Abstract:
A microcontroller integrated circuit includes an open-loop transimpedance amplifier (OLTA). An input lead of the OLTA is a terminal of the microcontroller. The cathode of a photodiode is connected to VDD and the anode is connected to the terminal. The OLTA maintains the photodiode in a strongly reverse-biased condition, thereby keeping diode capacitance low and facilitating rapid circuit response. The input of the OLTA involves a diode-connected field effect transistor that provides a low impedance. This low impedance decreases as the diode current increases, thus providing effective clamping of the voltage on the terminal. By this clamping, the amount of photodiode capacitance discharging necessary when transitioning from a high input current condition to a low input current condition is reduced, thereby further improving amplifier response time. The OLTA is small and consumes less than thirty microamperes and functions to mirror photodiode current and compare to a predetermined level.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of, and claims priority under 35 U.S.C. §120 from, nonprovisional U.S. patent application Ser. No. 11/479,037 entitled “Open-Loop Transimpedance Amplifier for Infrared Diodes,” filed on Jun. 30, 2006, now U.S. Pat. No. 8,269,562, the subject matter of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The described embodiments relate to infrared receiver circuits, and more particularly to an infrared receiver circuit that is fully integrated onto a microcontroller integrated circuit within a learning remote control device. 
     BACKGROUND INFORMATION 
     Manufacturers of electronic consumer devices (for example, televisions, radio tuners, home theatre and entertainment systems, digital video disc (DVD) players, video cassette recorders (VCR), compact disc (CD) players, set-top cable television boxes, set-top satellite boxes, video game controllers, home appliances, etc.) typically supply an infrared remote control device along with each electronic consumer device. Such an infrared remote control device is often a handheld battery-powered device with a set of keys and an infrared (IR) transmitter. The remote control device can control the associated electronic consumer device by sending an appropriate infrared operational signal to the electronic consumer device. The operational signal carries a key code. Each such key code corresponds to an associated function of the selected electronic consumer device. Such functions may include power on/off, volume up, volume down, play, stop, select, channel advance, channel back, etc. 
     If, for example, an individual in the home wishes to increase the volume of a television, then the individual presses the “volume up” key on the remote control device for the television. Circuitry in the remote control device detects the key press condition, accesses appropriate key code and modulation information stored in the remote control device, and uses the key code and modulation information to generate an appropriate control signal that is used to drive an infrared light emitting diode (LED). This control signal causes the LED to transmit the infrared operational signal to an infrared receiver in the television. The key code is carried by the operational signal. The infrared receiver in the television receives the infrared operational signal, detects the key code, and takes an action that is appropriate for the key code. In the present example where the “volume up” key was pressed, the appropriate action is to increase the audio output volume of the television. 
     A typical user in the home may have many different electronic consumer devices that are to be controlled. A user may, for example, have a digital video disc (DVD) player and a television. To view a movie on a DVD, the user may have to power on and control the DVD with a first remote control device that issues operational signals that the DVD player responds to. In addition, the user may have to power on and control the television with a second remote control device that issues operational signals that the television responds to. It is desired to reduce the number of remote control devices in this situation to one such that a single remote control device is usable to control both electronic consumer devices (the DVD player and the television). 
     A type of remote control device referred to as a “learning remote control device” may be employed to replace both remote control devices in the exemplary situation described above. The learning remote control device has infrared receiver circuitry as well as conventional infrared transmitter circuitry. The learning remote control device is placed such that the infrared receiver of the learning remote control device can receive infrared operational signals transmitted from one of the remote control devices to be replaced. A key on the remote control device to be replaced is then pressed. The infrared receiver in the learning remote receives the infrared operational signal and stores information about the operational signal such that the learning remote control device can later regenerate the operational signal using the infrared transmitter circuitry of the learning remote control device. This process of detecting and storing information that is usable to regenerate an operational signal is called “learning”. 
     After the learning remote control device has learned how to regenerate operational signals output from one remote control device to be replaced, the learning remote control device learns how to regenerate operational signals output from another remote control device to be replaced. Thereafter, the user can use the learning remote control device to emulate either the first or the second remote control device. The user controls which of the two remote control devices will be emulated by changing a mode of the learning remote control device. The learning remote control device is therefore now usable to control the both electronic consumer devices in the home, thereby replacing the multiple manufacturer-supplied remote control devices. 
