Design and Simulation of a Low Power Amplifier for CMOS MEMS Sensors Using Wide Swing Cascode Current Mirror

We present a low power amplifier integrated with a wide swing cascode current mirror (WSCCM) based on PMOS transistors. To derive the low power amplifier a current of 120 uA is required, which is achieved by varying the widths of the transistors. The best result is achieved by varying the widths up to 6um with an output saturated current response of 120.04 uA. SECTPRE of CADENCE design tool is used as the design software to carry out the simulations. The proposed circuits are designed and verified in MIMOS 0.35 um CMOS and BSIM3 transistor model technology. The low power amplifier achieves an output response of 44.71 mV with a minimum power consumption of 0.485 mW.


Introduction
MEMS integration with CMOS is the next evolutionary step in micro-technology (Gusev et al., 2010).The integration of both processes CMOS and MEMS is achieved by two techniques, hybrid and monolithic.The hybrid technique involves bonding of two wafers using bonding wires.The hybrid technique allows independent optimization of both the MEMS and CMOS components but the technique is not a cost effective solution in terms of packaging and assembly.The monolithic CMOS MEMS technique on the other hand increases the reliability of the overall system as it eliminate the number of off chip connections and bonding wires thus reducing parasitic effects.
The major disadvantages of monolithic CMOS MEMS integration include fabrication limitation and noise.Many MEMS sensors these days are based on capacitive sensing technique, scaling down is another challenge for these MEMS sensors when monolithically integrated on the same die.Sensors cannot be smaller than its sensitive components.Due to MEMS requirement of surface micromachining, all sensing capacitive components of sensors are located at the surface.The physical limitation of the sensors reduces the capacitance values in range of femto Farads, which bring up another challenge for the designers to design a highly sensitive capacitance detection circuitry as readout for these sensors (John & David, 2003).
In this paper we present a low power amplifier integrated with a PMOS based wide swing cascode current mirror as a sense detection circuitry for CMOS MEMS monolithic sensors.The capacitance range of CMOS MEMS sensors are generally in femto farad giving output sense signal up to few milli Volts.With a 6 mVpp test signal, the low power amplifier achieves a stable output with minimum power consumption through width adjustments of the MOS devices.
The architecture of current mirror is employed in cascode configuration to achieve characteristics such as current matching and high impedance at output and input stages.The useful aspect of the architecture is that it increases the resistance of the overall circuit by a factor of g m r o where g m is the transconductance and r o is the output resistance of the MOSFET (Bhawna, Maneesha, & Gupta, 2013;Allen & Holberg, 2004).These high performance characteristics of cascode current mirror achieves high priority over simple current mirrors as a choice for complex circuits such as variable gain low noise amplifiers etc (Yang, Kim, Yun, Y. Lee, & J. Lee, 2008;Eldbib & Musil, 2008;Laajimi & Masmoudi, 2012).Higher input output compliance voltages of the simple cascode current mirror provides a drawback for its selection in complex circuits (Toth & Meijer, 1992).The topology given in Figure 1(a) is an alternate topology for high swing cascode as it provides better performance by maintaining the same output resistance when compared to a standard cascode topology.Figure 1(a) topology also provides lower compliance voltage when compared with standard cascode structure.
The architecture of a high swing current mirror is based on a biasing circuit and a cascode current source (Shukla, Devashrayee, & Naik, 2011).Biasing is achieved by the two terminals in the current source, the rest of circuit employs 4 transistors Q1, Q2, Q3 and Q4.The fifth transistor Q5 ensures all the other transistors remain in saturation region for better performance of the circuit (Aziz, Mamun, Bhuiyan, & Bakar, 2013).WSCCM output impedance is kept high for high output swing.The small signal analysis behaviour of WSCCM is quite similar to the basic cascode, whereas biasing technique is achieved quite differently.The total drain to source voltage of Q2 and Q4 defines the biasing voltage which is usually kept at minimum value (Nojdelov & Nihtianov, 2009).Bias voltage and drain to source voltage of Q2 and Q4 contributes to the total output voltage.The widths and length ratio of the MOSFETs, always keep the transistor Q3 in on state and at the edge of saturation.The transistor Q5 added in series ensures the drain voltages of the other transistors to be identical and also remove any error caused by channel length modulation factor (Rosli, Mamun, Bhuiyan, & Husain, 2012).The simulations show that with increasing transistor widths, WSCCM achieves better performance.Figure 2 (b) shows the most stable current output achieving 120.04 uA while achieving a quicker response time in the saturation region when compared to other outputs.The results for 6 different transistor widths are summarized in Figure 3.The transistor widths for 6 transistors used in the proposed WSCCM are given in Table 1.

WSCCM Integration with Low Power Amplifier
Low power cascode amplifiers are used for differential capacitive sensing in many MEMS sensors and require stable biasing current sources.A block diagram representing the overall system is shown in Figure 4.The proposed PMOS based WSCCM is used as a stable and continuous current source with low power amplifier as shown in Figure 5.A stable 126.16 uA current input is achieved for the low power amplifier resulting in a 44.71 mV output with 6 mVpp input with a gain of 17dB.The mechanical comb fingers designed in the MEMS sensors provide a fully differential capacitive sensing scheme as an input to the proposed amplifier.The differential scheme employs two input signals as a result of change in capacitances between the fingers.The two input sensed signals are computed by Equation 1 (Wu, 2002).

Figure 1 .
Figure 1.Typical wide swing cascode current mirror vs proposed PMOS based WSCCM

Figure
Figure

Figure 5 .
Figure 5. Integration of the proposed WSCCM with low power amplifier Figure 9.