Abstract:
A circuit having a first transistor being a common gate connected transistor and a second transistor, the second transistor being M times the size of the first transistor, the first and second transistors having commonly connected gates and commonly connected drains, wherein an apparatus is provided to regulate the source voltage of the second transistor to track the source voltage of the first transistor, wherein the current gain of the circuit is M+1.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Great Britain Patent Application No. 1401215.7, with a filing date of Jan. 24, 2014, is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a current conveyor for providing a conduit for a signal current from a current source to a load. Such a current conveyor may be advantageously utilised in an amplification stage of an envelope tracking power amplifier system. 
     BACKGROUND OF THE INVENTION 
     Envelope tracking power amplifier systems are known in the art, and generally comprise the provision of an input signal to be amplified on an RF input path to a signal input of a power amplifier, and an envelope path for generating a modulated power supply based on the envelope of the input signal, with the modulated power supply being provided to a power supply input of the power amplifier. 
     In applications envelope tracking power amplifier systems are desired to be highly efficient. In order to be highly efficient, the amplifier component of such systems is required to be highly efficient. In particular efficient, low noise, high bandwidth amplifiers are required by envelope tracking systems. 
     A current conveyor acts as a conduit for current from one node to another, converting one voltage level to another voltage level, and acting as a buffer. The current conveyor buffers a given current between two nodes at a different voltage. 
     Current conveyors are used in high frequency applications where a conventional operational amplifier cannot be used because the conventional design is limited by gain-bandwidth products. 
     Current conveyors are an important component of high-efficiency amplifiers. A current conveyor is required in an amplifier to provide high gain across very high bandwidths, which may be needed to minimise the impact of the noise and spurious signals generated by switch mode circuits used elsewhere in the envelope tracking modulator. 
       FIG. 1  illustrates an ideal current conveyor. As illustrated in  FIG. 1 , an input signal is provided on a line  102  to one terminal of a voltage source  104 , the other terminal of which is connected to electrical ground. A current source  106  has one terminal connected to electrical ground, and the other terminal generates a signal on line  108 . The signal on line  102  is the input signal, and the signal on line  108  is the output signal. Current is detected in voltage source  104  and replicated in current source  106 , so that the voltage at the output can be different to the voltage at the input. 
     A current conveyor circuit should ideally have a zero input impedance to prevent loss in the input circuit. A current conveyor should also ideally have an infinite output impedance to prevent loss in the output circuit. 
     Whilst  FIG. 1  illustrates an ideal implementation of a current conveyor, with reference to  FIG. 2  there is illustrated a typical implementation of a current conveyor. 
     As is illustrated in  FIG. 2 , a CMOS transistor  202  has a source terminal connected to electrical ground, and a drain terminal connected to one terminal of a current source  208 . A CMOS transistor  204  has a source connected to one terminal of a constant current source  206 , the other terminal of the current source being connected to electrical ground. The drain terminal of the CMOS transistor  204  is connected to one terminal of a current source  210  which provides a high impedance. The other terminals of the current sources  208  and  210  are connected to a supply voltage V S . The gate of the CMOS transistor  204  is connected to the drain of the transistor  202 . The gate of the CMOS transistor  202  is connected to an input signal on line  212 . The input signal on line  212  is also connected to the source of the CMOS transistor  204 . An output signal is provided on a line  214  which is connected to the drain of the CMOS transistor  204 . 
     A typical current conveyor as illustrated in  FIG. 2  has unity gain. 
     The typical current conveyor as illustrated in  FIG. 2  uses a CMOS common gate transistor  204  with a regulated gate voltage provided by CMOS transistor  202  to achieve a low input impedance and a high output impedance. 
     As transistor  202  switches on, the voltage at the gate of transistor  204  decreases and the voltage at the source of transistor  204  also decreases, counteracting the input voltage increase, and lowering the input impedance. The input approximates a voltage source as indicated in  FIG. 1 . 
     With the current conveyor arrangement of  FIG. 2 , the optimal performance is achieved when the input current and the output current of the current conveyor are equal and thus the current gain is unity. An input impedance of greater than zero, or an output impedance of less than infinity, will reduce the current conveyor&#39;s gain to less than unity. 
