Abstract:
An apparatus is described comprising a bandgap reference circuit comprising: an amplifier including first and second inputs and an output; and a bandgap transistor coupled to the output of the amplifier at a control electrode thereof, the bandgap transistor being further coupled commonly to the first and second inputs of the amplifier at a first electrode thereof to form a feedback path. The apparatus further comprises a resistor coupled to the first electrode of the bandgap transistor.

Description:
BACKGROUND 
       [0001]    Many electronic circuits are designed for use with a constant current input or bias signal, which may be provided by a constant current source. For example, constant current sources are regularly employed in biasing input buffer circuits, delay circuits, and/or oscillator circuits. Traditional constant current sources employ a bandgap reference circuit using multiple amplifiers. The multiple amplifiers, however, consume substantial power and take up significant space in the circuit. Additionally, multiple amplifier bandgap reference circuits may still suffer from some current variation across operating temperatures. 
       SUMMARY 
       [0002]    An apparatus is described comprising a bandgap reference circuit comprising: an amplifier including first and second inputs and an output; and a bandgap transistor coupled to the output of the amplifier at a control electrode thereof, the bandgap transistor being further coupled commonly to the first and second inputs of the amplifier at a first electrode thereof to form a feedback path. The apparatus further comprises a resistor coupled to the first electrode of the bandgap transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a schematic diagram of a constant current source, in accordance with an embodiment of the present invention. 
           [0004]      FIG. 2  is a schematic diagram of a constant current source with a current mirror circuit, in accordance with an embodiment of the present invention. 
           [0005]      FIG. 3A  is a schematic diagram of a constant current source connected to an input buffer, in accordance with an embodiment of the present invention. 
           [0006]      FIG. 3B  is a schematic diagram of an input buffer, in accordance with the embodiment of  FIG. 3A . 
           [0007]      FIG. 4  is a schematic diagram of a constant current source, in accordance with an embodiment of the present invention. 
           [0008]      FIG. 5  is a graph depicting the output currents of a constant current source, in accordance with an embodiment of the present invention. 
           [0009]      FIG. 6  is a block diagram of a memory, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Certain details are set forth below to provide a sufficient understanding of embodiments of the invention. However, it will be clear to one skilled in the art that embodiments of the invention may be practiced without these particular details. Moreover, the particular embodiments of the present invention described herein are provided by way of example and should not be used to limit the scope of the invention to these particular embodiments. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
         [0011]    Constant current sources provide constant current under a variety of operating conditions. For example, during the operation of a current source, components of the current source may heat up. The change in temperature of the components may alter certain physical properties and result in an output current that changes as the current source heats up. Traditional circuits for generating constant current output signals include bandgap reference circuits. However, traditional bandgap reference circuits typically include multiple amplifiers which, in turn, draw substantial power. Embodiments of the present invention provide constant current sources that may exhibit less temperature dependency and have lower power and space consumption in comparison to traditional constant current sources. The reduced temperature dependency of the current source may be referred to as “temperature independent.” 
         [0012]      FIG. 1  is a schematic diagram of a constant current source, generally designated  100 , in accordance with an embodiment of the present invention. The current source  100  generally includes a bandgap reference circuit  102 , a resistor  114 , and an output circuit  116 . The output circuit  116  is illustrated in the embodiment of  FIG. 1  as p-type field effect transistor (pFET), however, it will be appreciated that other examples of output circuit  116  including different circuits than shown in  FIG. 1  may be used in other embodiments of the invention. 
