Abstract:
An embodiment of the present invention comprises a voltage regulator including a droop compensation circuit and a voltage regulator circuit. The droop compensation circuit compensates for an output that changes over a period of time. This variable output may be the result of voltage regulator droop and/or a load transistor droop. For instance, the voltage regulator droop may be associated with the decrease in regulator output voltage that occurs during the course of applying a load. The load transistor droop may correspond to the decrease in RF transistor gain that occurs over the course of a transmit pulse. The variable output could be attributable to other phenomena. Moreover, the variable output that is compensated for is not necessarily decreasing over time, but, in fact, could be increasing over time, or could exhibit increases and decreases over time.

Description:
GOVERNMENT RIGHTS 
     Because the invention was made under contract N00024-98-C-5200 (Department of the Navy), the Government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a regulator used to regulate the input voltage to transistor-based loads, such as to RF preamplifiers or RF amplifiers employed to drive solid state pulsed radar transmitters. 
     BACKGROUND OF THE INVENTION 
     Voltage regulators are used in a variety of contexts for providing and regulating the voltage applied to electronic circuit devices. Just by way of example, voltage regulators are an important system component in solid-state pulsed radar systems. In that context, voltage regulators are used to regulate the voltage supplied to various loads such as preamplifiers or amplifiers that power the radar transmitter circuitry. 
     One common problem with linear voltage regulators is that their voltage output will tend to decrease during loading (e.g., during transmit gate pulses) until the regulator senses the error and begins to regulate the output voltage. This decrease is sometimes referred to as “droop.” Generally, this problem is attributable to the voltage regulator output capacitance droop (dV=1/C*Idt). In the context of a pulsed radar system, this droop in the output voltage causes a decrease in the RF transistor power output. This results in suboptimal system performance. 
     This problem of a decrease in the RF transistor output may worsen in systems having multiple cascaded stages, such as in a solid-state radar system with cascaded amplifiers. Accordingly, the performance loss accumulates. 
     Additionally, it is common that RF transistor-based circuits will tend to exhibit their own natural droop over the course of a powering cycle. For example, high (e.g., microwave) frequency RF transistor-based circuitry in a pulsed radar system will tend to exhibit a droop in gain that worsens over the course of the transmit pulse. This may be attributable to heating of the RF transistor junction during an RF pulse, which, in turn, may cause a decrease in the transistor output power during the RF transmit pulse. The RF output power is no longer constant throughout the RF pulse, resulting in suboptimal performance. 
     When a voltage regulator exhibiting its own droop is used with a transistor-based circuit having its own internal droop, the overall performance loss exacerbates. That is, the droop in the regulator output voltage causes a decrease in the RF transistor power output, thereby adding to an already decreasing output power that is caused by the characteristics of the RF transistor. 
     Conventional voltage regulators, such as linear regulators in the prior art, may suffer the aforementioned drawbacks. In a linear regulator, the voltage is typically preset to a predetermined voltage (“reference voltage”) for normal operation. In some cases this reference voltage is adjustable so that a more accurate output voltage can be established when the load is applied. Otherwise, in a typical linear voltage regulator, the reference voltage does not change once it is set. Accordingly, the linear regulator output voltage may decrease during load pulses due to the droop in the output capacitor voltage. 
     FIG. 1 illustrates a conventional linear regulator circuit. Typical linear regulators may be “off-line” or “on-line.” An off-line linear regulator will tend to have a smaller input capacitance and a larger output capacitance. An on-line regulator tends to have larger input capacitance and a smaller output capacitance. Both types will tend to exhibit the problems discussed above. 
     FIG. 2 is an illustration (not to scale) of an example of the problem of internal transistor droop. In FIG. 2, graph A represents an ideal regulator output voltage, where the regulator output voltage is constant and is without droop. Graph B represents the output of a transistor-based circuit that is regulated by the ideal regulator output voltage. As can be seen, over the course of a gate pulse the transistor-based circuit exhibits an increasing transistor droop  200 . Once again, this may be attributable to heating of the RF transistor junction during an RF pulse, which, in turn, may cause a decrease in the transistor output power during the RF transmit pulse. The RF output power is no longer constant throughout the RF pulse, resulting in suboptimal performance. 
