Abstract:
A preamplifier used in a receiver is provided. The preamplifier comprises an input circuit and an output circuit. The input circuit receives an input differential voltage pair, pulls it down when the common voltage of the input differential voltage pair is higher than a reference voltage. The output circuit receives the input differential voltage pair outputted from the input circuit to pull high or low an output voltage accordingly.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to a preamplifier for a receiver and a method thereof, and more particularly to a preamplifier with wide range of the common voltage of the input differential voltage pair. 
   2. Description of the Related Art 
     FIG. 1  is a circuit diagram of a conventional rail-to-rail preamplifier for a receiver. The conventional rail-to-rail preamplifier  100  includes amplifiers  110  and  120  and an inverter  130 . The amplifiers  110  and  120  each amplify a differential voltage pair VN and VP and produce one voltage of the amplified differential voltage pair for the inverter  130 . The inverter  130  then pulls its output voltage Vo high or low based on these inputs. The transistors in amplifiers  110  and  120  are complementary, so that the preamplifier  100  is capable of amplifying the differential voltage VN and VP with wide common voltage range. 
   However, the amplifiers  110  and  120  are powered by a high voltage supply power HVDD, which is for analog power, while the inverter  130  is powered by a low voltage supply power LVDD, which is for digital power. The high supply voltage HVDD is around 3.3V, and the low supply voltage LVDD is around 1.8V. The low supply voltage LVDD is even lower than the high supply voltage HVDD minus a threshold voltage of a transistor  121  in amplifier  120 . Thus, the transistor  131  in the inverter  130  is cut off, which causes the rail-to-rail preamplifier to become disabled. 
   SUMMARY OF THE INVENTION 
   A preamplifier used in a receiver includes an input circuit, an output circuit. The input circuit receives an input differential voltage pair and pulls it down when the common voltage of the input differential voltage pair is higher than a reference voltage. The output circuit receives the input differential voltage pair outputted from the input circuit to pull high or low an output voltage accordingly. 
   A preamplifier used in a receiver includes an input circuit and an amplifier. The input circuit receives an input differential voltage pair, pulls it down when the common voltage of the input differential voltage pair is higher than a reference voltage, and keeps it unchanged when the common voltage is not higher than the reference voltage. The amplifier includes a first stage amplifier and a second stage amplifier. The first stage amplifier, powered by a high supply voltage, receives and amplifies the input differential voltage pair outputted from the input circuit to output an internal differential voltage pair. The second stage amplifier, powered by a low supply voltage, receives and amplifies the internal differential voltage pair to pull high or low an output voltage. 
   A method for preamplifying an input differential voltage pair, used in a receiver, includes the following steps. Firstly, the input differential voltage pair is pulled down when the common voltage of the input differential voltage pair is higher than a reference voltage. Next, the input differential voltage pair is amplified to output an internal differential voltage pair. Then, the internal differential voltage pair is amplified to pull high or low a first output voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a conventional rail-o-rail preamplifier for a receiver. 
       FIG. 2  shows a circuit diagram of the preamplifier according to a first embodiment of the invention. 
       FIG. 3  shows a circuit diagram of the preamplifier according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
     FIG. 2  shows a circuit diagram of the preamplifier  200  according to the first embodiment of the invention. The preamplifier  200  is for preamplifying an input differential voltage pair VIN and VIP to pull an output voltage Vo 1  for a receiver high or low. 
   The preamplifier  200  includes an input circuit  210  and an output circuit  220 . The input circuit  210  receives the input differential voltage pair VIN and VIP. In the first embodiment, when the common voltage Vcom of the input differential voltage pair VIN and VIP is higher than a reference voltage Vr, the input circuit  210  pulls the input differential voltage pair VIN and VIP down to produce a differential voltage pair VIN′ and VIP′ and then transfers the differential voltage pair VIN′ and VIP′ to the output circuit  220 . 
   When the common voltage Vcom of the input differential voltage pair VIN and VIP is not higher than the reference voltage Vr, the input circuit  210  directly transfers the input differential voltage pair VIN and VIP as the input differential voltage pair VIN′ and VIP′ to the output circuit  220  without pulling the input differential voltage pair VIN and VIP down. The output circuit  220  then takes the input differential voltage pair VIN′ and VIP′ and pulls the output voltage Vo 1  high or low. 
   The input circuit  210  is now described in detail. The input circuit  210  includes a comparator  230  and a level adjustment circuit  240  and switches  261 ,  262 ,  263  and  264 . The comparator  230  compares the common voltage Vcom of the input differential voltage pair VIN and VIP with the reference voltage Vr. The switches  261  to  264  are turned on or off based on the comparing result of the comparator  230 . 
