Electronic Circuits 1 Ch-4

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포스텍_전자회로1_4과_25강C

MOS current source

Cascode current source

- voltage range

- output resistance

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Biasing of discrete MOS amp circuit

① Resistors are widely used

② AC(capacitor) coupling is used to connect input & output

signal to amplifier

In IC(integrated circuit) chip, resistor and capacitor take large chip area ⇒

difficult to apply the biasing circuit of discrete MOS amp to IC

2. Biasing of IC MOS amp circuit

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Biasing in IC MOS amp

Current source biasing in IC MOS amp:

MOS transistors are widely used⇒ takes less chip area than R & C

VGS generated by M0, R & VDD delivered to M1, M2, …, Mn

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VG determined by VDD, R, M0 (the same VGS for M0, M1, M2, M3, …, Mn)

M0, M1, M2, M3, …, Mn must work in saturation region( )THG VVV 1

⇒

⇒

Biasing in IC MOS amp

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Change of current(I1) with voltage(V1) in a current source

Small-signal output resistance when M1 works in saturation region

Biasing in IC MOS amp

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current mirror (電流거울)

current source (電流源)

active load (能動負荷)

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Current mirror: used as an active load current source

VGS : a DC voltage, determined by Iref and M1 only

① simple current mirror circuit

Current mirror

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VG1 , VG2 : DC voltages determined by VG1 and VG2

⇒ VG1 , VG2 : DC voltages

VDS1 = VGS4 + VGS3 - VGS2

(close to DC voltage)

② Cascode current mirror circuit

Cascode current source

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Cascode current source

Output voltage range

When VO changes with time, VDS1 remains constant ⇒ IO remains constant.

⇒ : small ⇒ small-signal output resistance: large

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Cascode current source

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Cascode current source

Ro including gmb

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A current mirror and a cascode current sourceNMOS current mirror and a cascode current source* a simple NMOS current mirrorIr1 dd b1 1mmn1d b1 b1 0 0 n4007 W=170u L=10u Ad=8.5n As=8.5n Pd=440u Ps=440umnce1 o1 b1 0 0 n4007 W=170u L=10u Ad=8.5n As=8.5n Pd=440u Ps=440uvo1 out o1 dc 0* an NMOS cascode current sourceIr4 dd b4 1mmn4d b4 b4 b3 0 n4007 W=170u L=10u Ad=8.5n As=8.5n Pd=440u Ps=440umn3d b3 b3 0 0 n4007 W=170u L=10u Ad=8.5n As=8.5n Pd=440u Ps=440umncb4 o4 b4 d3 0 n4007 W=170u L=10u Ad=8.5n As=8.5n Pd=440u Ps=440umnce3 d3 b3 0 0 n4007 W=170u L=10u Ad=8.5n As=8.5n Pd=440u Ps=440uvo4 out o4 dc 0vdd dd 0 dc 10vvout out 0 dc 0.dc vout 0 10 1m.print dc i(vo1) i(vo4) v(b1) v(b3) v(b4) v(d3) .model n4007 NMOS LEVEL=1* the following 5 parameters extracted from measured data(MHEF4007)+VTO=1.669 GAMMA=2.273 PHI=0.92 KP=51.9U LAMBDA=8.66E-3 +TOX=40N UO=591 * Tox & following capacitance parameters borrowed from RIT web-site+ CJ=6.8E-8 MJ=0.5 PB=0.95 CJSW=1.26E-10 MJSW=0.5 + CGSO=3.4E-10 CGDO=3.4E-10 CGBO=5.75E-10 .end

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A current mirror and a cascode current source

Vb4=8.70V, Vb3=Vb1=3.14V, @Vout=5V

VTH= 1.67V, 4.07V,

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A current mirror and a cascode current source

Vb4=8.70V, Vb3=Vb1=3.14V, @Vout=5V, VTH= 1.67V, 4.07V

Valid output voltage range :

Vout > Vb1 - VTH = 3.14 - 1.67 = 1.47V for a current mirror

Vout > Vb4 – VTH = 8.70 – 4.07 = 4.63 V for a cascode current source

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A current mirror and a cascode current source

Vb4=8.70V, Vb3=Vb1=Vd3=3.14V, @Vout=5V, VTH= 1.67V, 4.07V

Calculate gm, gmb & ro values VGS-VTH=1.47V, 8.70 - 3.14 - 4.07 = 1.49V

gm = 2 * ID / (VGS - VTH),

gmb = gm * GAMMA*0.5 /sqrt(PHI-VBS),

ro = 1/(LAMBDA * ID)

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A current mirror and a cascode current source

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A current mirror and a cascode current source

Small-signal output resistance:

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A current mirror and a cascode current source

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A current mirror and a cascode current source

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A current mirror and a cascode current source

Vout > 1.48V for a current mirror

Vout > 4.54V for a cascode current source

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A current mirror and a cascode current source

Rout = 119k Ohm for a current mirror

Rout = 27.3Mega Ohm for a cascode current source

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