Baseband noise reduction

[0001] The present application relates to and claims benefit of priority of U.S. provisional patent application, entitled “Baseband Noise Reduction,” Ser. No. 60/753,878, filed on Dec. 22, 2005. The provisional patent application is hereby incorporated by reference in its entirety.

[0002] The present invention relates to a design of an electronic circuit. In particular, the present invention relates to using chopper-stabilized amplifiers in active filters to reduce baseband noise.

[0003] According to one embodiment of the present invention, an active circuit includes (a) a first chopper circuit receiving an input signal and a chopping signal having a frequency higher than a base band of the input signal, and providing a modulated input signal; (b) an amplifier that receives the modulated input signal and that provides an amplified signal resulting from amplifying the modulated input signal; and (c) a second chopper circuit that receives the amplified signal and the chopping signal to provide an output signal. The chopping signal has a frequency that may be dynamically adjusted to accommodate changes in impedance or output signal spectrum as a result of the operations of the chopper circuits. The active circuit further includes a low pass filter that receives the output signal and that attenuates components of the output signal above the base band of the input signal. In this manner 1/f noise introduced by the amplifier is eliminated or reduced.

[0004] The active circuit of the present invention may be implemented in active filters including multiple feedback filters. Alternatively, the active circuit may be implemented in a pre-amplifier of a base band filter, or as a gain stage prior to a filter with a high impedance input.

[0005] The active circuit of the present invention may be implemented in balanced filters that receive a differential signal.

[0006] In one embodiment, the active circuit has a chopping frequency greater than 50 MHz. The amplifier may have a unit gain for input signal up to 1 GHz.

[0007] The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.

[0012] To facilitate cross-reference among the figures, like elements in the figures are provided reference numerals.

[0013] The inventor observed that low-frequency CMOS amplifiers suffer from 1/f noise (i.e., noise that has a spectrum that decreases in magnitude with frequency). The present invention provides a chopper-stabilized amplifier that reduces the 1/f noise in a baseband filter or in a preamplifier for a baseband filter. According to one embodiment of the present invention, 1/f noise introduced in baseband lowpass filters can be reduced by replacing the amplifiers with chopper-stabilized amplifiers.

[0014] FIG. 1 illustrates the operation of such a chopper-stabilized amplifier (chopper-stabilized amplifier 100). As shown in FIG. 1, an input signal v_i(t) is modulated at chopper circuit or modulator 101 with a chopping signal v_ch(t), which includes a pulse train at a predetermined chopping rate that is substantially higher in frequency than the base band of input signal v_i(t). The resulting signal v_m(t) is provided to amplifier 102, where signal v_m(t) is amplified to provide amplified signal v_ma(t). In FIG. 1, amplifier 102 provides a high gain (e.g., 50). Amplified signal v_ma(t) is mixed in chopper circuit or modulator 103 with chopping signal v_ch(t) to provide output signal v_d(t), Output signal v_d(t) is then filtered using low-pass filter 104, which has a stop band frequency less than the chopping rate of signal v_ch(t). In this process, any low-frequency noise introduced by amplifier 102 is up-converted to the chopping frequency and filtered out by low-pass filter 104.

[0015] FIG. 2 illustrates in detail chopping circuit 200, which may be used to implement each of chopper circuits 101 and 103. In chopping circuit 200, switches S₁ and S₃ are controlled by clock signal φ₁ to open and close at the same time. Similarly, switches S₂ and S₄ are controlled by clock signal φ₂ to open and close at the same time. Clock signals φ₁ and φ₂ are preferably non-overlapping. If amplifier 102 has low distortion, lowpass filter 104 may be implemented by as simply as a capacitor that holds the output voltage when all switches are off.

[0016] In one embodiment, chopping circuit 200 may operate at a frequency greater than approximately 50 MHz. It may also be desirable, in some instances, to dynamically set the chopping frequency (i.e., the frequencies of clock signals φ₁ and φ₂), such that harmonics of the chopping frequency are outside of a received signal's spectrum. FIGS. 3 and 4 illustrates applying chopper-stabilized amplifier technique to a balanced filter. FIG. 3 shows a conventional balanced filter 300. As shown in FIG. 3, balanced filter 300 for a differential input signal represented by voltage source 306 includes multiple feedback (MFB) filters implemented by amplifiers 301a, 301b, capacitors 302a, 302b, 303a and 303b and resistors 304a, 304b, 305a and 305b. Such an MFB filter may be used where greater than approximately 1 GHz unity gain bandwidth is used. FIG. 4 shows balanced filter 400 achieved by introducing chopper circuits 401 and 402 to provide chopper-stabilized amplifiers, in accordance with one embodiment of the present invention. Chopper circuits 401 and 402 may each be implemented by, for example, chopper circuit 200 of FIG. 2. Charge holding capacitors (not shown) may be provided at the output terminals of chopper-stabilized balanced filter 400.

[0017] In applying the chopper-stabilized amplifier technique, in some cases, for example, where a filter has a high impedance at the amplifier input terminals, low noise performance may be achieved by providing a chopper stabilized gain stage after the mixer and before the filter to set noise performance. Thus, many filter designs are suitable for chopper-stabilization. Chopper-stabilization may be applied also in a baseband preamplifier, in addition to filters. The chopping frequency is preferably dynamically selected, to accommodate any change in performance due to converting an amplifier to a chopper-stabilized implementation, or bandwidth shifting due to the internal impedances changes as the switches change state. Noise introduced at the switching frequency can be reduced further by an additional filter at the output.

[0018] The detailed description above is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is set forth in the following claims.