     The circuitry in the learning remote control device generally includes a microcontroller integrated circuit, an infrared photodiode, and an infrared receiver circuit. The infrared receiver circuit receives a signal from the infrared photodiode and outputs a digital output signal onto a serial input terminal of the microcontroller integrated circuit. The infrared receiver circuit is typically a fairly expensive circuit that consumes a substantial amount of power when it is functioning. Traditional techniques involve realizing the infrared receiver in discrete circuit components (including discrete resistors and/or capacitors) located outside the microcontroller integrated circuit. In one example, the infrared receiver circuit includes multiple operational amplifier gain stages, each including a feedback loop having resistors. The operational amplifier circuit consumes three hundred microamperes or more when it is receiving an infrared signal from an infrared photodiode. In another example, a cascode bipolar transistor amplifier circuit involves multiple resistors and a capacitor in a biasing network. If either of these traditional infrared receiver circuits were to be integrated into the microcontroller integrated circuit, then the resistors and capacitors and/or the complex operational amplifier circuitry would consume an undesirably large amount of die area, thereby increasing the cost of the microcontroller integrated circuit. Accordingly, integrating an infrared receiver circuit onto a microcontroller integrated circuit that is to see general usage in non-learning remote control devices can be recognized to be unacceptably expensive. Moreover, the hundreds of microamperes of power consumed by such a traditional infrared receiver would be undesirable. An improved microcontroller integrated circuit that has an improved, fully-integrated infrared receiver is desired. 
     SUMMARY 
     It is recognized that the high sensitivity afforded by conventional photocurrent operational amplifier circuits is unnecessary in a learning remote control device where the remote control device from which an infrared operational signal is to be learned is generally placed in close proximity to the photodiode of the learning remote control device. In addition, the photodiode in a learning remote control device involves a parasitic capacitance. Ordinary amplifier circuits used in learning remote control devices to amplify photocurrents can have significant input impedances and can therefore be slow in discharging the photodiode&#39;s parasitic capacitance, thereby contributing to slow response times. 
     A novel microcontroller integrated circuit is disclosed that includes a novel open-loop transimpedance amplifier (OLTA). An input node of the OLTA is an input terminal of the microcontroller. In one embodiment, when the microcontroller is used in a learning remote control device, the cathode of a photodiode external to the microcontroller integrated circuit is connected to a supply voltage VDD and the anode of the photodiode is connected to the input terminal. The photodiode supplies a photocurrent onto the input terminal of the microcontroller (and therefore onto the input node of the OLTA). If the input photocurrent is less than a “trip point input current” (for example, due to dark conditions), then the OLTA forces a digital output signal DATA on an OLTA output lead to a digital logic high value. If the input photocurrent is more than the “trip point input current” (for example, due to the infrared photodiode receiving infrared radiation from an operational signal), then the OLTA forces the signal DATA to a digital logic low value. 
     The OLTA includes a diode-connected N-channel transistor circuit as an input stage. The diode-connected N-channel transistor circuit biases the voltage on the input terminal under dark conditions (photodiode not activated by light) at approximately one N-channel transistor Vt (threshold voltage) above ground potential. Because the cathode of the photodiode is coupled to VDD and the anode is coupled to the input terminal, the photodiode is biased in a strongly reverse-biased condition. Keeping the photodiode strongly reverse-biased minimizes the parasitic capacitance of the photodiode and thereby facilitates fast response times of the OLTA. 
     The diode-connected N-channel transistor of the input stage of the OLTA also causes the input terminal of the microcontroller to have a low input impedance. In one example, the input impedance is forty ohms or less when input currents of eight milliamperes of more are being received into the input terminal. 
     In another novel aspect, the input impedance of the input terminal decreases as the diode current flowing into the terminal increases, thus providing effective clamping of the voltage on the input terminal of the microcontroller. The input voltage on the input terminal is clamped for photocurrents over a wide dynamic range (for example, from zero photocurrent to approximately 10 milliamperes of photocurrent). By this clamping, less discharging of photodiode parasitic capacitance when transitioning from a high input current condition to a zero input current condition is required, thereby further improving amplifier response time. The OLTA involves no feedback loop, no large resistors and/or capacitors and/or operational amplifiers, and therefore can be integrated into a small die area. In one example, the OLTA: 1) is realized in 20,000 square microns of integrated circuit die area, and 2) consumes less than 30 microamperes of supply current when receiving a photodiode input current signal having an amplitude of greater than 100 microamperes and having a signal rate from zero of up to at least five hundred kHz. The OLTA functions over a wide supply voltage range and it requires no voltage references or power supply voltage dividers. 