     In an amplifier where high gain across a wide bandwidth is advantageous, the usefulness of the unity gain current conveyor is limited. The unity gain provided by such typical current conveyor arrangements is a significant disadvantage in amplifier designs where high gain across a wide bandwidth is required. In existing systems, in order to provide high gain, the current conveyor&#39;s limited gain has to be compensated for by increasing the gain requirements of the amplifier sections that precede and follow the current conveyor, in order to achieve a required amplifier gain. These amplifier sections then require an excessive current to provide this gain across a wide bandwidth, and increasing complexity and die area, and reducing efficiency. 
     It is an aim of the present invention to provide an improved current conveyor for an amplifier. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a circuit having a first transistor being a common gate connected transistor and a second transistor, the second transistor being M times the size of the first transistor, the first and second transistors having commonly connected gates and commonly connected drains, wherein an apparatus is provided to regulate the source voltage of the second transistor to track the source voltage of the first transistor, wherein the current gain of the circuit is M+1. 
     The apparatus may comprise a differential amplifier provided to receive the source voltages of the first and second transistors and regulate the source voltage of the second common gate transistor. The output of the differential amplifier may control a transistor which provides the source voltage of the second transistor. 
     The drain of the first transistor may provide the output of the current conveyor. The drain of the first transistor may be connected to one terminal of a current source. The other terminal of the current source may be connected to a supply voltage. The current source may provide a biasing current for the current conveyor of I CC . The current source may provide a bias current of I CC *M/(1+M) to the second transistor. The current source may provide a bias current of I CC /(1+M) to the first transistor. 
     The source of the first transistor may be connected to receive an input to the current conveyor. The circuit may further comprise a third transistor having a gate connected to receive the input to the current conveyor, and for controlling the gate of the first transistor. The drain of the third transistor may be connected to the gate of the first transistor. The drain of the third transistor may be connected to one terminal of a current source. The other terminal of the current source may be connected to a supply voltage. The source of the third transistor may be connected to electrical ground. 
     The circuit may be termed a current controlled source. The circuit may be termed a current conveyor. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention is now described by way of example with reference to the Figures, in which: 
         FIG. 1  illustrates an ideal current conveyor; 
         FIG. 2  illustrates a typical implementation of a current conveyor; 
         FIG. 3  illustrates an improved current conveyor; and 
         FIG. 4  illustrates an envelope tracking power amplification system including a power amplifier in which an improved current conveyor may be advantageously utilised. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is now described by way of reference to preferred examples and preferred implementations. However one skilled in the art will appreciate that the present invention is not limited to its application to the specific examples as set out herein below. 
     The present invention is described herein by way of a circuit termed a current conveyor. However in general there is provided a circuit, which may be termed a controlled current source or may be termed a current conveyor. The term current conveyor is a term which is used in the art to describe a circuit having the functionality of current conveying as described herein, and may be also referred to as a controlled current source. 
     With reference to  FIG. 3 , there is illustrated a multiplying current conveyor in accordance with a preferred implementation. 
     The multiplying current conveyor includes a reference section  302  and a multiplier section  304 . 
     The multiplying current conveyor comprises a CMOS transistor  306  having its source connected to electrical ground and its drain connected to one terminal of a current source  308 . 
     A current source  310  of the reference section  302  has one terminal connected to electrical ground and another terminal connected to a source of a CMOS transistor  312  of the reference section  302 . The drain of the CMOS transistor  312  is connected to one terminal of a current source  314  of the reference section  302 . The current sources  308  and  314  have second terminals connected to a supply voltage V S . 
     The gate of the CMOS transistor  312  of the reference section  302  is connected to the drain of the transistor  306 . The gate of the CMOS transistor  306  is connected to the first terminal of the current source  310  of the reference section. 
     The CMOS transistor  306 , the current source  308 , and the elements of the reference section  302  (the current source  314 , the CMOS transistor  312 , and the current source  310 ) form a unity gain current conveyor according to  FIG. 2 . 
     An output signal is generated on line  322  at the drain of the CMOS transistor  312 . An input signal on line  320  is received at the source of the CMOS transistor  312 . 
     The gate of the CMOS transistor  312  of the reference section  302  is additionally connected to the gate of a CMOS transistor  316  of the multiplier section  304 . 