     FIG. 3 is an illustration (not to scale) of an example of the problem of regulator droop in conjunction with load transistor droop. In FIG. 3, graph A represents a regulator output voltage (e.g., a linear regulator with an output capacitance) that exhibits a regulator droop  300  over the course of a gate pulse. This regulator droop  300  aggregates or combines with the load transistor&#39;s internal droop to render an even greater overall amplifier output droop  350 . This represents that a further degraded system-level performance when compared to FIG.  2 . Not only is the power output of FIG. 3 not constant over time (like FIG.  2 ), but the average power output of FIG. 3 is even less than that of FIG.  2 . This is a significant drawback. 
     In sum, so-called “real world” voltage regulators used with real world transistor-based circuits tend to suffer significant performance losses associated with regulator droop and/or load transistor droop. In high stability systems, like solid state pulsed radar systems, these performance losses can be a significant problem. This problem can be mitigated somewhat by using custom transistors (or by screening commercial-off-the-shelf [COTS] transistors), but this may greatly increase the costs of production. In many markets, such as for low- to medium-production military applications, these cost increases may not be acceptable. 
     Finally, it can be readily appreciated that the problem of voltage droop exists in other contexts. For example, instead of an RF transistor load for the regulator, there may be some other component, device, or system, whose output response (e.g., voltage, gain, power, etc.) exhibits some undesirable variation over time. Also, the variation of this output response or gain may increase over some time period, decrease over some time period, or increase and decrease at points over a time period. The common problem is that of how to control (or compensate for) the time variable output response in order to render the desired output response. Generally, the desired output response is constant or flat over some period of time for the system at issue. Sometimes, a non-flat response may be desired. 
     Other problems and drawbacks also exist. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention comprises a voltage regulator including a droop compensation circuit and a voltage regulator circuit. The droop compensation circuit compensates for an output that changes over a period of time. This variable output may be the result of voltage regulator droop and/or a load transistor droop. For instance, the voltage regulator droop may be associated with the decrease in regulator output voltage that occurs during the course of applying a load. The load transistor droop may correspond to the decrease in RF transistor gain that occurs over the course of a transmit pulse. The variable output could be attributable to other phenomena. Moreover, the variable output that is compensated for is not necessarily decreasing over time, but, in fact, could be increasing over time, or could exhibit increases and decreases over time. 
     The voltage regulator of the present invention may be used to regulate the voltage supplied to a preamplifier circuit or an amplifier circuit. The droop compensation circuit may include a control circuit, an offset circuit, and a discharge circuit. The droop compensation circuit may be designed to render a desired output response of the preamplifier circuit or the amplifier circuit. According to one aspect of the invention, the desired output response is substantially flat over the course of a transmit cycle. According to another aspect of the invention, the desired output response is not flat over the course of a transmit cycle. The transmit cycle may correspond to the transmit cycle of a pulsed radar system. The voltage regulator may be an on-line linear regulator. 
     According to another aspect of the invention, a droop compensation circuit is disclosed. The droop compensation circuit may include means for controlling a variable voltage to be input as a reference voltage. The droop compensation circuit may also include means for generating the variable voltage. The variable voltage can be input to a voltage regulator as the reference voltage. 
     According to another aspect of the invention, a method for regulating voltage supplied to a preamplifier circuit or an amplifier circuit is disclosed. A regulator voltage output is supplied to the preamplifier circuit or the amplifier circuit. A variable output is counteracted by compensating for a voltage regulator droop and/or a load transistor droop. The compensation may begin before the regulator voltage output is supplied to the preamplifier circuit or the amplifier circuit in order to quicken recovery and/or to reduce oscillations or instability. The method may also include the step of discharging the regulator voltage output in order to quicken recovery. 
     Accordingly, it is one object of the present invention to overcome one or more of the aforementioned and other limitations of existing systems and methods for regulating voltage supplied to an electronic circuit, such as a transistor-based preamplifier circuit or amplifier circuit. 
     It is another object of the invention to provide a voltage regulator that solves or mitigates the problem of droop in electronic circuits. 
     It is another object of the invention to provide a voltage regulator that solves or mitigates the problem of droop so that COTS RF transistors can be used in the regulator load without special screening procedures. 
     It is another object of the invention to provide a voltage regulator for use in high stability radar systems so that the system response during a transmit cycle remains substantially constant. 
