Amplifiers – With periodic switching input-output
Reexamination Certificate
2000-10-02
2001-10-23
Pascal, Robert (Department: 2817)
Amplifiers
With periodic switching input-output
C330S069000
Reexamination Certificate
active
06307430
ABSTRACT:
FIELD OF THE INVENTION
This patent application and the embodiments in it relate generally to the field of operational amplifier (“op-amp”) circuits. More specifically described herein is a method and circuitry for reducing noise in an op-amp circuit employing chopper stabilization.
SUMMARY
Chopper-stabilized amplifiers have the property of reducing the contribution of random offset voltage and noise, especially 1/f noise. Such circuits operate by modulating the signal to a higher frequency, then sending it to the amplifier, which adds unwanted low-frequency components, then synchronously demodulating the signal and modulating the offset voltage and 1/f noise. This is accomplished by using a biphasic non-overlapping clock, which during one phase enables the input signal to be passed to the amplifier circuit in first orientation and during the other phase allows the input signal to be passed to the operational amplifier in the opposite orientation relative to its inputs. The same biphasic signal is used to synchronize a set of switches at the output of the op-amp circuit in phase with the switching of the input signals at the input to the op-amp circuit.
An op-amp circuit employs feedback to control the closed loop gain of the operational amplifier. This gain is controlled by a current or voltage feedback which adjusts the input current or voltage depending on the output voltage of the circuit. Thus, if there is noise at the output, this noise will be mathematically referred back to the signal input of the amplifier circuit. Since amplifiers are employed to amplify smaller signals into larger ones, it is desirable to have as wide a dynamic range of voltage at the output of the amplifier circuit as possible, but this dynamic range is constrained by the voltage power supply limits for the amplifier circuit itself. For low-voltage systems, in which those power supply voltages may be a range of approximately 3 volts, is contrasted to a higher-voltage system where the voltage range of the amplifier may be in the five-volt range or higher, the dynamic range of the output from the amplifier circuit is limited. It would be possible to compensate for this lower voltage range by employing smaller amplitude signals throughout the system, however, the noise poses an effective lower limit on the amplitude of the signals within the amplifier circuit. Thus, the noise effects an unwanted diminishing of the circuit's dynamic range.
One approach for constructing a high-gain amplifier using transistors is sometimes referred to as a double cascode circuit. The difficulty with the double cascode circuit, especially in a low voltage system, is that there are typically three transistors between the power supply rails and the output of the double cascode amplifier circuit. This reduces the output swing of the amplifier, which is especially important in a low supply voltage application. It is possible to use a gain-boosted cascode amplifier circuit in which two of the transistors are replaced with a single transistor with the gate of the single transistor being fed by another amplifier. This would reduce the number of voltage drops across the transistors from three to two. An amplifier circuit having this gain-boosted cascode circuit relative to both the positive and negative supply rails to the amplifier might be referred to as a folded cascode amplifier using a gain-boosted cascode circuit.
The preferred implementation of the chopper output modulation is by current steering. Relative to a normal folded cascode amplifier, a chopper amplifier using this circuitry would further require switches at the output to properly steer the current through the correct output current path. Embodiments described in this application, however, incorporate the analog clock or biphasic clock used for this switching within the gain-boosted cascode circuit. This is accomplished by adding additional amplifiers to the main folded-cascode amplifier described in the embodiments in this application. This approach uses a single amplifier to provide both the output currents steering and some measure of additional gain for the amplifier circuit.
This chopper-stabilized amplifier circuit preferably has first and second signal inputs, or in other words, is a “differential” amplifier. Within the amplifier are chopper switches, which feed the balanced input signals into balanced input signals for an amplifier portion of the chopper amplifier circuit. These chopper switches periodically reverse the polarity of the signals being applied to the amplifier portion of the chopper amplifier circuit. Within the amplifier portion of the chopper amplifier circuit is a sub-amplifier circuit connected to the positive and negative signal inputs of the amplifier portion. The sub-amplifier circuit has at least one transistor amplifier which is driven by a gain-boosting amplifier. The gain-boosting amplifier preferably provides an output signal which comprises one component that is an amplified signal based on the differential voltage between the positive and negative signal inputs and another component which is a clock signal oscillating over a voltage range that is less than the power supply voltage provided to the transistor amplifier (defined for convenience as “analog clock”).
Often there might be further circuitry between the input chopper switches and the inputs to the amplifier portion of the chopper-stabilized amplifier. Also, the clock signal component of said amplified signal is preferably derived from the clock signal that is used to operate the chopper input switches. Within the amplifier sub-circuit, the output voltages are preferably supplied at the drain of the MOS transistor amplifier. That MOS transistor amplifier preferably receives its current from a transistor current source interposed between the source of the MOS transistor amplifier and the power supply. By using the gain-boosted approach described above, the output voltage can be made to swing closer to the supply voltage than would be possible using the double cascode amplifier circuit. Specifically, the output voltage can come within two V
DS
voltage drops of the supply voltage.
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Ammisetti Prasad
Kejariwal Murari
Thomsen Axel
Baker & McKenzie
Choe Henry
Cirrus Logic Inc.
McCormack Brian C.
Pascal Robert
LandOfFree
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