Abstract:
A switched capacitor anplifier circuit is provided with two independent reference voltages, one which provides an appropriate bias level for the amplifiers, and one which sets a common mode input level for the amplifiers, thereby allowing the dynamic range to be maximized.

Description:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9930388.5 filed in United Kingdom on Dec. 22, 1999; the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to an electronic circuit, and in particular to a low voltage differential amplifier incorporating switched capacitors. 
     BACKGROUND OF THE INVENTION 
     A standard, fully differential, operational amplifier has specific requirements as regards the bias voltage which must be supplied thereto, in order for it to operate effectively. Specifically, the amplifier usually includes at its input a single pair of differential transistors. If PMOS input devices are used, then, in order to provide suitable bias voltages to these transistors at low supply voltages, the input reference voltage level needs to be nearer to the negative supply voltage than to the positive supply. This restricts the voltage swing which can be handled by the input stage, and hence restricts the maximum available dynamic range at the output of the operational amplifier. Likewise, if NMOS input devices are used, the input reference voltage level needs to be nearer to the positive supply voltage than to the negative supply. 
     U.S. Pat. No. 5,565,813 discloses a low voltage differential amplifier, in which the voltage on a capacitance circuit, which is switched in and out of the circuit, is used to provide a bias voltage. 
     SUMMARY OF THE INVENTION 
     The prior art has the problem discussed above that, at low supply voltages, the maximum available dynamic range at the output of the operational amplifier is restricted. 
     One possible solution to this problem is to use a voltage boosting circuit such as a charge pump, to provide a supply voltage to the amplifier circuit which is higher than the actual supply voltage level. With a higher amplifier supply voltage, the input reference voltage level can be set close to half way between the negative supply voltage and the positive supply, while still providing suitable bias voltages to the input stage transistors. This maximises the dynamic range of the circuit. However, this solution has the disadvantage that it wastes power to boost the supply voltage in this way. 
     According to the invention, a switched capacitor circuit provides the bias voltages for a differential amplifier circuit, supplied from two reference voltages. More specifically, a first reference level is used to bias the amplifier input stage, and a second reference level is used to set the centre of the dynamic excursion range of the amplifier output stage. Thus, the first reference level can be set to a level determined by the biassing requirement of the amplifier input The second reference level can be set to one half of the supply voltage. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a switched capacitor amplifier circuit according to the invention, in a first phase of operation. 
     FIG. 2 shows the switched capacitor amplifier circuit of FIG. 1 in a second phase of operation. 
     FIG. 3 shows a differential amplifier forming part of the amplifier circuit of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a switched capacitor amplifier circuit, having two operational amplifier circuits  10 ,  12  cascaded together. That is, each amplifier circuit has a pair of differential input terminals in+, in−, and a pair of differential output terminals out+, out−, and the output terminals out+, out− of the first amplifier circuit  10  are connected to the corresponding input terminals of the subsequent amplifier circuit  12 . Any desired number of amplifier circuits can be cascaded in this way. Each amplifier circuit also hag an input pin CM, by means of which the common mode output voltage may be set. 
     Located between each positive output terminal and the subsequent input terminal is a first capacitor  14 , and located between each negative output terminal and the subsequent input terminal is a second capacitor  16 . As is conventional in switched capacitor circuits, these capacitors can be connected to either one of the respective output terminal or the respective input terminal, by means of switches. Thus, there is a first pair of switches  18   a ,  18   b , connected respectively between the output terminals out+, out− of the operational amplifier  10  and the respective capacitors  14 ,  16 . Further, there is a second pair of switches  20   a ,  20   b , connected respectively between the respective capacitors  14 ,  16  and the input terminals in+, in− of the operational amplifier  12 . 
     Moreover, there is a third pair of switches  22   a ,  22   b , connected between the positive and negative rails, from nodes between the respective switch  18   a ,  18   b  of the first pair and the respective capacitor  14 ,  16 , with an input terminal  24  between the switches  22   a  and  22   b . Further, there is a fourth pair of switches  26   a ,  26   b , connected between the positive and negative rails, from nodes between the respective capacitor  14 ,  16  and the respective switch  20   a ,  20   b  of the second pair, with an input terminal  28  between the switches  26   a  and  26   b.    
     To this extent, the circuit of FIG. 1 is conventional. However, an important aspect of the invention is that, whereas a first voltage reference vref 1  is applied to the input terminal  24 , an independent second voltage reference vref 2  is applied to the input terminal  28 . Thus, the first and second voltage references can be different. The importance of this will become apparent from the following description of the operation of the circuit. 
     In a first phase, as shown in FIG. 1, switches  18   a ,  18   b ,  26   a ,  26   b  are closed, and switches  20   a ,  20   b ,  22   a ,  22   b  are open. As a result, the capacitors  14 ,  16  are charged by the output stage of the amplifier  10  to the level of the second voltage reference vref 2 . 
     In order to allow the charging of the capacitors  14 ,  16  to be balanced and symmetrical, the second voltage reference vref 2  is made equal to the common mode output voltage of the amplifier, which is itself defined by the voltage on the input pin CM of the amplifier. The second voltage reference vref 2  can be set to be VDD/2, where VDD is the supply voltage, to maximise the output dynamic range of the amplifier. 
     In a second phase, as shown in FIG. 2, switches  18   a ,  1   b ,  26   a ,  26   b  are open, and switches  20   a ,  20   b ,  22   a ,  22   b  are closed. The voltages on the capacitors  14 ,  16  then bias the input terminals in+, in− respectively of the subsequent amplifier  12 . Because the first voltage reference vref 1  is set not to be equal to the second voltage reference vref 2 , the dynamic range of the input of the amplifier  12  can be maximised, for reasons described below. 
     Also in the second phase, the capacitors  14 ,  16  can discharge through integrator output capacitors  30 ,  32 , and hence the voltage levels on the capacitors can shift. Specifically, the capacitors  14 ,  16  are rereferenced to the level of the first voltage reference vref 1 . 
     Thus, this first voltage reference vref 1  can be set to a level which provides appropriate biassing for the amplifier. For example, FIG. 3 shows schematically an amplifier with a p-input stage, having first and second p-type transistors  50 ,  52  connected to respective differential input terminals in+, in−. For an amplifier of this type, the first voltage reference vref 1  can be set to: 
     
       
         vref 1 =[VDD−(Vgs 1 +Vdsat 2 )], 
       
     
     in which: 
     
       
         Vgs 1 =Vth+Vdsat 1   
       
     
     where Vth is the threshold voltage of an input device, Vdsat 1  is the saturation voltage of an input device, and Vdsat 2  is the saturation voltage of a current source  54  connected to the input pair. 
     It will be appreciated that any desired bias voltage level can be applied. The provision of different voltage levels for the two voltage reference allows appropriate biassing of the amplifier input stage, while maintaining the maximum available dynamic range for output signals.