         [0013]    The bandgap reference circuit  102  may generally be any bandgap reference and provide a reference voltage (an output voltage). In some embodiments, the bandgap reference circuit  102  may provide a reference voltage of 1.25V. In the embodiment of  FIG. 1 , the bandgap reference circuit  102  includes an amplifier  104 , an output transistor  106 , resistors  120 , and diodes  122 A and B (collectively referred to as “diodes  122 ”). The diodes  122  (resistive elements) may exhibit a temperature dependency, such as having a current that varies based on the temperature. In some embodiments, the diodes  122  exhibit an increasing current for increasing temperature. In other words, resistance values of the diodes  122  may represent negative temperature coefficients. In various embodiments, the amplifier  104  may be an operational transconductance amplifier (OTA) or an operational amplifier (op-amp). The amplifier  104  includes non-inverting (+) and inverting (−) inputs, and an output, and is configured to provide an output based on the inputs provided to the non-inverting and inverting inputs. Those skilled in the art will appreciate that embodiments implemented with an op-amp may further include compensation components, such as capacitors. The output transistor  106  is illustrated in the embodiment of  FIG. 1  as a pFET, but other transistors may be used in other embodiments. 
         [0014]    In the depicted embodiment, the output of the amplifier  104  is coupled to the gate of the output transistor  106 . The source of the output transistor  106  is coupled to a supply voltage V PP . The drain of the output transistor  106  may be coupled a node  124  (a current output node) and provide to an output signal  108 . In the depicted embodiment, a first branch  130  of the node  124  provides a feedback signal  110 , which may carry a constant voltage of 1.25V, and a current that is proportional to absolute temperature (“PTAT”), I PTAT  (a first current). Those skilled in the art will appreciate that I PTAT  increases as temperature increases, as discussed in further detail below with respect to  FIG. 4 . 
         [0015]    The current, I PTAT , may be determined based on components to which the feedback signal  110  is provided. In the depicted embodiment, the feedback signal  110  is provided to a positive feedback loop  126  (a first current path) and a negative feedback loop  128  (a second current path). The positive feedback loop  126  includes two resistors  120  and a diode  122 B coupled in series to ground. The resistors  120  may have an associated resistance, R 1 . The resistance, R 1  may represent a positive temperature coefficient. The non-inverting input of the amplifier  104  is coupled to a node between the two series resistors  120  in the positive feedback loop  126  and receives an input voltage V IN2 . The negative feedback loop  128  includes a resistor  120 , having resistance R 1 , and a diode  122 A coupled in series to ground. The inverting input of the amplifier  104  is coupled to the negative feedback loop  128  between the resistor  120  and the diode  122  and receives an input voltage V IN1 . The current, I PTAT , of the feedback signal  110  may be determined based on Ohm&#39;s Law, I PTAT =2×ΔV/R 1  where ΔV is the difference between V BE1  and V BE2  which are voltages of diodes  122 A and  122 B, respectively and depends on the values of the diodes  122 A and  122 B. For example, as previously discussed, the diodes  122 A and  122 B may exhibit an increasing current for increasing temperature. As a result, ΔV may be directly proportional to temperature (e.g., V∝kT/q, where k is Boltzmann&#39;s constant, T is the absolute temperature, and q is the magnitude of the electron charge). Therefore, I PTAT  may also be directly proportional to temperature (as indicated by the acronym PTAT). Those skilled in the art will appreciate that the bandgap reference circuit  102  depicted in  FIG. 1  is provided merely as an example, and other bandgap reference circuits may be used without departing from the scope of this disclosure. 
         [0016]    A second branch  112  of the node  124  is coupled to a resistor  114  having a resistance, R 2 , and to ground. The resistance, R 2 , may represent a positive temperature coefficient. The second branch of the node  124  may provide a current that is complementary to absolute temperature (“CTAT”), I CTAT  (a second current). The current, I CTAT , is equal to the voltage at the node  124  (e.g., 1.25V) divided by the resistor  114  (e.g., R 2 ). In various embodiments, the resistance R 2  of resistor  114  may be selected such that the current, I CTAT , has an opposite temperature dependence to the current I PTAT . For example, I PTAT  may linearly increase with temperature (e.g., I PTAT  increases by 0.1 μA per 100K). In such a case, the resistor  114  is selected such that the current through the resistor  114 , I CTAT , decreases at the same rate (e.g., I CTAT  decreases by 0.1 μA per 100K). In one embodiment, the resistor  114  may have a resistance R 2 =225 kΩ. By providing currents I PTAT  and I CTAT  to have equal and opposite temperature dependencies, the current of the output signal  108  (the output current I STAB ) may remain constant over varying temperatures at I STAB . That is, as the temperature increases, the current through the feedback signal  110  increases and the current through the second branch  112  decreases at the same rate. Therefore, because the sum of I PTAT  and I CTAT  (e.g., the total current leaving the node  124 ) is constant with temperature, the current of the node  124  (e.g., I STAB ) is also constant with temperature. 