     The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. It will become apparent from the drawings and detailed description that other objects, advantages and benefits of the invention also exist. 
    
    
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the system and methods, particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The purpose and advantages of the present invention will be apparent to those of skill in the art from the following detailed description in conjunction with the appended drawings in which like reference characters are used to indicate like elements, and in which: 
     FIG. 1 is an electrical schematic of a conventional voltage regulator. 
     FIG. 2 is an exemplary graph illustrating the problem of transistor droop presented by prior art approaches to voltage regulation of transistor-based circuits. 
     FIG. 3 is a exemplary graph illustrating the problems of voltage regulator droop and transistor droop presented by prior art approaches to voltage regulation of transistor-based circuits. 
     FIG. 4 is an exemplary amplifier system according to an embodiment of the invention. 
     FIG. 5 is a graph illustrating the outputs of an exemplary amplifier system employing a droop compensation circuit that compensates for voltage regulator droop. 
     FIG. 6 is a graph illustrating the outputs of an exemplary amplifier system employing a droop compensation circuit that compensates for both voltage regulator droop and transistor droop. 
     FIG. 7 is a graph illustrating the outputs of an exemplary amplifier system employing a droop compensation circuit that compensates for both voltage regulator droop and transistor droop, and further employing an energy discharge circuit. 
     FIG. 8 is a graph illustrating the outputs of an exemplary amplifier system employing a droop compensation circuit that overcompensates for droop(s). 
     FIG. 9 is a graph illustrating the outputs of an exemplary test prototype according to an embodiment of the invention for providing a droop-compensating regulator that has not been loaded. 
     FIG. 10 is a graph illustrating the outputs of a test prototype according to an embodiment of the invention for providing a droop-compensating regulator that has been loaded with a pulsed resistive loading. 
     FIG. 11 is a graph illustrating the outputs of a test prototype of a droop compensating regulator according to an embodiment of the invention whereby the application of the pulsed resistive loading is delayed for a period of time after the gate pulse starts. 
     FIG. 12 is a flow diagram according to an embodiment of the invention for compensating for droop in a preamplifier circuit or amplifier circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4 is an exemplary amplifier system according to an embodiment of the invention for addressing the problem of voltage droop. Exemplary amplifier system  400  may be a preamplifier system, amplifier system, or a combination of the two. Amplifier system  400  includes voltage regulator  410 , amplifier  420 , and droop compensator  450 . 
     According to an embodiment, the operation of amplifier system  400  is as follows. Droop compensator  450  provides a voltage to an input of voltage regulator  410 . Preferably, this voltage is an adjusted voltage comprising a nominal reference voltage adjusted by an offset voltage. The offset voltage can be a positive offset, a negative offset, or a zero offset. Preferably, when droop compensator  450  is enabled during a transmit cycle, the offset voltage will be a positive offset so that the adjusted voltage supplied to voltage regulator  410  will be adjusted upwards. This upward adjustment can compensate for a droop in either the regulator output voltage, the RF transistor output, or both. 
     The droop may be associated with the voltage regulator  410 , with the amplifier  420 , with both the voltage regulator  410  and the amplifier  420 , or with another system component. The droop may be an expected or an anticipated droop that is based on empirical measurements or theoretical calculations. The values for the expected droop (or for the adjusted voltage used to compensate for the expected droop) may be selected or computed by droop compensator  450 . For example, values for the expected droop (or for the adjusted voltage used to compensate for the expected droop) could be selected (e.g., selected from a memory) by droop compensator  450 , which are then used to cause the addition of an appropriate offset voltage. In another example, the values for the expected droop (or for the adjusted voltage used to compensate for the expected droop) could be computed by droop compensator  450  based on an equation. 
     According to another embodiment, the values for the droop (or for the adjusted voltage used to compensate for the droop) could be based on a measured or projected droop based on a feedback measurement. For example, the droop of voltage regulator  410  could be measured or projected based on a feedback (not shown) from regulator output voltage  418 . Similarly, the droop of amplifier  420  could be measured or projected based on a feedback (not shown) from RF output  430  or detected RF output  435 . Accordingly, such a feedback could be used by droop compensator  450  to determine how much droop compensation (e.g., voltage offset) is appropriate for the circumstance. 
