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FGMOS CURRENT MIRROR :BW
ENHANCEMENT
JayPee Institute of Information Technology, Noida.
BY: PROJECT TO: GOPI KRISHNA DR. MANISH KUMAR [14317188] MR.SHAMIM AKHTAR M.TECH (MET)
INTRODUCTION FGMOS
To presents a high performance, resistively compensated low voltage current mirror using floating gate MOSFETs (FGMOS).
Current mirrors (CMs) have been used as basic circuit element for the design of various low voltage circuit structures .
Floating gate MOS is similar to conventional MOSFET. The gate of FGMOS is electrically isolated , and a no. of secondary gates or inputs are deposited over the floating gate and are electrically isolated from it.
CONTD..
FG is completely surrounded by highly resistive material, the charge contained in it remains unchanged for long period of time.
Usually Hot-Carrier injection scheme is applied to modify the charge stored in the FG.
INTRODUCTION CURRENT MIRROR
Current mirror is a circuit designed to copy current through one active device and replicating it to another active device , keeping the output current constant regardless of loading.
Important feature of current
mirror is its high output resistance.
It provides bias current and
active loads in amplifier stages.
WORKING (a)
In FGMOS it is possible to programme the threshold voltage of FGMOS. For two input FGMOS with Vs=Vb=0 and C1,C2 >> Cd. Then VFG isVFG= πΆ1πΆπ‘ππ‘ππ π1+ πΆ2πΆπ‘ππ‘ππ π2+ πΆπ·πΆπ‘ππ‘ππ ππ· [1] Now the drain current (ID) in saturation is πΌπ· = π½2 (ππΉπΊ β ππ)2 = π½2[( πΆ1πΆπ‘ππ‘ππ π1+ πΆ2πΆπ‘ππ‘ππ π2)- ππ]2 [2] = π½2 π12 [π1 β (ππβπ2π2π1 )]2 = π½2 π12[π1 β ππ(ππππππ‘ππ£π)]2 [3]
CONTD.. Thus ππ(effective) is decided by π2,π1and π2and
hence it is possible to programme ππ according to application.
In simple FGMOS based current mirror one of the gates is used to programme the threshold voltage.
Bias voltage (Vb) decreases the threshold voltage and so on current voltage characteristics is also changed.
Circuit has low compliance voltage at low current levels.
FGMOS CM CIRCUIT
MODIFIED CM
Basic structure of FGMOS cuurent mirror is modified by adding a resistance (R) in series with capacitance (C2) .
The output short circuit transfer function of the modified current mirror is given by
π»αΊπ α»= ππ2(π + 1π πΆ2)3πΆ[π 2 + π αππ1πΆ2 +α
3πΆ+ πΆ2π α3πΆπΆ2 α+ ππ13π πΆπΆ2]
MODIFIED CM CIRCUIT
CONTD..
Transfer function has a zero at (-1/RC2) and complex poles (P1,2) at
π1,2 = βαΊππ1π πΆ2+3πΆ+πΆ2α»Β±ΰΆ₯αΊππ1π πΆ2+3πΆ+πΆ2α»Β²β12ππ1π πΆπΆ26π πΆπΆ2
Location of poles depends on the value of resistance (R).
If resistance (R) is very high then transfer function becomes
π»αΊπ α»= 1( 3πΆππ π +1)
CONTD..
If gm1 = gm2 = gm then one of the pole is cancel out by zero , because zero approaches towards one of poles.
Pole of transfer function is at S= -(gm/3C) . If the input current increases then the value of gm also
increases , resulting pole at higher frequency. As a result of this frequency response of the system is improved.
Capacitance (C) is gate to source capacitance which is process dependent and its value is low when transistor dimensions are low.
SENSITIVITY
Sensitivity of output current for the change in resistance R is given as
When R is very high then
Thus, as R increases sensitivity decreases.
ππ ππ = 1π απ + 1π πΆ2α[πΆ2 + 4πΆ+ πΆ1π π πΆ2 + (4πΆ+ πΆ1)]
ππ ππ = 1π π (4πΆ+ πΆ1)
REFRENCES:-
(1) S. Sharma, L.K. Mangotra , S.S. Rajput and S. S. Jamuar, βFGMOS Current mirror: Bandwidth enhancementβ SPRINGER, Analog Integrated Circuits and Signal Processing, 46, 281β286, 2006.
(2) S.S. Rajput and S.S. Jamuar, βDesign techniques for low voltage analog circuit Structuresβ in Proc. NSM 2001/IEEE, Malaysia, Nov. 2001.
(3) S. S. Rajput and S.S. Jamuar, βLow voltage analog circuit design techniques.β IEEE Circuits and Systems Magazine, vol. 2, no. 1, pp. 24β42, 2002.
(4) Design of Analog CMOS Integrated Circuits, Behzad Razavi, Tata McGraw-Hill 2002 .
THANK YOU