     Further details and embodiments are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  is a diagram of a learning remote control device that includes a photodiode  3  and a novel microcontroller integrated circuit (the microcontroller integrated circuit is contained in the plastic enclosure of the remote control device). The novel microcontroller integrated circuit includes a novel OLTA (open-loop transimpedance amplifier) in accordance with one novel aspect. 
         FIG. 2  is a circuit diagram showing the photodiode  3  of  FIG. 1  coupled to the novel OLTA  101  within microcontroller integrated circuit  100 . 
         FIG. 3  is a waveform diagram showing operation of the OLTA of  FIGS. 1 and 2  when a photodiode input current having a digital amplitude of 100 microamperes flows into the OLTA. 
         FIG. 4  is a waveform diagram showing operation of the OLTA of  FIGS. 1 and 2  when a photodiode input current having a digital amplitude of 10 milliamperes flows into the OLTA. 
         FIG. 5  is a diagram of an IV (current-to-voltage) input characteristic of the OLTA of  FIGS. 1 and 2 . 
         FIG. 6  is a flowchart of a method in accordance with one novel method. 
         FIG. 7  is a circuit diagram of another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of a learning remote control device  1  that includes a novel microcontroller integrated circuit. Learning remote control device  1  is usable to receive and “learn” an infrared operational signal  2  transmitted from another infrared remote control device. Operational signal  2  is received by an infrared photodiode  3 . The novel microcontroller integrated circuit (not shown in  FIG. 1 ) is disposed within the plastic housing of the learning remote control device  1 . The microcontroller integrated circuit is coupled to infrared photodiode  3  as explained in further detail below. Learning remote control device  1  also includes infrared transmitter circuitry and an infrared light emitting diode (LED)  4  for transmitting an infrared operational signal  5 . An infrared operational signal that is “learned” by learning remote control device  1  in association with a key  6  can be regenerated and transmitted from learning remote control device  1  at a later time by pressing key  6  when learning remote control device  1  is in an appropriate mode of operation. 
     An infrared operational signal can be “learned” in the sense that timing information on when the received operational signal transitions low and high is stored and then the microcontroller later uses this timing information to regenerate a facsimile of the received operational signal. Alternatively, an infrared operational signal can be “learned” by detecting when the received operational signal transitions low and high and then by using this information to search a group codesets to identify which codeset contains information for generating an operational signal having similar high and low transition timing. Once the proper codeset is identified, then the microcontroller uses a key code and modulation information and other information in the codeset to regenerate the operational signal. 
       FIG. 2  is a simplified circuit diagram showing the incoming infrared operational signal  2 , infrared photodiode  3 , and a novel microcontroller integrated circuit  100  that includes a novel open-loop transimpedance amplifier (OLTA)  101 . The vertical dashed line  102  in  FIG. 2  represents a boundary of microcontroller integrated circuit  100 . The cathode of diode  3  is coupled to a power supply (VDD) terminal  103  of microcontroller integrated circuit  100 . The anode of diode  3  is coupled to photodiode current input terminal (PD)  104 . An input lead  105  of OLTA  101  is directly coupled to an input terminal (PD)  104  of the microcontroller and to an OLTA input node N 1 . In the illustrated specific embodiment, an output lead  106  of OLTA  101  is coupled to a data input lead of a timer  107 . A digital processor  108  reads and controls timer  107 , thereby obtaining timing information about when the various edges of a digital signal passing into the timer  107  occurred. Timer  107  and processor  108  are specific to the particular embodiment depicted here. Other suitable circuitry for interfacing a digital processor of a microcontroller to a photocurrent amplifier circuit that outputs a digital signal can be employed. In the example of  FIG. 2 , microcontroller integrated circuit  100  and photodiode  3  are powered by two AA batteries  109  and  110 . The proper operating range of the supply voltage VDD between supply terminal  103  and ground terminal  145  is from 1.8 volts to 3.6 volts. 