     Once the offset voltage is computed or determined, droop compensator  450  provides the adjusted voltage to the input of voltage regulator  410 . This adjusted voltage (which can be referred to as V REFERENCE ′) controls the output voltage  418  of voltage regulator  410 . Output voltage  418  of regulator  410  is provided as an input to amplifier  420 . Preferably, output voltage  418  is provided as an input to one or more transistors (or like semiconductor devices) in amplifier  420 . For example, output voltage  418  may be provided as an input voltage to the RF transistor circuits (e.g., amplifier  420 ). There may also be a RF input  422  to amplifier  420 . Generally, the characteristics of RF input  422  depend on the system specification for a particular application, as does the operational frequency of the RF transistor circuits (e.g., amplifier  420 ). 
     Because the adjusted voltage compensates for droop, the output response of the system component at issue can be adjusted to provide a desired output response. For example, the desired output response may be a flat or a non-flat response. Preferably, the desired output response is a flat response. 
     According to one embodiment, the desired output response is a flat response for the output voltage  418  of regulator  410 . According to another embodiment, the desired output response is a flat response for the RF output  430  or the detected RF output  435  of amplifier  420 . According to yet another embodiment, the desired output response is a non-flat response for amplifier  420 , such as an output response that ramps up or increases during the course of a transmit cycle. Those of skill in the art will readily appreciate that many different variations of the desired output response can be accommodated without departing from the spirit and scope of the present invention. 
     According to an embodiment, droop compensator  450  comprises a control circuit  460  and an offset circuit  470 . Control circuit  460  controls a variable voltage to be input to voltage regulator  410 , i.e., an adjusted voltage. Offset circuit  470  generates the variable voltage. For example, offset circuit  470  may generate an offset voltage to be combined with a reference voltage. Offset circuit  470  may also generate the nominal reference voltage, or the nominal reference voltage may be received as an input to offset circuit  470 . 
     According to a preferred embodiment, droop compensator  450  further includes a discharge circuit  485 . Discharge circuit  485  permits discharging of output voltage  418  of voltage regulator  410 . This can be employed to accelerate the return of output voltage  418  to a nominal value, such as to the nominal reference voltage. Accordingly, inclusion of such a discharge circuit  485  can improve the so-called “recovery time” of voltage regulator  410  following the end of a transmit cycle. 
     According to an embodiment, control circuit  460  has an input trigger  452  for activating/deactivating droop compensator  450 . Control circuit  460  also has a transmit gate  464  output that is coupled to offset circuit  470 . Transmit gate  464  may comprise a transmit gate that activates/deactivates droop compensation. Control circuit  460  may also include a discharge trigger  466  that is used to activate/deactivate the discharge circuit  485 . 
     Control circuit  460  can be implemented in a multitude of fashions, and the design of control circuit  460  for a particular application is well within the skill of the ordinary artisan. For example, control circuit  460  may include circuitry for controlling the sequencing of offset circuit  470  and discharge circuit  485 . Control circuit  460  may include input components (e.g., line receivers), timing generators (e.g., one shot devices, D/A signal converters, and so forth), and sensing circuits (e.g., resistive divider, current sensors, and so forth). 
     According to one embodiment, control circuit  460  includes one shot device  467 , line receiver  468 , and one shot device  469 . After receiving trigger input  452 , line receiver  468  enables one shot device  467  to activate/deactivate droop compensation. Line receiver  468  may also enable one shot  469  to activate/deactivate discharge circuit  485 . 
     According to an embodiment, offset circuit  470  has a nominal reference voltage input  462 , a transmit gate  464  that is input from control circuit  460 , and an adjusted output voltage  472  that is provided to voltage regulator  410 . 
     A variety of implementations of offset circuit  470  could be used, and the design of offset circuit  470  for a particular application is well within the skill of the ordinary artisan. For example, offset circuit  470  may include input signal circuits (e.g., line receivers, differential inputs, and so forth), ramp generators (e.g., RC networks, adders, and so forth), and discharge circuits for ramp generators (e.g., active type energy dump circuits and so forth). 
     According to one embodiment, offset circuit  470  comprises a waveform (ramp) generation circuit. According to a preferred embodiment, offset circuit  470  includes a D-A converter to generate a waveform according to a desired output wave shape. This wave shape may be linear or nonlinear. In one configuration, offset circuit  470  includes a ramp generator comprised of capacitor  473  and resistance  474 , an active type ramp discharge  475 , and input resistance  476 . This configuration is provided merely as an example of many implementations that could be employed for offset circuit  470  without departing from the spirit and scope of the invention. 
