Abstract:
The present invention relates to a conversion circuit that converts a single-ended signal to differential signals. According to an embodiment of the present invention, crosstalk is avoided by insuring that none of the transistors in the conversion circuit are directly connected to ground. By not having a transistor directly connected to ground, ground current is avoided and crosstalk associated with ground current is eliminated.

Description:
This is a divisional of application Ser. No. 09/296,142 filed filed Apr. 21, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a conversion circuit that may be used in a preamplifier circuit. In particular, the present invention relates to conversion of single-ended to differential signals. 
     BACKGROUND OF THE INVENTION 
     Preamplifier circuits are typically low-noise amplifiers incorporated into disk drives for the purpose of amplifying signals used in the disk drive. In meeting the low-noise requirements of the preamplifier, a single-ended signal may be converted to differential signals in an attempt to reduce or eliminate crosstalk. A single-ended signal is typically a signal defined by one voltage or current. A differential signal is typically a signal defined by the difference of two currents. Crosstalk is an undesired transfer of signals between system components. 
     Any noise on the current supply is typically noticeable on a single-ended signal since the current supply affects the single-ended signal without compensation. However, noise on the supply is typically not noticeable on a signal produced by differential signals since the noise is reflected on both the differential signals and therefore the resulting difference of the two signals is preserved. Accordingly, converting a single-ended signal to differential signals typically reduces crosstalk. 
     In a preamplifier circuit, there is typically some amplification (often referred to as gain) to a single-ended signal prior to the conversion of the single-ended signal to the differential signals. This single-ended gain may affect the current supply which in turn may affect the single-ended signal through the supply, causing crosstalk. Accordingly, this crosstalk typically limits the single-ended signals that could be passed through the single-ended to differential converter. Due to cross-talk, the current amplification of the preamplifier typically shuts off at high frequencies, since the impedance may become too high for the circuit to carry the high frequency signals. 
     Additionally, there may also be some crosstalk due to a current flowing into ground, commonly referred to as ground current. When current flows into ground, the ground may fluctuate. Since signals are measured in relation to ground, fluctuation of ground may cause fluctuation in the signal, causing cross-talk. 
     It would be desirable to have a single-ended to differential converter that prevents cross-talk. It would also be desirable for the single-ended to differential converter to process signals at higher frequencies. The present invention addresses such needs. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a conversion circuit that converts a single-ended signal to differential signals. According to an embodiment of the present invention, crosstalk is avoided by insuring that none of the transistors in the conversion circuit are directly connected to ground. By not having a transistor directly connected to ground, ground current is avoided and crosstalk associated with ground current is eliminated. 
     Additionally, according to an embodiment of the present invention, the conversion circuit also amplifies the signal by a gain greater than one. Accordingly, the amplification which is typically performed prior to the signal being input into the conversion circuit may now be performed in the conversion circuit. By shifting the amplification from occurring prior to the conversion circuit to occurring in the conversion circuit, crosstalk between the current source and the single-ended input signal may also be avoided. 
     A system according to an embodiment of the present invention for converting a single-ended signal to differential signals is presented. The system comprises a first device configured to convert a current to voltage. The system also includes a second device coupled to the first device. The system further includes a third device coupled to the first device and second device, wherein not one of the first, second, and third device is directly connected to ground and wherein the current is amplified by a gain of more than two. 
     A method according to an embodiment of the present invention for converting a single-ended signal to differential signals is also presented. The method comprises converting a current to voltage; inputting a differential voltage to a differential pair; and amplifying the current by a gain of more than two, wherein approximately no ground current is produced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a conventional single-ended to differential signal conversion circuit. 
     FIG. 2 is a schematic diagram of a single-ended to differential signal conversion circuit according to an embodiment of the present invention. 
     FIG. 3 is another schematic diagram of the single-ended to differential signal conversion circuit according to an embodiment of the present invention. 
     FIG. 4 is a flow diagram of a method according to an embodiment of the present invention of converting a single-ended signal to differential signals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is presented to enable one of ordinary skill in the art to make and to use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. 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. 
     FIG. 1 is a schematic diagram of an example of a conventional single-ended to differential signal conversion circuit. The conversion circuit  100  is shown to include a current supply  102 , a volt meter  104 , a ground  110 , and transistors  106  and  108 . An example of the type of transistors  106  and  108  is an enhanced n-type metal oxide semiconductor (NMOS). Enhanced NMOS transistors typically have positive threshold voltages. 
     An amplified single-ended current is input into the conversion circuit  100 . The input current (I in ) meets impedance caused by transistor  106 . This impedance converts (I in ) into voltage. Transistor  108  sees this voltage as a positive voltage and transistor  106  receives this voltage as a negative voltage. By definition, the amplification of the current through transistor  106  has a gain of one and the current gain of transistor  108  matches the gain of transistor  106 . Accordingly, the current gain of transistor  108  is also one. Differential signals I outP    112   a  and I outN    112   b  are of the same magnitude. Accordingly, the differential circuit  100  has a current gain of two. 
     Since the single-ended signal is typically amplified prior to entering the differential circuit  100 , there may be some crosstalk caused by the single-ended gain affecting the current supply. The effect on the current supply may in turn affect the signal-ended signal. This crosstalk may shut off the current gain of conversion circuit  100  at high frequencies, such as at a frequency of approximately 160 MHz. 
     There may also be some crosstalk in the conventional conversion circuit  100  due to a ground current flowing from transistor  108  to ground  110 . As the signal is sent to ground  110 , the ground  110  may fluctuate. Since all the signals are measured in terms of ground  110 , all the signals also fluctuate, causing cross talk. 
