Abstract:
A circuit is disclosed that comprises a capacitor gain-boost circuit and an amplifier coupled to capacitor gain-boost circuit. A capacitor gain-boost circuit comprises of capacitor, gain-boost amplifier and biasing circuit. The gain-boost amplifier and capacitor provides optimum biasing operation and performance. Accordingly, through the use of capacitor gain-boost circuit, the supply voltage range and power consumption of an amplifier is optimized while the gain of amplifier is improved.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to integrated circuits and more specifically to analog amplifier circuits. 
   BACKGROUND OF THE INVENTION 
   The invention relates to analog amplifiers. Since the introduction of transistors and integrated circuits, analog amplifiers have been used in many applications Example applications range from video processing, audio processing, radio-frequency circuits, networking and sensor related fields. Amplifiers are often the fundamental building block of many analog systems. These systems include signal amplification, digital-to-analog converter, analog-to-digital converter, output drivers and sensor interfacing circuits, and they require amplifiers. 
   In these systems or applications, the overall performance is highly dependent on the performance capability of the individual amplifiers. Often, the system performance is limited by the amplifiers. Therefore, it is important to maximize the amplifier performance, and one of the critical performance factors is the gain. 
   Various amplifier designs have been developed in an attempt to meet high performance requirements. These include folded-cascode amplifiers, which are known in the art, and need not be described here. The closest prior art is thought to be represented by U.S. Pat. No. 6,362,688, and U.S. Pat. No. 6,828,856, both disclose prior gain boosting circuitry. Yet, such designs have not been addressing the increasing demand for high performance with flexible operating supply voltages and biasing conditions. Thus there is a need for a gain-boosting amplifier that requires simple configuration and offers wide range of supply voltage and biasing condition. The present invention addresses such a need. 
   SUMMARY OF THE INVENTION 
   A circuit is disclosed that comprises capacitor gain-boosting circuit and an amplifier coupled to a capacitor gain-boosting circuit. The gain-boosting circuit consists of amplifier and capacitor. 
   One advantage of a preferred embodiment of the present invention is that it performs with a high gain in comparison to existing amplifier circuits. Another advantage of a preferred embodiment of the present invention is that it is suitable to operate in many conditions, one example is wide supply voltage range. 
   Other objects, features, and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional objects, features, and advantages be included herein within the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a single-stage amplifier. 
       FIG. 2  shows a capacitor gain-boost circuit. 
       FIG. 3  shows capacitor gain-boost circuits implemented on complementary p-channel and n-channel amplifier. 
       FIG. 4  shows a capacitor gain-boost circuit with bias circuit. 
       FIG. 5  shows a dual-capacitor gain-boost circuit. 
       FIG. 6  shows a dual-capacitor gain-boost circuit with bias circuit. 
       FIG. 7  shows a switched-capacitor gain-boost circuit. 
       FIG. 8  shows an example implementation of gain-boost circuit. 
       FIG. 9  shows a gain-boost circuit in a p-channel amplifier arrangement. 
       FIG. 10  shows a capacitive MEMS sensor electrical equivalent model. 
       FIG. 11  shows an application example for capacitive MEMS interface circuit. 
       FIG. 12  shows an application example for analog integrator circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates generally to integrated circuits and more specifically to analog amplifier circuits utilized in such circuits. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
   A capacitor gain-boost circuit applied to an analog amplifier is disclosed. The capacitor gain-boost circuit can be configured to increase the gain of the amplifier. To describe the features of the present invention in more detail, refer now to the following description in conjunction with the accompanying Figures. 
     FIG. 1  shows an ordinary single-stage n-channel analog amplifier, its input is Vin and its output is Vout. It has an n-channel transistor  102  and a current source  104 . The voltage gain of this amplifier  100 , can be expressed as: A=g m r o , where g m  is the transconductance of  102 , and r o  is the parallel output resistance of  102  and  104 . 
   The capacitor gain-boosting circuit is shown in  FIG. 2 . It employs an additional n-channel transistor  204 , a negative gain amplifier  206  and a capacitor  202 . The output of amplifier  206  connects to the Gate of  204 . The input of amplifier  206  is coupled to the source of  204  through capacitor  202 . A gain-boost feedback loop is formed. Amplifier  206  continuously regulates the gate of transistor  204  such that the source of transistor  204  is maintained constant. The total effective gain of this amplifier can be simplified and approximated as: A total =A original *A boost,  where A total  is the new gain using the gain-boost circuit, A original  is the voltage gain of the original amplifier  100  shown in  FIG. 1 , and A boost  is the gain of the gain-boost amplifier  206 . 
   The capacitor gain-boost circuit allows a fully adjustable bias voltage for both amplifier  206  and transistor  204 . This gives the design flexibility for component selection and operating region, which in turn may offer high frequency bandwidth, fast settling time and small IC silicon area. The operating supply voltage can also be minimal by setting the suitable bias voltage. As a result, it is able to operate in a low supply voltage system. 
