Review of Basic Micromixer

The down-conversion mixer is a key building block in a receiver system. Its main function is to translate the incoming RF signal to an intermediate frequency for further processing. It dominates the system linearity and determines the performance requirements of its adjacent blocks. Among many proposed active mixers the Gilbert-cell mixer has been widely used because of it’s LO suppression at the IF output. However the circuit linearity is limited by MOSFET transistor linearity, which is the common source MOSFET transconductance [24]. The small-signal linearity of the input stage, and thus the third-order intercept point, can be greatly improved using several techniques, notably, source degeneration, the multi-tanh doublet and triplet.

However the 1-dB gain compression point still falls short of what may be required in handling large input signals without significant intermodulation. Further these RF stages do not provide an accurate match to the source [25]. Therefore the micromixer was proposed in [25] to overcome these problems. The topology of the basic micromixer is shown in Figure 4.1.1.

The micromixer follows the general form of Gilbert-cell mixer except for the use of a bisymmetric class-AB RF stage based on the translinear principles while the mixer core is identical to the Gilbert-cell mixer. The class-AB RF stage provides

Figure 4.1.1 Basic micromixer

Although the micromixer does not have inherent gain compression in RF stage, the 1-dB compression point of the micromixer will often be determined by limitations on the output IF signal amplitude, rather than by the RF stage. The noise figure of the micromixer depends on design details and is acceptable for many receiver applications although it is generally not as low as in mixers specially optimized for noise performance.

In Figure 4.1.1, Q1 can be viewed as a grounded-base stage. It delivers its output I1

to the mixer pair QM1-QM2 in phase. It can, in principle, handle unlimited amounts of current during large negative excursion of VGEN. On the other hand, the current mirror sub-cell Q2-Q3 can handle essentially unlimited amounts of current during positive excursion of VGEN both at its input node and at its inverted-phase current output I , which drives QM3-QM4. Acting together, these two sub-cells provide an

overall transfer characteristic which is symmetric to both positive and negative inputs, and which is in principle not limited by the choice of bias level. The differential current output I1-I3 is linear with IRF, although the individual currents are quite nonlinear. [25]

Because of the advantage of easily matching and wide dynamic range the micromixer is also applied to the CMOS process in recently years [26]. Replacing the BJT with MOSFET, we can derive two simple expressions for low-frequency small-signal input resistance and voltage gain under the assumption of ideal transistors and neglecting parasitic effects for simplifying [27]. The low frequency small-signal input resistance of RF input stage is approximately

( )

which implies the micromixer RF input stage can be matched to 50Ω as long as we choose proper bias current. Assume perfect impedance matching to 50Ω, the low frequency small-signal voltage gain is approximately

L

These two equations will be very helpful when designing the micromixer.

4.2 Low-Voltage Micromixer

In recent years low-voltage circuit design has become an important issue because of the consideration of battery design and power reduction. However the traditional micromixer is inapplicable for the low voltage design due to the stack of the four stage cascode architecture. Here we propose a modified micromixer applicable for

The main improvement of the low-voltage micromixer is the RF stage, while the switch-stage of the low-voltage micromixer is identical to the basic micromixer. The RF signal is feed in between R1 and M2, and coupled to the RF stage by CcRF1 and CcRF2. We bias the transistors M1 and M2 separately using Vg1 and Vg2. The improved RF stage overcomes the bias-relative problem and retains the characteristic of class

Figure 4.2.1 Low-voltage micromixer CcLOn

CcLOp

LLOn

LLOp Rbn

Rbp

CLOn2

CLOp2 CLOn1

CLOp1 LOn

LOp

VbLO VbLO

LOng

LOpg

Figure 4.2.2 LO matching network

AB stage in the basic micromixer. The pi-matching network is added at the LO port for the narrow band input matching to 50 ohms for measurement consideration.

Figure 4.2.2 shows the topology of the LO port on-chip pi-matching network composed of two MIM capacitors and one spiral inductor. The LO stage bias voltage is feed with bias resistors in the matching network. To keep the output IF waveform symmetric the two resistors R1 and R2 in the RF stage adjust the transconductance and current balance of M1 and M2.

In the low-voltage micromixer we adopt the charge injection method to improve the gain [28]. According the relationship of transconductance and IP3 with current in the traditional mixer architecture

2

which imply the mixer gain and IP3 are proportional to the bias current flowing in the input MOSFETs, I_SS . Because the micomixer has identical operational model as the Gilbert-cell mixer, the two equations are also applicable to the micromixer. The charge injection method can improve the micromixer gain and linearity, compensating the disadvantage of low supply voltage and low transconductance in CMOS process.

In Figure 4.2.1, M7 and M8 work as current sources, and provide extra charge current feeding into the RF stage. R7 and R8 provide high impedance to prevent the small signal from going to the current sources so that the charge injection stage will not interfere with the function of low-voltage micromixer.