     It would be desirable to have a single-ended to differential signal conversion circuit that avoids such crosstalk. It would also be desirable for the single-ended to differential converter to process signals at higher frequencies. The present invention addresses such needs. 
     FIG. 2 is a schematic diagram of a single-ended to differential signal conversion circuit according to an embodiment of the present invention. FIG. 2 shows an example of a single-ended to differential signal conversion circuit  200  which is shown to include three transistors  214 ,  208 ,  206 , a voltage source  204 , a current supply  202 , and a ground  210 . An example of the type of transistors  214 ,  208 ,  206  to be used are NMOS transistors. The primary function of transistors  208  and  206  are to act as a differential pair. Voltage is input into transistors  208  and  206  and the voltage is converted into current to result in an output of a differential current. 
     A current is input (I in ) into the conversion circuit  200 . An example of I in  is approximately 0.5 milli-Amps with a signal of approximately 10 micro-Amps or about 1% of I in . Transistor  214  converts I in  to voltage. An example of the voltage converted by transistor  214  is approximately 10 milli-Volts at the input. There is a voltage drop at transistor  214  such that the voltage at the common transistor source  216   a,    216   b,  and  216   c  is ½ V, where V is the input voltage. For example, ½ V at the common transistor source  216   a,    216   b,  and  216   c  may be 5 milli-Volts. The current flows through transistor  214 , adds to the current at transistor  208 , and flows through source  216   c  of transistor  206  to flow out at I out P    212   b.  An example of I out P  is approximately two milli-Amps of direct current (DC), with approximately forty micro-Amps of signal current. 
     To produce I out N , I in  flows through transistor  214 , adds to the current at transistor  208 , and is sent out of the circuit as I out N    212   a.  I out N  and I out P  are compliments of each other, accordingly, an example of I out N  is approximately two milli-Amps of DC, with approximately forty micro-Amps of signal current. An example of the current at the current source  202  is approximately five milli-Amps. Note that in this conversion circuit  200 , there is no current flowing into ground  210  since no device is directly connected to ground. Accordingly, there is no cross-talk from a ground current. 
     A further advantage of this conversion circuit  200  is that a significant current gain may be accomplished. For example, a current gain of eight may be accomplished by setting the ratio of the drain  218   b  of transistor  208  and the drain  218   a  of transistor  214  at a ratio of four to one, and the ratio of drain  218   c  of transistor  206  to the drain  218   a  of transistor  214  at a ratio of four to one. If drain  218   b  and drain  218   c  are set four times higher than drain  218   a,  then a current gain of four I occurs at transistor  208  and a current gain of four I occur at transistor  206 , providing a total current gain of eight for the differential signal. 
     Accordingly, the single ended signal does not need to be amplified prior to being input into the conversion circuit  200 . Since the single ended signal is not an amplified signal, there is no gain prior to the conversion circuit  200  to cause cross-talk with the current source. Additionally, the conversion circuit  200  is able to process signals at higher frequencies, such as frequencies up to approximately 200 MHz. 
     FIG. 3 is a schematic diagram of an example of the single-ended to differential signal conversion circuit  200  as incorporated into a larger conversion circuit, according to an embodiment of the present invention. An input voltage, such as 2 volts, is input into a conversion circuit  300 . A transistor  302  converts the voltage into current. Transistor  304  passes the alternating current (AC) and transistor  303  balances the direct current (DC) component. An example of the current output of transistor  302  is approximately 1000 micro-Amps DC and 10 micro-Amps AC. 
     The current passes through transistor  304  which protects transistor  214  from capacitance. Transistor  304  acts as a cascode device which causes transistor  214  to see very low impedance and low gain. Cascode devices may be common gate transistors that pass current from source to drain with a voltage gain. The cascode devices may provide a low gain and low capacitance at the drains of transistors, such as transistor  214 , an protect the drains of the transistors from an output voltage. Details of the workings of cascode devices are well known in the art. Once the current is input into circuit  200 , events occur as described in conjunction with FIG.  2 . 
     As previously described, a current is input (I in ) into the conversion circuit  200 . Transistor  214  converts I in  to voltage. There is a voltage drop at transistor  214  such that the voltage at the common transistor source  216   a,    216   b,  and  216   c  is ½ V, where V is the input voltage. The current flows through transistor  214 , adds to the current at transistor  208 , and flows through source  216   c  of transistor  206  to flow out at I out P    212   b.  To produce I out N , I in  flows through transistor  214 , adds to the current at transistor  208 , and is sent out of the circuit as I out N    212   a.    
     A transistor  306  may be coupled with circuit  200  in order to balance transistor  214 . The current gain at transistor  306  is the negative of the current gain at transistor  214 . For example, if transistor  214  has a current gain of 1, then transistor  306  has a current gain of −1. When a circuit is balanced, the current on transistors  208  and  206  are equal and the input current operates at the same average current as the current source  202 . 
     Transistors  308 - 312  may also be coupled with circuit  200  to protect the output voltage from capacitance for transistors  208 ,  206 , and  214 , respectively, by acting as cascode devices which causes transistors  208 ,  206 , and  214  to see very low impedance and low gain. Additionally, transistors  308 - 310  may be used as multiplexing switches that can be used to tristate the output into an off state. The use of such a cascode device as a tristate device is also well known in the art. 
     FIG. 4 is a flow diagram of a method according to an embodiment of the present invention for converting a single-ended signal to differential signals. An initial current is converted to a voltage (step  400 ). This voltage is used to create a differential voltage, and the differential voltage is input into a differential pair to produce differential currents (step  402 ). The initial current is also amplified by a gain of more than two, wherein approximately no ground current is produced (step  404 ). 
     Although the present invention has been described in accordance with the embodiment shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiment and these 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 appended claims.