ECE 607(Edgar Sanchez-Sinencio) Low Voltage Power Supply Current Source A M S C Simple implementation of a current source in many applications including a tail current yields a low output impedance. Cascode implementations of current sources yields a larger output impedance, however the trade-off is the reduction on the headroom. One alternative implementation of a current source with higher output impedance without sacrificing significant overhead is presented next. TAMU-AMSC
The current source discussed here has an output resistance about 25 times larger than that of a single transistor current source. This current source improves the common-mode input range and the CMRR of differential pairs. Another use of this LV CS is for the voltage follower topologies.
Low Voltage CURRENT-SOURCE This current source is applied at the PMOS differential pair ( i and i) at node Y V dd M 2 Z M 1 V x X A Y I R 0S Vi+ i I R 0S (a) I B R 0B i Vi- (b) Analog and Mixed-Signal Center (AMSC) TAMU A conceptual Schematic of the low voltage current source. (a) Current source representation (b) Architecture You, F., Embabi, H.K., Duque-Carrillo, J.F., Sanchez-Sinencio, E., An improved tail current source for low voltage applications, IEEE Journal of Solid-State Circuits, Volume: 32, Issue: 8, Aug 1997, Page(s): 1173 1180
Ros 1 gm1a o /(go1 gob) go2(1 Aogm1 /(go1 g ) Aogm2 / go2) ob (1) where gm1 g m2, g o1 g o2 are the transconductance and output conductance of 1 M 2 respectively. A is the DC gain of the error amplifier A and g is the output o conductance (resistance) of the reference current source go1 g o 2, equation (1) can be simplified as: R ob R ob I B. Assuming that 1 os R ob gm g m 2 M, and (2) Note that the resistance is negative and is equal to the resistance of the reference source I B. R os g g o3 m4 g o4 (3) Analog and Mixed-Signal Center (AMSC) TAMU
V dd M 2 Z M 1 X Cc I o R 0S Y M 4 V b IB ID M 3 V Current Ref. V ss Error Amplifier A Full implementation of the LV current source. Analog and Mixed-Signal Center (AMSC) TAMU
FREQUENCY RESPONSE OF THE TAIL CURRENT OUTPUT IMPEDANCE (a) WITHOUT DIFFERENTIAL PAIR AND (b) WITH DIFFERENTIAL PAIR
This LV Current Sources has one negative feedback loop and one positive feedback loop. The negative feedback ( consisting of the error amplifier A and transistor )must be greater than the positive( which consists of A and transistor ) to have stability. The analysis is based on the small signal equivalent circuit shown in the next slide. Cx and Cy are the parasitic capacitances at nodes X and Y. gob is the conductance of the current source IB The error amplifier is characterized by one dominant pole at CZ/goa
Open loop transfer function with Cc
The frequency response is simulated for two cases; i) With a simple transistor tail current and ii) using the proposed LV current source. The effect of Rc is shown in curve ( c ) Fig 5 (a) does not have Cc, thus its poor phase margin. Adding Rc to CC in series yields Fig 5( c )
V dd I V b V i1 Mi1 M i 2 V i2 C c M6 V out M3 M4 V ss M5 Use of the proposed current source in simple two stage amplifier. Cc has a resistor in series Rc to provide better stability. Analog and Mixed-Signal Center (AMSC) TAMU
Frequency response of two stage amplifiers using LV current source (dashed) and simple current source (solid). Analog and Mixed-Signal Center (AMSC) TAMU
Measured output current of the simple (curve A) and LV (Curve B) current source. Analog and Mixed-Signal Center (AMSC) TAMU
Measured I-V characteristics of the LV current source. Analog and Mixed-Signal Center (AMSC) TAMU
LOW VOLTAGE CURRENT SOURCES V i+ M 1i I B2 X I Bref V Z A R os r Bref M 2i Y I B1 V i- Amplifier A forces the V DS of and to be equal. Thus I B1 = I B2. R os R Bref MAX MAX VREF Vy VDD VSG,AUX VSD,AUX Large variations of V Z are caused when and enter in triode mode. Using an auxiliary differential amplifier LV Current Source MA4 MA3 V Z Vy Vx A v z V X MA1 MA2 A s A 1 sc o Z g Oa I B 14
An Alternative Approach for a LV Current Source* The auxiliary amplifier is implemented by NMOS cascoding transistor MCP connected between node Z and node y. Thus v Y MCP Vc p V Z A V y V V Z Y V CP g m V MCP SG r o MCP V BC MBC Then V V MAX y icm V DD V DS can be extended SAT to the upper rail V MAX icm V DD * J. Ramirez-Angulo, R. G. Carvajal and A. J. Lopez-Martin, Single Transistor High Impedance Tail Current Source with Extended Common-Mode Input Range and Reduced Supply Requirements, IEEE Trans. on Circuits and Systems II, Vol. 54, No. 7, pp. 581-585, July 2007.
V BC I BC M CB V Z To reduce mismatch betweeen V DS1 and V DS2 a diode connected NMOS MDP is inserted. M DP M CP V y I BREF + I BC - M 1i M BP V v BP in i + v in Implementation of an enhanced LV current source where amplifier is implemented by a simple cascoding (two transistor MCB and MBP) W L 1 i,2i W L W L 1, 2 2 W L W L 4 W L MPB
Potential LV Current-Mirrors Goals: To reduce the input impedance and to increase the output impedance, while keeping the voltage operation Iin Iin Vref Vcas Mc Mm Aact Iin Vref Mc Iin Vref Mc Mm Vmirror Mm 17
Iin I B I B Vcas Iin I B I B M5 M4 Iin X M4 M3 M3 M3 Iin V shift Iin M3 Vdd I B1 M4 R OB Iin I B2 A FB 18