         [0017]    The output of the amplifier  104  may also be coupled to the output circuit  116 . The output circuit  116  may have a source coupled to the supply voltage, V PP , and provide an output signal  118  (an output current I OUT ) at the drain having a current, I OUT . In the depicted embodiment, the output circuit  116  is configured as a current mirror with the transistor  106 . That is, I OUT  is the mirror current of I STAB . In some embodiments, the output circuit  116  and the transistor  106  may be matched (e.g., have the same electrical characteristics and performance). In other embodiments, the channel size (a ratio of the channel width to the channel length) of the output circuit  116  may be adjusted relative to that of the output transistor  106  to compensate for differences between the current of the output signal  118  and the output signal  108 . In some embodiments, the channel size of the output circuit  116  may be N times greater or less than that of the output transistor  106  in order to cause I OUT  to be N times greater or less than I STAB . By selecting the resistor, R 2 , of the resistor  114  to create a current, I CTAT , that complements the temperature variability of the current I PTAT , and mirroring the current, I STAB , of the output signal  108  to the current, I OUT , of the output signal  118 , the current source  100  provides a temperature independent, constant current output which may be provided to any other component or circuit that requires a constant current source. 
         [0018]      FIG. 2  is a schematic diagram of a constant current source, generally designated  200 , in accordance with an embodiment of the present invention. The current source  200  generally includes a bandgap reference circuit  202 , a resistor  214 , an output circuit  216 , and a current mirror circuit  230 . The output circuit  216  is illustrated in the embodiment of  FIG. 2  as p-type field effect transistor (pFET), however, it will be appreciated that other examples of output circuit  216  including different circuits than shown in  FIG. 2  may be used in other embodiments of the invention. 
         [0019]    In various embodiments, the bandgap reference circuit  202  may be implemented as the bandgap reference circuit  102  described above with respect to  FIG. 1 . For instance, the amplifier  204  may be implemented as the amplifier  104 , the output transistor  206  may be implemented as the output transistor  106  to provide an output signal  208 . As described above with respect to the node  124 , a first branch  238  of the node  224  may provide a feedback signal  210  to a positive feedback loop  226  and a negative feedback loop  228 . The positive feedback loop may include resistors  220  and a diode  222 B, which may be implemented as resistors  120  and diode  122 B, as described above with respect to  FIG. 1 . The negative feedback loop  228  may include a resistor  220  and a diode  222 A, which may be implemented as resistor  120  and diode  122 A, as described above with respect to  FIG. 1 . Each of the positive and negative feedback loops  226  and  228  may be coupled to the amplifier  204  as described above with respect to the positive and negative feedback loops  126  and  128  in  FIG. 1 . A second branch  212  of the node  224  may include the resistor  214 , which may be implemented as described above with respect to the resistor  114  to have a current I CTAT  to complement the current, I PTAT  on the feedback signal  210 . The output of the amplifier  204  may be provided to the output circuit  216  as described above with respect to the output circuit  116 . 