     According to an embodiment, discharge circuit  485  includes a discharge trigger  466  input from control circuit  460 . Preferably, discharge circuit  485  also includes a connection  487  to output voltage  418  so that energy can be dissipated when output voltage  418  is to be reduced. 
     Discharge circuit  485  can be implemented in a various fashions and its design for a particular application is well within the skill of the ordinary artisan. Discharge circuit  485  may include activate components, such as transistors, and associated passive components, such as resistors, which are used to discharge excessive voltage from the regulator output prior to the beginning of the next load on time period. According to a preferred embodiment, discharge circuit  485  is an active switch circuit where the output capacitance  414  can be discharged in a finite amount of time. For example, discharge circuit  485  may include a power MOSFET with low Rdson being gated by a control signal. Using such an active device in discharge circuit  485  decreases the amount of power wasted during nonload times. 
     Continuing with FIG. 4, according to an embodiment, voltage regulator  410  may comprise a linear regulator. Preferably, voltage regulator  410  is an on-line linear regulator having a first input capacitor  404 , a second output capacitor  414 , and an error amplifier or differential amplifier  408 . Preferably, the first input capacitor  404  has a value greater than the second output capacitor  414 . According to an embodiment, voltage regulator  410  further comprises an output resistance. The output resistance may include a first resistance  412  and a second resistance  413  forming a sampling circuit to send a sample of the output voltage back to the input of the error amplifier  408 . 
     According to an embodiment, amplifier  420  is an RF transistor-based circuit having an RF input  422 , multiple transistors  426 , and an RF output  430 . There may be a detected RF output  435  for measuring the output of amplifier  420 . Preferably, amplifier  420  also includes a divider  424  and a combiner  428 . 
     In the preferred embodiment of droop compensator  450 , compensation is provided to offset the droop caused by the regulator output capacitance (e.g., capacitor  414 ) and the inherent droop of amplifier  420  (e.g., the droop created by transistors  426 ). In one embodiment, the adjusted output voltage  472  may be ramped up in order to provide this compensation. 
     FIGS. 5-8 are graphs illustrating the performance of an exemplary droop compensation circuit under varying circumstances. FIGS. 5-8 are not necessarily drawn to scale, but are provided to illustrate the operation the exemplary droop compensator. 
     FIG. 5 is a graph illustrating the outputs of an amplifier system employing a droop compensation circuit according to a first embodiment of the invention that compensates for voltage regulator droop. In FIG. 5, graph C represents the output voltage of the exemplary droop compensator. For example, this output voltage could be the adjusted voltage  472  of FIG.  4 . In FIG. 5, graph D represents the transmit gate (waveform). For example, graph D could represent the activation/deactivation of the transmit cycle in a pulsed radar system. Preferably, the transmit gate waveform is a rectified square wave, although other waveforms could be employed. Preferably graph D could be the transmit gate  464  input to offset circuit  470  in FIG.  4 . In FIG. 5, graph A represents the regulator output voltage after droop compensation. For example, graph A could be the regulator output voltage  418  of FIG.  4 . Finally, graph B of FIG. 5 represents the output of a transistor-based circuit supplied with the regulator output voltage. For example, graph B could be RF output  430  or detected RF output  435  of FIG.  4 . 
     According to FIG. 5, the transmit gate is activated (on) during time periods  502 . During time periods  502 , the droop compensation circuit ramps up or otherwise increases its output voltage  510  so as to provide an adjusted voltage (e.g., V REFERENCE ′). This adjusted voltage compensates for regulator droop so as to render a flat regulator output voltage  505 . Otherwise, during time periods  502  the regulator output voltage  505  would be decreasing (exhibiting droop). As can be seen from graph B, the output of the transistor-based circuit still exhibits some droop  520  during time periods  502 . This is attributable to internal transistor droop. For example, this droop may be the droop caused by the normal droop of microwave solid state transistors. 
     FIG. 6 is a graph illustrating the outputs of an exemplary amplifier system employing a droop compensation circuit according to a second embodiment of the invention that compensates for both voltage regulator droop and transistor droop. Graphs A-D of FIG. 6 are similar to graphs A-D of FIG.  5 . 