   When the common voltage Vcom is higher than the reference voltage Vr, the switches  263  and  264  are turned on to transmit the input differential voltage pair VIN and VIP to the level adjustment circuit  240 . Meanwhile, the switches  261  and  262  are turned off with the result that the input differential voltage pair will not be directly transmitted to the output circuit  220 . The level adjustment circuit  240  then pulls the input differential voltage pair VIN and VIP down. The pulled-down input differential voltage pair VIN and VIP is then transferred to the first stage amplifier  270  in the output circuit  220 . 
   In this embodiment, the level adjustment circuit  240  includes source followers  241  and  242 , which are for pulling down the voltages VIN and VIP, respectively, of the input differential voltage pair when the common voltage Vcom is higher than the reference voltage Vr. The source follower  241  includes transistors  243  and  244 . When the common voltage Vcom is higher than the reference voltage Vr, the gate of the transistor  243  receives the voltage VIP. The drain of the transistor  243  receives the high supply voltage HVDD and its source is connected to the drain of the transistor  244 . The transistor  244  has its source grounded. 
   When the transistor  243  receives the voltage VIP, the transistor  243  pulls down the voltage VIP by an amount equal to its gate-source cross voltage to produce the voltage VIP′ at its source which is connected to the output circuit  220 . 
   Similarly, when the common voltage Vcom is higher than the reference voltage Vr, the gate of the transistor  245  receives the voltage VIN. Then the transistor  245  pulls down the voltage VIN by an amount equal to its gate-source cross voltage to produce the voltage VIN′ at its source which is connected to the output circuit  220 . 
   When the common voltage Vcom is not higher than the reference voltage Vr, the switches  261  and  262  are turned on to transfer the voltages VIN and VIP to the output circuit  220  as the voltages VIN′ and VIP′, respectively. Meanwhile, the switches  263  and  264  are turned off with the result that the level adjustment circuit  240  will not receive the input differential voltage pair VIN and VIP. Therefore, when the common voltage Vcom is not higher than the reference voltage Vr, the input differential voltage pair VIN and VIP is not pulled down but directly transferred to the output circuit  220 . 
   In this embodiment, the input circuit  210  further includes a voltage divider  250  to generate the common voltage Vcom of the input differential voltage pair VIN and VIP to the comparator  230 . In the first embodiment, the voltage divider  250  is a resistor string including resistors  251  and  252  which are serially connected. The voltage divider  250  is coupled between the voltage VIN and the voltage VIP to divide the voltage therebetween. In the first embodiment, the resistances of the resistors  251  and  252  are the same. Thus, the common voltage Vcom of the input differential voltage pair VIN and VIP is produced at the connection of the resistors  251  and  252 . 
   The output circuit  220  is now described in detail. In the first embodiment, the output circuit  220  is an amplifier. The output circuit  220  includes a first stage amplifier  270  and a second stage amplifier  280 . In the first embodiment, the first stage amplifier  270  is powered by the high supply voltage HVDD, which is for analog power, while the second stage amplifier  280  is powered by the low supply voltage LVDD, which is for digital power. The first stage amplifier  270  receives and amplifies the input differential voltage pair VIN′ and VIP′ produced by the input circuit  210  and produces an internal differential voltage pair Vin and Vip. The second stage amplifier  280  receives and amplifies the internal differential voltage pair Vin and Vip to pull the output voltage Vo 1  high or low. 
   The first stage amplifier  270  includes transistors  271 ,  272 ,  273 . The transistor  271  is for receiving the high supply voltage HVDD and providing a bias current for the first stage amplifier  270 . The transistors  272  and  273  are for receiving the voltages VIN′ and VIP′, respectively. The source-drain cross voltage of the transistor  271  is Vsd. The source-gate cross voltage of the transistors  272  and  273  are Vsg. 
   The second stage amplifier  280  includes transistors  281 ,  282 ,  283 , and  284 . The transistors  281  and  282  form a current mirror. The transistors  283  and  284  receive the voltage Vip and Vin, respectively. 
   In the first embodiment, the output circuit  220  further includes an inverter  290  for amplifying the output voltage Vo 1  to produce an inverted output voltage Vo 2 . 
   When the voltage VIP is higher than the voltage VIN, that is, the voltage VIP′ is higher than the voltage VIN′, the amplifier  280  pulls the output voltage Vo 1  low, and the inverter  290  then pulls the output voltage Vo 2  high. When the voltage VIP is lower than the voltage VIN, that is, the voltage VIP′ is lower than the voltage VIN′, the amplifier  280  pulls the output voltage Vo 1  high and the inverter  290  then pulls the output voltage V 02  low. 
   Therefore, the preamplifier  200  receives the input differential voltage pair VIN and VIP and pulls the output voltages Vo 1  and Vo 2  high or low according to the input differential voltage. 
   However, when the common voltage of the input differential voltage pair VIN′ and VIP′ is higher than the voltage HVDD-Vsd-Vsg, the transistors  272  and  273  will be cut off, which causes the output circuit  220  to be disabled. Therefore, by applying the input circuit  210 , which pulls down the input differential voltage pair VIN and VIP when the common voltage Vcom of the input differential voltage pair VIN and VIP is too high, the transistors  272  and  273  are therefore enabled to receive the pulled-down input differential voltage VIN′ and VIP′ so as to be kept turned on. 