     Input terminal  104  has an input impedance less than forty ohms when an input current  111  of eight milliamperes or more is flowing into input terminal  104 . OLTA  101  has a non-linear IV (current-to-voltage) characteristic. As the input current  111  increases, the voltage on input node N 1  increases proportional to the square root of the input current increase. Accordingly, the input impedance decreases as input current  111  (in an input current operating range) increases. The input current operating range in the example of  FIG. 2  extends from zero microamperes (when no infrared signal is being received onto diode  3 ) to approximately ten milliamperes (when a strong infrared signal is being received onto diode  3 ). Because the input impedance decreases more rapidly than the input current increases over this input current range, the voltage on PD input terminal  104  is effectively clamped to an input voltage range of less than approximately 0.7 volts for levels of input current  111  in the normal input current operating range (from zero to 10 milliamperes) of the circuit. If input current  111  is less than a “trip point input current” of approximately 32.0 microamperes, then a digital logic zero signal is output onto output lead  106 . If input current  111  is more than the “trip point input current”, then a digital logic high value is output onto output lead  104 . This 32.0 microampere trip point input current level is referred to here as the “sensitivity” of the amplifier. The higher the trip point input current level, the less “sensitive” the amplifier is said to be. In one advantageous aspect, OLTA  101  has a sensitivity less than 20 microamperes (the “trip point input current” is higher than 20 microamperes) but nonetheless functions acceptably in the application of “learning” an infrared operational signal emitted from a remote control device. 
     OLTA  101  receives a 0.67 microampere bias current (IIN)  112  on a bias current input lead  113  from elsewhere on the microcontroller integrated circuit. If OLTA  101  is enabled by virtue of an enable signal (EN) on enable input lead  114  being a digital logic high, then the passgate formed by transistors  115  and  116  is conductive. The bias current  112  that flows through N-channel transistor  117  is mirrored and multiplied by twelve by the current mirror formed by N-channel transistors  117  and  118 . A mirrored current  119  of approximately 8.0 microamperes therefore flows through N-channel transistor  118 . This mirrored current  119  also flows through P-channel transistor  120 . The 8.0 microamperes of current  119  flowing through P-channel transistor  120  is in turn mirrored by P-channel mirroring transistors  121 - 124  into four corresponding mirrored currents  125 - 128 . The relative sizes of P-channel transistors  121 - 124  to P-channel transistor  120  determines the relative magnitudes of the currents  125 - 128 . In the example of  FIG. 2 , the four currents  125 - 128  are 8.0 microamperes, 4.0 microamperes, 4.0 microamperes, and 8.0 microamperes, respectively. 
     Dark Condition Operation: 
     In operation, when there is a dark condition (substantially no infrared radiation is being received by diode  3 ) and diode photocurrent  111  is approximately zero, then substantially no current is flowing into input terminal  104  and to node N 1 . The 8.0 microamperes of current  125  from P-channel transistor  121  therefore flows from drain to source through a so-called “diode-connected N-channel transistor”  129 . 
     Diode-connected N-channel transistor  129  is not a real diode in the sense that a real diode or a diode-connected bipolar transistor has adjacent oppositely doped semiconductor regions and has an exponential current-to-voltage relationship for forward voltages across the junction. Rather, diode-connected N-channel transistor  129  is an N-channel transistor whose drain is connected to its gate to form a two terminal device. The gate-drain node is a first terminal. The source node is a second terminal. In an N-channel transistor in the saturation region of operation, the drain current is roughly proportional to the square to the gate-to-source voltage when a forward gate-to-source voltage on the transistor is greater than a threshold voltage (Vth). Accordingly, if the drain and gate of an N-channel transistor are connected as a so-called “diode-connected N-channel transistor”, and if a forward current is made to flow from drain to source through the transistor, then increases in the drain current will roughly be proportional to the square of the corresponding increases in gate-to-source voltage. Due to this operation, which is similar to the operation of a real diode in some respects, an N-channel transistor used in this way whose drain is connected to its gate is referred to here as a “diode-connected N-channel transistor”. 
     In OLTA  101  of  FIG. 2 , transistors  130  and  131  form a passgate. This passgate is conductive when the enable signal EN is a digital logic high. The gate  132  of N-channel transistor  129  is therefore connected by the passgate to the transistor&#39;s drain at node N 1 . The body of N-channel transistor  129  is coupled to ground potential. Due to the action of P-channel pullup transistor  121 , the gate-to-source voltage across diode-connected N-channel transistor  129  is approximately the threshold voltage (approximately 0.5 volts) of N-channel transistor  129 . N-channel transistor  129  therefore conducts the drain current supplied by P-channel transistor  121 . Further increases in forward drain current  134  (above the drain current at which transistor  129  begins to conduct) result in much smaller gate-to-source voltage increases. As set forth above, in a diode-connected N-channel transistor the drain current increases proportionally to the square of the increase in gate-to-source voltage. Diode-connected N-channel transistor  129  therefore functions to bias the voltage on input terminal  104  to approximately one threshold voltage (approximately 0.5 volts) above ground potential on ground conductor  146 . 
