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A voltage sampled on the gate of an MOS capacitor rises when the channel charge is withdrawn [1]. This little-used effect is the basis of a parametric amplifier that draws no dc current and is ideally noiseless. This paper reports detailed measurements of this phenomenon and, based on it, demonstrates that micropower, low-gain, low-noise, large-signal amplification is possible.The MOS capacitor is implemented using a MOSFET with drain and source shorted together. The drain and source thus function as a single terminal and their combination will be referred to as "source" (S) hereafter. This configuration is a three-terminal MOS varactor. The basic operation of the parametric amplifier is explained in Fig. 23.1.1 using an NMOS for simplicity. At time t 1 , as shown in Fig. 23.1.1a, S of the MOS capacitor is connected to its bulk. A switch connects an input voltage V IN to the gate with V IN large enough to strongly invert the channel. The sum of the inversion charge (Q I ) and the depletion charge (Q B ) equal the negative of the gate charge (Q G ) for charge neutrality, assuming negligible interface charge.At time t 2 , after the switch has been opened, the input voltage is sampled on the gate of the capacitor as shown in Fig. 23.1.1b. The charge on the gate can no longer change. The inversion and depletion charge distribution in the bulk remains the same as at t 1 . The gate voltage (v GB ) is the sum of the voltage drops across the oxide and the bulk. It can be shown that for a fixed gate charge the drop across the oxide is fixed. It can also be shown that the voltage drop across the bulk is an increasing function of the magnitude of the depletion charge.For t 3 with the switch still open, a large positive voltage (V PULL ) is applied to the source terminal, as shown in Fig. 23.1.1c. This eliminates the entire inversion charge leaving only depletion charges. To balance the gate charge with the charge in the bulk, the depletion charge magnitude increases by the same amount that the inversion charge magnitude decreases. The increase in depletion charge magnitude mandates an increase in the voltage drop across the bulk. As the voltage drop across the oxide is fixed, this increase in voltage reflects directly as an increase in the gate voltage. The output is the gate voltage during the boosting phase. During this phase the small-signal capacitance of the structure decreases since the depletion region is effectively in series with the oxide capacitor. With two structures operated differentially, a small input difference results in a larger difference between their boosted values [1], as in classical parametric amplifiers [2]. While this implies small-signal gain, it is demonstrated that large signal amplification can be obtained as well.To test the characteristics of the parametric amplifier, a chip is implemented in a standard 0.25µm CMOS process. The chip uses 3.3V PMOS devices for the parametric amplifier (with W=84µm and L=4.2µm) and for the output buffers. The test setup is shown in Fig. 23.1.2. The voltage v S...

This paper presents a 4 th order low-pass baseband filter for WLAN applications, implemented in a 1.5 V 3 V 0.15 µm CMOS process. The active-RC configuration uses single amplifier biquads (SABs) to save power and has resistor and capacitor arrays to provide variable gain and bandwidth trimming. The biquads are implemented using three-stage opamps and a hybrid π-to-Tee transformation network for the resistor arrays, so that higher linearity than conventional Sallen-Key circuits may be obtained. The filter achieves a measured THD of -72 dB at 1 MHz.

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