   In the first embodiment, the reference voltage Vr is set to about HVDD/2, which ensures that the pulled down input differential voltage pair VIN′ and VIP′ are not to cut off to the transistors  272  and  273 . 
   Conversely, when the common voltage Vcom of the input differential voltage pair VIN and VIP is not higher than the reference voltage Vr, the input circuit  210  simply reproduces VIN and VIP as the differential voltage pair VIN′ and VIP′ at its output, in which case the transistors  272  and  273  will not be cut off. 
   Therefore, even if the common voltage Vcom of the input differential voltage pair VIN and VIP is high, the common voltage of the input differential voltage pair VIN′ and VIP′ produced by the input circuit  210  will not be higher than the voltage HVDD-Vsd-Vsg, so that the output circuit  220  works properly. Thus, the preamplifier  200  according to the first embodiment is capable of properly preamplifying the input differential voltage pair even with wide range of common voltage. 
   Second Embodiment 
     FIG. 3  shows a circuit diagram of the preamplifier  300  according to the second embodiment. In the second embodiment, when the common voltage Vcom′ of the input differential voltage pair DIN and DIP is lower than a reference voltage Vr′, the input circuit  310  pulls the input differential voltage pair DIN and DIP up to produce a differential voltage pair DIN′ and DIP′ and then transfers the differential voltage pair DIN′ and DIP′ to the output circuit  320 . 
   When the common voltage Vcom′ of the input differential voltage pair DIN and DIP is not lower than the reference voltage Vr′, the input circuit  310  directly transfers the input differential voltage pair DIN and DIP as the input differential voltage pair DIN′ and DIP′ to the output circuit  320  without pulling the input differential voltage pair DIN′ and DIP′ up. The output circuit  320  then takes the input differential voltage pair DIN′ and DIP′ and pulls the output voltage Vo 1 ′ high or low. 
   The detailed description of the input circuit  310  is explained as follows. In the input circuit  310 , when the common voltage Vcom′ is lower than the reference voltage Vr′, the switches  363  and  364  are turned on to transmit the input differential voltage pair DIN and DIP to the source followers  342  and  341  in the level adjustment circuit  340 , respectively. Meanwhile, the switches  361  and  362  are turned off. The transistors  346  and  344  in the source followers  342  and  341  then pulls the voltages DIN and DIP up, respectively, by amounts equal to the gate-drain cross voltages of the transistors  346  and  344 . The pulled-up input differential voltage pair DIN′ and DIP′ is then transferred to the first stage amplifier  370  in the output circuit  320 . 
   In the input circuit  310 , when the common voltage Vcom′ is not lower than the reference voltage Vr′, the switches  361  and  362  are turned on to transmit the voltage DIN and DIP as the voltage DIN′ and DIP′ directly to the output circuit  320 . Meanwhile, the switches  363  and  364  are turned off, so that the level adjustment circuit  340  will not receive the input differential voltage pair DIN and DIP. 
   In the output circuit  320 , the first and second stage amplifiers  370  and  380  are both powered by the same supply voltage, for example, one of the high and low supply voltages HVDD and LVDD. The functions of transistors  371  to  373  in the first stage amplifier  370  in  FIG. 3  are similar to the function of the transistors  271  to  273  in the first stage amplifier  270  in  FIG. 2 , respectively. The functions of the transistors  381  to  384  are similar to those of the transistors  281  to  284 , respectively. The corresponding transistors in the output circuits  320  and  220  are complementary. For example, the transistor  371  is NMOS, while its corresponding transistor  271  is PMOS. The function of the inverter  390  is similar to that of the inverter  290 . 
   In the output circuit  320 , the drain-source cross voltage of the transistor  371  is Vds, and the gate-source cross voltage of the transistors  372  and  373  are Vgs. When the common voltage of the input differential voltage pair DIN′ and DIP′ is lower than the voltage equal to Vds+Vgs, the transistors  372  and  373  will be cut off, which causes the output circuit  320  to be disabled. Therefore, by applying the input circuit  310 , which pulls up the input differential voltage pair DIN and DIP when the common voltage Vcom′ of the input differential voltage pair DIN and DIP is too low, the transistors  372  and  373  therefore receive the pulled-up input differential voltage DIN′ and DIP′ so as to be kept turned on. 
   Consequently, even if the common voltage Vcom′ of the input differential voltage pair DIN and DIP is too low, the common voltage of the input differential voltage pair DIN′ and DIP′ produced by the input circuit  310  will not be lower than the voltage Vds+Vgs, so that the output circuit  320  works properly. Thus, the preamplifier  300  according to the second embodiment is capable of properly preamplifying the input differential voltage pair even with wide range of common voltage. 
   While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.