         [0020]    The current mirror circuit  230  provides an output current, I OUT , that is based on the temperature independent current, I STAB  provided by the output transistor  206 . The current mirror circuit  230  may include an amplifier  232  and a transistor  236 . In one embodiment, the amplifier  232  is an OTA. The transistor  236  is illustrated in the embodiment of  FIG. 2  as pFET, however, it will be appreciated that other circuits may be used in other embodiments of the invention. The transistor  236  may be matched to the transistors  206  and a transistor of the output circuit  216 . The amplifier  232  may have a non-inverting input terminal coupled to the node  224 . As described above with respect to node  124  in  FIG. 1 , node  224  may have a constant voltage equal to the bandgap reference voltage (e.g., 1.25V). The inverting input of the amplifier  232  may be coupled to the output circuit  216 , which provides a constant voltage equal to the bandgap reference voltage, V bgr =1.25. The output of the amplifier  232 , is coupled to the transistor  218 . The source of the transistor  236  may be coupled to the output circuit  216 , and the drain of the transistor  236  may provide an output signal  218  having a current, I OUT . In the depicted embodiment, the current mirror circuit  230  mirrors the current, I STAB , from the drain of the transistor  206  to the current of the output signal  218 , I OUT . The amplifier  232  provides a voltage at a gate of the transistor  236  to maintain the source of the transistor  236  at the same voltage of the node  224 , thereby ensuring that the current I OUT  is the same as the current I STAB . If the voltage at the source of the transistor  236  varies, the amplifier  232  adjusts the voltage provided to the gate of the transistor  236  to return the source voltage to that of the node  224 . Those skilled in the art will appreciate that in embodiments where the transistor of the output circuit  216  is the same as the output transistor  206 , a signal provided by the output circuit  216  may not mirror the current of the output signal  208 . Therefore, it may be beneficial to include the current mirror  230  to ensure that the output current of the current source  200  mirrors the current of the output signal  208 . 
         [0021]      FIG. 3A  is a schematic diagram of a constant current source, generally designated  300 , coupled to an input buffer  342 , in accordance with an embodiment of the present invention. Those skilled in the art will appreciate that the input buffer  342  may be replaced by a delay circuit, an oscillator, or any other circuit that can be implemented with a current source having reduced temperature dependence. In various embodiments, the output of the current sources  100 ,  200 , and  300  may be coupled to any type of circuit that uses a constant current. The current source  300  generally includes a bandgap reference circuit  302 , a resistor  314 , and output circuit  316 , and a current mirror circuit  330 , which provides a current to the input buffer  342  via a current mirror circuit including transistors  338  and  340 . 
         [0022]    In various embodiments, the bandgap reference circuit  302  may be implemented as described above with respect to bandgap reference circuits  102  and  202 . The bandgap reference circuit  302  may include an amplifier  304 , a transistor  306  coupled to the output of the amplifier  304 . The transistor  306  may have a source coupled to a voltage, V PP , and may provide an output signal  308  having a current, I STAB , that is provided to a node  324 . A first branch  344  of the node  324  may provide a feedback signal  310 , having a current, I PTAT , that is coupled to a positive feedback loop  326  and a negative feedback loop  328 . The positive feedback loop may include two resistors  320  and a diode  322 B coupled in series to ground. A non-inverting input of the amplifier  304  may be coupled to the positive feedback loop  326  between the resistors  320  and provide a voltage, V IN2 . The negative feedback loop  328  may include a resistor  320  coupled in series with a diode  322 A to ground. An inverting input of the amplifier  304  is coupled to the resistor  320  and is provided a voltage, V IN1 . 
         [0023]    A second branch of the node  324  may be coupled through a resistor  314  to ground. The current through the resistor  314  may be complementary to absolute temperature and have a value, I CTAT . In various embodiments, the current I CTAT  decreases as temperature increases. The current, I PTAT , provided on feedback signal  310  increases with temperature. The currents I CTAT  and I PTAT  change with temperature at equal and opposite rates. Therefore, because I CTAT  and I PTAT  complement each other with changing temperature, the input current, I STAB , remains constant with changing temperature. 