     The 8.0 microamperes of current  134  flowing through input transistor  129  due to P-channel transistor  121  is mirrored by a current mirror formed by N-channel transistors  129  and  133 . Input transistor  129  has a width/length ratio of 600/1 whereas mirroring transistor  133  has a width/length ratio of 60/1. Mirrored current  135  flowing through transistor  133  is therefore one tenth as small as current  134 . Current  135  is therefore 0.8 microamperes. Because P-channel transistor  122  is to supply the 4.0 microampere current  135  into node N 2  and because N-channel transistor  133  only sinks 0.8 microamperes of current  135  to ground, the voltage on node N 2  is pulled high. The voltage on node N 2  increases until the drain of P-channel transistor  122  increases to the point that transistor  122  no longer acts as a current mirror. 
     When the voltage on node N 2  is adequately high, N-channel transistor  136  becomes conductive. N-channel transistor  136  can sink more than 4 microamperes of current to ground potential, so transistor  136  being made conductive overdrives P-channel transistor  123  and pulls the voltage on node N 3  to a digital logic low. When the voltage on node N 3  falls, N-channel transistor  137  is made non-conductive. P-channel transistor  124  therefore pulls the voltage on node N 4  upward until the drain of P-channel transistor  124  increases to the point that transistor  124  no longer acts as a current mirror. When the enable signal EN is a digital logic high, P-channel transistor  138  is non-conductive and digital logic high values are present on the upper input leads of NAND gates  139  and  140 . The digital logic high on node N 4  is therefore inverted by NAND gate  139 , is again inverted by NAND gate  140 , and is inverted again by inverter  141 . The resulting digital logic low value is output as digital signal DATA onto OLTA output lead  106 . 
     Input Current Trip Point Operation: 
     If 32.0 microamperes of photocurrent  111  is flowing into PD input terminal  102  and to node N 1 , then this photocurrent adds to the 8.0 microamperes of current  125  such that current  134  flowing through diode-connected N-channel transistor  129  is 40.0 microamperes. Due to the operation of the current mirror of transistors  129  and  133 , mirrored current  135  is one tenth of 40.0 microamperes or 4.0 microamperes. This 4.0 microamperes of current  135  matches the 4.0 microamperes of current  126  supplied by P-channel transistor  122 . If current  135  is any greater than 4.0 microamperes, then the voltage on node N 2  will be pulled down and OLTA  101  will force the output signal DATA to a digital logic high value. If current  135  is any smaller that 4.0 microamperes, then the voltage on node N 2  will be pulled up and OLTA  101  will force the output signal DATA to a digital logic low value. Accordingly, 32.0 microamperes of input current  111  represents the input current trip point between outputting a digital logic low value of signal DATA (representing a dark condition) onto OLTA output lead  106  and outputting a digital logic high value of signal DATA (representing an infrared radiation detected condition) onto OLTA output lead  106 . 
     Infrared Radiation Detected Condition: 
     When adequate infrared radiation is detected by diode  3 , then input photocurrent  111  has a magnitude that exceeds 32.0 microamperes. Consider a situation where the input photocurrent is 100 microamperes. 108 microamperes are therefore flowing into node N 1 , and current  134  flowing through diode-connected N-channel transistor is 108.0 microamperes. Mirrored current  135  is therefore 10.8 microamperes. N-channel transistor  133  overdrives the current mirror of transistor  122 . Node N 2  is therefore pulled to ground potential. The low voltage on the gate of N-channel transistor  136  makes N-channel transistor  136  non-conductive such that P-channel transistor  123  pulls the voltage on node N 3  high. The voltage on node N 3  stops rising when P-channel transistor  119  is saturated and is no longer acting as a current mirror. The high voltage on node N 3  makes N-channel transistor  137  adequately conductive that it overdrives the current mirror of P-channel transistor  124 . The voltage on node N 4  is therefore below the switching voltage of NAND gate  139 . The digital logic low on the lower input lead of NAND gate  139  is inverted three times by NAND gate  139 , NAND gate  140  and inverter  141  such that a digital logic high value of the signal DATA is output onto output lead  106 . 