     The CMOS transistor  316  of the multiplier section  304  has its drain connected to the drain of the CMOS transistor  312  of the reference section  302 . The CMOS transistor  316  has its source connected to the drain of a CMOS transistor  320 . The source of the CMOS transistor  320  is connected to electrical ground. 
     The source of the CMOS transistor  312  provides an inverting input to a difference amplifier  318 . 
     The gate of the CMOS transistor  320  is connected to an output of the difference amplifier  318 . The difference amplifier  318  has its non-inverting input connected to the source of the CMOS transistor  316 . 
     The multiplying current conveyor of  FIG. 3  implements a current gain of (1+M): a gain of unity provided by the reference section  302  plus a gain of M provided by the multiplier section  304 . 
     The current source  314  is biased with a bias current of I CC . 
     The ratio of the current flowing in the drains of the respective transistors  312  and  316  is determined by the physical size of the transistors  312  and  316 . The size of transistor  316  is a multiple of the size of transistor  312 . Because the transistor  316  is M times the size of transistor  312 , it has M times the gain. 
     The reference section  302  implements a current gain of unity. The reference section  302  uses the conventional current conveyor topology of  FIG. 2 . The reference section accepts a signal current I S  on line  320 . The reference section  302  is biased with a current of I CC /(1+M). 
     The multiplying section  304  is biased with a current of I CC *M/(1+M), and implements a current gain of M. The relative size of the multiplying section  304  in comparison to the reference section  302  (which includes the common gate transistor) is equal to the desired multiplication factor M. 
     The source voltage of the common gate transistor  316  of the multiplying section  304  is made identical to the source voltage of the common gate transistor  312  of the reference section by the regulation effect achieved by the difference amplifier  318 . The difference amplifier compares the signal at the source to the transistor  312  to the signal at the source of the transistor  316 , and the difference between the two is used to control the gate of the transistor  320  to control the voltage at the source of the transistor  316 , to make the voltages at the sources of the transistors  312  and  316  equal. 
     Hence, the terminal voltages of the CMOS transistors  312  and  316  are identical: they have a common gate connection, a common drain connection, and a difference amplifier arranged to equalise their source voltages. 
     As the terminal voltages of the reference CMOS transistor  312  and multiplying CMOS transistor  316  are identical, their current densities are also identical. If the reference section  302  conveys a current I S , then the multiplying transistor conveys a current of M·I S , and the combined current gain of the multiplying current conveyor is 1+M. 
     The multiplying current conveyor scheme provided by the implementation of  FIG. 3  provides a gain multiplying advantage over conventional current conveyors, whilst preserving the current conveyor benefits of low input impedance and high output impedance across a wide bandwidth. 
     By providing a current gain, the multiplying current conveyor allows the gain requirements of any preceding and following gain stages to be reduced. 
     By reducing the gain requirements of the preceding and following gain stages, the power requirements of these gain stages will be reduced. 
     By reducing the gain requirements of the preceding and following gain stages, the die area required by these gain stages will be reduced, and overall power conversion efficiency will be increased. 
     With reference to  FIG. 4 , there is illustrated the architecture of a radio frequency (RF) envelope tracking power amplifier system including a power amplifier, which power amplifier may be adapted to include the advantageous current conveyor of  FIG. 3 . 
     As exemplified in  FIG. 4 , an RF input signal on line  408  provides an input to an RF amplifier  402 , which provides an amplifier RF output signal on line  410 . An envelope shaping stage  404  receives the input signal on line  408 , and provides an envelope of the input signal to a supply modulator  406 . The supply modulator includes a first path including an amplifier  412  for providing a main supply voltage. The amplifier  412  may select between switched supply levels. The supply modulator includes a second path including an error amplifier  414 . The outputs of the amplifiers  412  and  414  are combined in a combiner  416 . The output of the combiner  416  provides a second input to the error amplifier  414 . The output of the combiner  416  forms the output of the supply modulator  406 , and provides the modulated supply voltage to the amplifier  402 . 
     The present invention has been described by way of example to its implementation in various exemplary systems. One skilled in the art will understand that the invention may be provided in systems other than those systems which are illustrated.