         [0024]    The current, I STAB , is mirrored to the output circuit  316 , which is coupled to the output of the amplifier  304 . The output circuit  316  is further coupled to the voltage V PP . The output circuit  316  may be coupled to a current mirror circuit  330 . The current mirror circuit  330  may be implemented as the current mirror circuit  230 , as described above with respect to  FIG. 2 . The current mirror circuit  330  may include an amplifier  332  and a transistor  336 . The output circuit  316  may be coupled to an inverting input of the amplifier  332  and to a source of the transistor  336 . The non-inverting input of the amplifier  332  may be coupled to the node  324 . The output of the amplifier  332  is provided to the gate of the transistor  336 , which provides an output signal  318 . The output signal  318  has a current, I OUT , which is equal to the current, I STAB . The output signal  318  may be provided to diode coupled transistor  338 , which is coupled to the gate of a second transistor  340 . The transistor  340  may provide a constant current signal to the input buffer  342  mirrored by the transistors  338  and  340  based on the current I OUT  provided by the current mirror circuit  330 . In the embodiment of  FIG. 3 , a particular application of the current source  300  is shown as a bias current to an input buffer. For example, the input buffer  342  may be an input buffer for a dynamic random access memory (DRAM) device as discussed in further detail below with respect to  FIG. 6 . 
         [0025]      FIG. 3B  is a schematic diagram of the input buffer  342 , in accordance with the embodiment of  FIG. 3A . In the embodiment of  FIG. 3B , the input buffer  342  is a two stage input buffer configured to receive a bias signal from the current source  300  in  FIG. 3A . The input buffer  342  generally includes a first buffer stage  348 , a second buffer stage  346 , and mirror transistors  350  and  352 . As discussed above with respect to  FIG. 3A , the output signal  318 , which may have reduced temperature dependency, may be mirrored to the input buffer  342  by transistors  338  and  340 . The output signal  318  may provide a biasing signal to the mirror transistors  350  and  352 . In the embodiment of  FIG. 3B , the mirror transistor  350  may mirror the output signal  318  to the first buffer stage  348 . The first buffer stage  350  may be configured to receive an input signal, I N , and a reference signal V REF  and provide an output signal to the second stage  346  based on the output signal  318 . The second stage  346  may be configured to receive signals from the first stage  348  and provide a buffered signal based on the output signal  318  provided to the mirror transistor  352 . 
         [0026]      FIG. 4  is a schematic diagram of a current source, generally designated  400 , in accordance with an embodiment of the present invention. The current source  400  may include a bandgap reference circuit  402 , a resistor  414 , and an output circuit  416 . The bandgap reference circuit  402  may include an amplifier  404 , an output transistor  406 , resistors  420  having resistances, R 1 , and transistors  422 A and  422 B. In the depicted embodiment, the amplifier  404  provides a signal to the output transistor  406  and the transistors  422 A and  422 B. The output transistor  406  may receive a voltage, V PP , and provide an output signal  408  to a node  424  based on the output signal of the amplifier  404  and the voltage, V PP . The node  424  may be coupled to a first branch  430  and a second branch  412 . The first branch may provide a feedback signal  410 , which may carry a current, I PTAT , which is proportional to absolute temperature. 
         [0027]    The feedback signal  410  may be provided to the resistors  420  in a positive feedback loop  426  and a negative feedback loop  428 . The positive feedback loop  426  may include a resistor  420  coupled in series to the transistor  422 A, and two additional resistors  420 . The positive feedback loop  426  may provide a signal V IN2  to a non-inverting input of the amplifier  404 . The negative feedback loop  428  may include a resistor  420  coupled in series to the transistor  422 B and a resistor  420 . The negative feedback loop  428  may provide a signal V IN1  to an inverting input of the amplifier  404 . 