       FIG. 3  is a waveform diagram. Waveform  200  represents the voltage on input terminal  104  when an input current  111  having a digital logic low level of zero amperes and a digital logic high level of 100 microamperes is supplied onto OLTA input lead  105 . Waveform  201  represents the resulting DATA signal output onto OLTA output lead  106 . 
     When diode  3  is highly illuminated with infrared radiation, diode  3  can output a significant amount of current. In the present example, diode  3  can output ten milliamperes or more. The photodiode has a parasitic capacitance between its cathode and anode terminals. If a traditional infrared photocurrent amplifier circuit having a high input impedance were directly connected to the cathode terminal of diode  3 , and if the infrared amplifier circuit were a voltage detecting device, then when diode  3  suddenly switched from a highly illuminated condition to a dark condition, the voltage on the input node of the amplifier would remain on the input node due to the capacitance of the diode  3  itself. Only as the voltage on the input node decays to the voltage trip point of the infrared receiver circuit would the amplifier receiver circuit detect the dark condition. The result would be a slow response time of the infrared amplifier receiver circuit when going from an illuminated condition to a dark condition. The slow response time is made worse under conditions of very high illumination. Due to the substantial input impedance of the infrared receiver circuit, the high diode current corresponding to the very high illumination would cause the voltage on the input of the receiver circuit to rise a significant amount. When the diode suddenly stops outputting photocurrent due to a dark condition, the high voltage on the input of the receiver circuit would have to discharged down to the trip point of the receiver circuit before the receiver circuit could detect a dark condition. The higher the voltage on the input node, the longer it would take to discharge the diode capacitance on the input of the receiver circuit. 
     PD input terminal  104 , however, has a low input impedance. When diode current  111  falls rapidly upon a transition from an illuminated diode condition to a dark diode condition, the low input impedance of terminal  104  facilitates discharging of the diode capacitance through the input terminal  104  and thereby decreases amplifier response time. In the example of  FIG. 2 , the input impedance of input terminal  104  is less than forty ohms when an input current  111  of eight milliamperes or more is flowing into input terminal  104 . 
     Not only is response time decreased due to the low input impedance of input terminal  104 , but OLTA  101  also decreases response time by preventing large voltages on input terminal  104  during high diode current conditions. As set forth above, transistor  129  is a diode-transistor N-channel transistor having a non-linear IV (current to voltage) characteristic. Due to this IV characteristic, a large increase in current  111  gives rise to only a small increase in the voltage on node N 1 . At a first approximation, the current  111  increases as the square of the voltage on node N 1  (a diode-connected field effect transistor is sometimes referred to as a “square-law device”). Accordingly, the voltage on node N 1  does not increase linearly even with large magnitudes of diode current  111 . In the example of  FIG. 2  where the highest diode input current  111  under normal operating conditions is approximately 10 milliamperes, the voltage on input terminal  104  (and on input node N 1 ) is clamped to be in a range between 0.5 volts and approximately 1.2 volts. Because the voltage on the input terminal is clamped under high diode current conditions, only a relatively low voltage on the input terminal  104  need be discharged in order for OLTA  101  to detect a dark condition. As illustrated in  FIG. 3 , the response time from when input current  111  begins to decrease from its high current level to its low current level until the OLTA digital output signal DATA begins to transition from a digital high to a digital low is approximately 0.20 microseconds. 
       FIG. 4  is a waveform diagram that shows the voltage  300  on input terminal  104  when input current signal  111  has a first current level of zero amperes and a second current level of 10 milliamperes. Waveform  301  represents the resulting digital signal DATA on output lead  106 . When  FIGS. 3 and 4  are compared, it is noted that: 1) a zero input current condition results in 0.5 volts on input terminal  104 ; 2) a 100 microampere input current condition results in 0.63 volts on input terminal  104 ; and 3) a 10 milliampere input current condition results in 1.13 volts on input terminal  104 . Accordingly, a digital input current signal having a 100 microampere digital amplitude on input terminal  104  results in a 0.13 volt digital amplitude signal on input terminal  104 . A digital input current signal having a 10 milliampere (100 times higher than 100 microamperes) digital amplitude on input terminal  104  results in a 0.63 volt digital amplitude signal on input terminal  104  (only about 4.5 times higher than 0.13 volts). This non-linear input impedance of input terminal  104  allows for faster recovery to the no signal level (i.e., dark condition) than a fixed resistive input impedance would allow for. As illustrated in  FIG. 4 , the response time from when input current  111  begins to decrease from its high current level (the 10 milliampere level) until the OLTA digital output signal DATA begins to transition from its digital high level is approximately 0.25 microseconds. 
