 = 12 k′n( WL)2(V GS −Vtn)2 (8.3)where we have neglected channel-length modulation. Equations (8.1) and (8.3) enable us torelate the output current IO to the reference current IREF as follows:I OIREF= (W/L)2(W/L)1
Question
8.2.1 The Basic MOSFET Current SourceFigure 8.1 shows the circuit of a simple MOS constant-current source. The heart of the circuitis transistor Q1 , the drain of which is shorted to its gate, 1 thereby forcing it to operate in thesaturation mode withID1 = 12 k′n( WL)1(V GS − Vtn)2 (8.1)where we have neglected channel-length modulation. The drain current of Q1 is supplied byVDD through resistor R, which in most cases would be outside the IC chip. Since the gatecurrents are zero,I D1 = IREF = VDD − VGSR (8.2)where the current through R is considered to be the reference current of the current source andis denoted IREF . Equations (8.1) and (8.2) can be used to determine the value required for R.Now consider transistor Q2 : It has the same VGS as Q1 ; thus, if we assume that it is operatingin saturation, its drain current, which is the output current IO of the current source, will beIO = ID2 = 12 k′n( WL)2(V GS −Vtn)2 (8.3)where we have neglected channel-length modulation. Equations (8.1) and (8.3) enable us torelate the output current IO to the reference current IREF as follows:I OIREF= (W/L)2(W/L)1
Solution
The text provided is a detailed explanation of how a basic MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) current source works. The transistor Q1 is the main component of the circuit, and it operates in the saturation mode. The drain current of Q1 is supplied by VDD through resistor R. The current through R is considered to be the reference current of the current source and is denoted IREF.
The equations (8.1) and (8.2) are used to determine the value required for R. The transistor Q2 has the same VGS as Q1 and if it is operating in saturation, its drain current will be the output current IO of the current source.
The equations (8.1) and (8.3) are used to relate the output current IO to the reference current IREF. The ratio of IO to IREF is equal to the ratio of (W/L)2 to (W/L)1, where W is the width of the transistor and L is the length.
This explanation assumes that channel-length modulation is neglected.
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