     FIG. 3  shows capacitor gain-boost circuits apply to a complementary push-pull amplifier. In this circuit, a second capacitor gain-boost circuit is used, which consists of a negative gain amplifier  308 , a capacitor  306  and a p-channel transistor  304 . 
     FIG. 4  illustrates a capacitor gain-boost circuit with an established bias condition. Resistor  402  is connected between the input and output of amplifier  206 . A bias condition is established for the input of amplifier  206 . The bias voltage is stored and memorized in capacitor  202 . This is an illustrative example, and one of ordinary skill in the art readily recognizes the bias network theory and may be modified for different applications. 
     FIG. 5  shows a dual-capacitor gain-boost circuit. A second capacitor  502  is coupled between the output of amplifier  206  and the gate of transistor  204 . In this scheme, the output of amplifier  206  can be independently biased and is level shifted to connect to the gate of transistor  204 . This allows optimal bias and operating points for multiple components simultaneously. 
     FIG. 6  illustrates a dual-capacitor gain-boost circuit in a bias condition. Resistor  602  provides the bias operating point for the gate of transistor  204 . The bias voltage is stored and memorized in capacitor  502 . This is an illustrative example, and one of ordinary skill in the art readily recognizes the bias network theory and may be modified for different applications. 
     FIG. 7  illustrates a switched-capacitor gain-boost circuit. Four switches ( 702 ,  704 ,  706  and  708 ) are added to the circuit. There are two clock phases: Initialization phase (PH 1 ), and Amplification phase (PH 2 ). During the initialization phase (PH 1 ), switch  702  and switch  706  are closed, and switch  704  and switch  708  are open. The output and input of amplifier  206  are connected together in an auto-zero configuration to establish the bias operating point. The bias voltage is stored and memorized in capacitor  202 . During the amplification phase (PH 2 ), switch  704  and switch  708  are closed, and switch  702  and  706  are open. The gain-boost feedback loop is formed. Amplifier  206  continuously regulates the gate of transistor  204  such that its source voltage is maintained at a virtual constant level. The bias voltage can be set at any level thus the drain voltage of transistor  102  can be regulated to any desired voltage level. 
     FIG. 8  shows an implementation of the gain-boost amplifier  206 . A single-input single-output amplifier is shown as an example. It consists of an n-channel transistor  802  and a constant current source  804 . This amplifier circuit offers low power consumption. One of ordinary skill in the art readily recognizes this gain-boost amplifier  206 , and that many alternative circuit approaches can be used, such as differential input amplifier architecture. 
     FIG. 9  illustrates a switched-capacitor gain-boost circuit applies to a p-channel amplifier. In this example, the same gain-boost amplifier  206  is used, or it can be modified to meet different needs. 
   The gain-boost circuit can be employed in different analog or mixed-mode designs. To illustrate its application usage, a few examples, but not limited to these examples, are demonstrated below. 
   Application Example I: MEMS sensor interface circuit. MEMS sensors can be used to measure pressure, motion, acceleration, magnetic field or temperature etc. For a capacitive type MEMS sensor, its electrical equivalent model can be shown as  FIG. 10 . In this model, there are three terminals and two discrete capacitors:  1002  and  1004 . When the input stimulus changes (such as pressure, acceleration or temperature), the values of capacitors  1002  and  1004  change accordingly. By measuring the capacitance difference between  1002  and  1004 , the input stimulus can be interpreted. 
   An analog interface with amplification circuit is used to convert the MEMS sensor&#39;s output into a usable electrical signal. An example MEMS interface circuit is shown in  FIG. 11 . The amplifier  1102  requires a high voltage gain for this application. A capacitor gain-boost circuit is used in amplifier  1102  to improve its gain. The operation can be described as follows. There are two clock phases: PH 1  and PH 2 . During PH 1 , both capacitors  1002  and  1004  are shorted together. At the same time, the bias voltage is also set up for gain-boost amplifier  206  and stored in capacitor  202 . During PH 2 , one of the terminals of capacitor  1002  is connected to ground. The switched-capacitor gain-boost circuit is also formed. The change in the output voltage reflects the changes in capacitors  1002  and  1004 . The output has a direct relation to the input stimulus. 
   Application Example II: Voltage Integrator. The second application example is an analog voltage integrator.  FIG. 12  shows a first-order active integrator. Capacitor  1202  is the integration capacitor. Capacitor  1206  is the switched-capacitor equivalent integration resistor. An operational amplifier  1208  is also used. The input and output of the integrator are Vin_integ and Vout_integ, respectively. For the purpose of illustration, a single-stage gain-boost circuit is used for operational amplifier  1208 . It is understood by one of ordinary skill in the art that, it is a charge transfer amplifier circuit, and other types of charge transfer circuit may possibly be used, and that would be within the spirit and scope of the present invention. 
   Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the invention.