       FIG. 5  is a diagram showing an IV characteristic of OLTA  101  of  FIG. 2 . OLTA  101  has a input current operating range from a minimum input current value of approximately zero milliamperes to a maximum input current value of approximately 10 milliamperes. Due to the non-linear current-to-voltage characteristic of OLTA  101 , the input voltage on input terminal  104  is effectively clamped to less than 1.2 volts as illustrated. As the diode current  111  increases, the drain voltage on diode-connected transistor  129  increases very little past the threshold voltage of transistor  129 . Input transistor  129  clamps the voltage on node N 1  to a reasonable level even if tens of milliamps of photocurrent are flowing into terminal  104 . This is important because transistor  129  functions to pull the voltage on node N 1  back down when the diode current  111  falls at the beginning of a dark condition. Because diode-connected transistor  129  is a square-law device, the voltage on node N 1  is pulled down faster than if a simple fixed resistance (that is practical for an amplifier in this application) were used. 
     In one advantageous aspect, OLTA  101  involves no resistors or capacitors that if realized in integrated form would consume large amounts of die area. OLTA  101  includes no bipolar transistors and is realized in integrated form in 20,000 square microns in a standard 0.5 micron CMOS process. In contrast to traditional infrared diode receiver circuits involving operational amplifiers, OLTA  101  involves no feedback loop. In operation, not all of current-mirroring P-channel transistors  121 - 124  are sourcing the indicated currents because some of the nodes N 1 -N 4  are at high voltage levels and the associated current sources are current starved. In contrast to a typical infrared diode operational amplifier receiver circuit that may consume 300 microamperes or more, OLTA  101  of  FIG. 2  consumes less than 30 microamperes when receiving a 500 KHz digital input current signal having a 100 microampere amplitude. The reference here to “a 100 microampere amplitude” means the input signal has a first current level that corresponds to zero milliamperes of input current and has a second current level that corresponds to 100 microamperes of input current. Capacitors  142  and  143  are optional and may be included in OLTA  101  to provide a measure of noise filtering such that short-lived changes in input current  111  do not pass through OLTA  101  and appear on output lead  106 . Capacitors  142  and  143  may, for example, be 0.5 picofarad capacitors. 
     Disable: 
     When OLTA  101  is not being used to “learn” an operational signal, OLTA  101  is disabled to stop OLTA from consuming power. Processor  108  may, for example, write to an OLTA enable bit in a control register. The value of the OLTA enable bit determines the digital logic value of the enable signal EN on input lead  114 . If the enable signal EN is a digital logic low, then the passgate formed by transistors  115  and  116  is non-conductive and N-channel transistor  144  is conductive. N-channel transistor  144  being conductive couples node N 5  to ground potential, thereby disabling the current mirror formed by transistors  117  and  118 . Current  119  is cut to zero. In addition, the passgate formed by transistors  130  and  131  is made non-conductive, thereby decoupling the gate  132  of transistor  129  from node N 1 . Rather than coupling the gate  132  of transistor  129  to the drain of transistor  129 , the gate  132  is coupled through conductive transistor  140  to ground potential. Grounding the gate  132  of transistor  129  makes transistor  129  non-conductive and cuts the currents  134  and  135  to zero, thereby conserving power. When the enable signal EN is a digital low, P-channel transistor  138  is enabled, thereby coupling node N 4  and the lower input lead of NAND gate  139  to supply voltage VDD. The output signal DATA on output lead  106  is therefore maintained at a digital logic low value. 