         [0028]    The second branch  412  may include a resistor  414  having a resistance R 2  coupled to ground. The resistance R 2  may be selected such that the current, I CTAT , through the resistor  414  is complementary to absolute temperature. That is, the current I CTAT  through the resistor  414  has temperature dependency that is equal in magnitude and opposite in direction to the temperature dependency of the feedback signal  410 . Because the currents I PTAT  and I CTAT  through the first branch  430  and second branch  412  have equal and opposite temperature dependency, the current I STAB  through the output signal  408  may demonstrate reduced temperature dependency. 
         [0029]    The output signal of the amplifier  404  may also be provided to an output circuit  416  which may include, for example, a transistor having similar channel size to the output transistor  406 . The output circuit  416  may provide an output signal  418  having a current, I OUT . In some embodiments, the current of the output signal  418  may mirror the current of the output signal  408 . That is, the current I OUT  may have reduced temperature dependency compared to traditional current sources. In other embodiments, the transistor in the output circuit  416  may have a channel size that is adjusted relative to the channel size of the output transistor  406  such that the current of the output signal  418  mirrors the current of the output signal  408 . As described above with respect to  FIG. 1 , the output signal  418  may be provided to any of a number of circuits including input buffers, oscillator circuits, delay circuits, or any other type of circuit that may benefit from a signal having reduced temperature dependence. 
         [0030]      FIG. 5  is a graph depicting the output currents of a temperature independent constant current source, in accordance with an embodiment of the present invention. The graph shows temperature on the horizontal axis and current on the vertical axis. As described above, I PTAT  is proportionally related to temperature, such that the current increases as temperature increases. I CTAT  is inversely proportionally related to temperature, such that current decreases as temperature increases. The temperature dependencies of I PTAT  and I CTAT  are equal and opposite such that when I PTAT  and I CTAT  are added together, a temperature independent, constant current, I STAB , is produced. The temperature independent, constant current, I STAB , may be provided to any electrical components that benefit from the use of a temperature independent, constant current. 
         [0031]      FIG. 6  is a block diagram of a memory, according to an embodiment of the invention. The memory  600  may include an array  602  of memory cells, which may be, for example, volatile memory cells (e.g., dynamic random-access memory (DRAM) memory cells, static random-access memory (SRAM) memory cells), non-volatile memory cells (e.g., flash memory cells), or some other types of memory cells. The memory  600  includes a command decoder  606  that may receive memory commands through a command bus  608  and provide (e.g., generate) corresponding control signals within the memory  600  to carry out various memory operations. For example, the command decoder  606  may respond to memory commands provided to the command bus  608  to perform various operations on the memory array  602 . In particular, the command decoder  606  may be used to provide internal control signals to read data from and write data to the memory array  602 . Row and column address signals may be provided (e.g., applied) to an address latch  610  in the memory  600  through an address bus  620 . The address latch  610  may then provide (e.g., output) a separate column address and a separate row address. 
         [0032]    The address latch  610  may provide row and column addresses to a row address decoder  622  and a column address decoder  628 , respectively. The column address decoder  628  may select bit lines extending through the array  602  corresponding to respective column addresses. The row address decoder  622  may be connected to a word line driver  624  that activates respective rows of memory cells in the array  602  corresponding to received row addresses. The selected data line (e.g., a bit line or bit lines) corresponding to a received column address may be coupled to a read/write circuit  630  to provide read data to an output data buffer  634  via an input-output data path  640 . Write data may be provided to the memory array  602  through an input data buffer  644  and the memory array read/write circuit  630 . The input data buffer  644  may receive a signal from a constant current source according to an embodiment of the present invention, for example, a constant current source as described above with respect to  FIGS. 1-4 . For example, the input data buffer  644  may use a constant current bias in one or more input buffer stages. 
         [0033]    Those of ordinary skill would further appreciate that the various illustrative logical blocks, configurations, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software executed by a processor, or combinations of both. Various illustrative components, blocks, configurations, modules, circuits, and steps have been described above generally in terms of their functionality. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.