       FIG. 6  is a flowchart of a method in accordance with one novel aspect. Initially (step  400 ), a photodiode is coupled to an input terminal of an open-loop transimpedance amplifier (OLTA). In one example, this photodiode is photodiode  3  of  FIG. 2  and the input terminal is input terminal  104  of microcontroller integrated circuit  100 . A photocurrent from the photodiode is received (step  401 ) onto a diode-connected N-channel transistor within the OLTA. In one example, this diode-connected N-channel transistor is transistor  129  of  FIG. 2 . Operation of the diode-connected N-channel transistor effectively clamps the voltage on the input terminal. In the example of  FIG. 2 , the voltage on input terminal  104  is clamped within a range of approximately 0.7 volts (0.5 volts to 1.2 volts) for photocurrents in an input photocurrent operating range (from zero microamperes to ten milliamperes). The diode-connected N-channel transistor causes the input terminal to have a low input impedance that decreases with increases in the photocurrent. A current flowing through the diode-connected N-channel transistor is mirrored (step  402 ) to obtain a mirrored current. In the example of  FIG. 2 , the current  134  flowing through diode-connected transistor  129  is mirrored by transistor  133  to produce mirrored current  135 . If the mirrored current is greater than a predetermined current (step  403 ), then the OLTA output signal DATA has a first digital logic value (step  404 ). In the example of  FIG. 2 , if mirrored current  135  is greater than the predetermined current level of 4 microamperes flowing through P-channel transistor  122 , then OLTA outputs a digital logic high value onto output lead  106 . If the mirrored current is not greater than the predetermined current (step  403 ), then the OLTA output signal DATA has a second digital logic value (step  405 ). In the example of  FIG. 2 , if mirrored current  135  is not greater than the predetermined current level of 4 microamperes flowing through P-channel transistor  122 , then OLTA outputs a digital logic low value onto output lead  106 . 
       FIG. 7  is a circuit diagram of another embodiment. Rather than using transistor  133  to mirror the current  134  flowing through the diode-connected N-channel transistor  129  as in the circuit of  FIG. 2 , transistor  133  is also diode-connected as illustrated in  FIG. 7 . The current  126  flowing through transistor  133  does not change with changes in photocurrent  111 . The voltage on node N 2  is therefore substantially fixed. The voltage on node N 1 , however, does change slightly with changes in photocurrent  111  as explained above in connection with  FIG. 2 . The currents  125  and  126  and/or the sizes of transistors  129  and  133  are chosen such that the voltages on nodes N 1  and N 2  are identical when the photocurrent  111  flowing into input terminal  104  is the desired “trip point input current”. An open-loop voltage comparator  500  compares the voltages on nodes N 1  and N 2 . If the voltage on node N 1  is higher than the voltage on node N 2  (a high photocurrent condition), then comparator  500  forces signal DATA to a digital logic high value. If the voltage on node N 1  is lower than the voltage on node N 2  (a low photocurrent condition), then comparator  500  forces signal DATA to a digital logic low value. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Although OLTA  101  of  FIG. 2  has a single “trip point input current” for detecting both low-to-high input current transitions and for high-to-low input current transitions, the OLTA in another embodiment has a first “trip point input current” for detecting low-to-high input current transitions and has a second “trip point input current” for detecting high-to-low input current transitions. Numerous techniques for providing hysteresis can be employed. For example, the digital signal on node N 4  can be used to switch in an additional P-channel current mirror that supplies additional current to node N 2  (in addition to current  126 ) such that the total current supplied to node N 2  is different depending on whether the voltage on node N 4  is high or low. 
     In one embodiment, the “trip point input current” is adjusted based on the photocurrent level. For example, where the photocurrent level is detected to be high, the trip point input current is adjusted to be a higher trip point input current. Using this higher trip point input current improves amplifier response time because the parasitic capacitance of the photodiode need not be discharged as far (when transitioning from a high photocurrent condition to a low photocurrent condition) in order for the voltage on node N 1  corresponding to the trip point input current to be reached. The amplifier can therefore detect the low photocurrent condition more rapidly due to the trip point input current having been adjusted higher. The OLTA is usable to realize a reduced current-consumption sleep mode in which receiving of the beginning of an infrared signal can wake-up a device such as a microcontroller from the sleep mode. One way to accomplish this is by selectively reducing currents through selected ones of the P-channel current sources. The OLTA is more sensitive in the sleep mode. After waking up, standard P-channel current source selections would be enabled to receive the remainder of the infrared signal or another infrared signal. The photodiode and OLTA can be integrated onto the same integrated circuit die. Although an OLTA is set forth above where the photodiode is coupled between VDD and the input of the amplifier, another example of the OLTA involves a photodiode coupled between the input of the amplifier and ground. In such a case, the N-channel transistors of the OLTA are replaced with P-channel transistors, and the P-channel transistors of the OLTA are replaced with N-channel transistors. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.