     In FIG. 6, during time periods  602 , the droop compensation circuit ramps up or otherwise increases its output voltage  610  so as to provide an enhanced (or overcompensated) adjusted voltage. This enhanced adjusted voltage compensates for regulator droop and transistor droop so as to render a flat output  620  of the transistor-based circuit. Not only is the output flat, but the average power over the cycle is greater than that provided in the first embodiment of FIG.  5 . In short, the regulator output voltage  605  during time periods  602  has been enhanced beyond nominal in order to compensate for the transistor droop in the next stage. Otherwise, during periods  602  the output  620  of the transistor-based circuit would be exhibiting droop. 
     The second embodiment of the invention reflected by FIG. 6 also exhibits a slow recovery time for the regulator output voltage  630  to return to nominal after the end of the transmit gate. Full recovery will be dependant on the timing of transmit gate waveform D and the RC time constant of the applied load (e.g., the transistor-based circuit). 
     FIG. 7 is a graph illustrating the outputs of an exemplary amplifier system according to a third embodiment of the invention employing a droop compensation circuit that further employs an energy discharge circuit that improves the recovery time of the exemplary voltage regulator. In FIG. 7, graph E represents a discharge circuit enable signal that activates or enables the discharge circuit at select times, such as at the end of a transmit cycle  702 . For example, the discharge circuit enable signal could be the discharge trigger  466  input from control circuit  460  of FIG.  4 . 
     FIG. 7 (like FIG. 6) shows that the regulator output voltage  705  is overcompensated by the output voltage  710  of the droop compensation circuit. This causes a flat output  720  of the transistor-based circuit during time periods  702 . Additionally, in FIG. 7 the discharge circuit enable signal of graph E enables the discharge circuit during time periods  700 , which results in a faster recovery time for the regulator output voltage  730  of graph A to return to nominal. The use of a discharge or so-called “active bleeder” circuit not only speeds recovery, but it permits a larger variety of duty cycles. 
     FIG. 8 is a graph illustrating the outputs of an exemplary amplifier system employing a droop compensation circuit according to a fourth embodiment of the invention that overcompensates for droop(s) so that there is an increasing output response. In FIG. 8, the output voltage  810  of the droop compensation circuit more than compensates for the system-wide droop, e.g., regulator droop and transistor droop. Accordingly, the output  820  of the transistor-based circuit exhibits a positive droop. The average power is increased in comparison to the third embodiment of FIG.  7 . FIG. 8 also depicts the accelerated recovery time of the regulator output voltage  830  of graph A that is caused by the discharge circuit enable signal of graph E during times  800 . The purpose of FIG. 8 is to illustrate that the voltage output  810  of the droop compensation circuit could be nearly any type of waveform that could be programmed in order to render a desired output response. 
     FIGS. 9 through 11 are captured waveforms of a breadboard regulator unit illustrating the regulator output voltage (e.g., element  418  of FIG.  4 ), the input trigger (e.g., element  452  of FIG.  4 ), and the regulated load current (e.g., current into amplifier  420  of FIG.  4 ). The regulator output was connected to a pulsed resistive load representing the loading of an RF transistor. Referring to FIG. 4, the exemplary droop compensation circuit was modeled after droop compensator  450 . The exemplary linear voltage regulator was modeled after voltage regulator  410  of FIG.  4 . In particular, the linear voltage regulator included an SG1532 voltage regulator, IRF4905 power MOSFETs for pass transistors, LS123 components for one-shot timing circuits, a 2N2222 transistor for the ramp discharge circuit, and various resistors and capacitors for scaling the output voltage. The pulsed resistive load was comprised of an N-channel power MOSFET in series with a low value resistance that was representative of the equivalent load resistance of an RF amplifier. The MOSFET was gated on by a high current pulse generator. 
     Referring now to the results of the test of FIG. 9, graph  945  (CH  3 ) is the trigger input waveform, which is in the on state during negative excursion  902 . Graph  955  is the load current, which, in this case, is fixed at 0 A because no load has been applied. Graph  950  is the regulator output voltage. (FIG. 9 can be considered roughly analogous to FIG.  6 .) As can be seen from FIG. 9, when the trigger input gate enters the on state, the regulator output voltage  905  is ramping up or increasing due to the effect of the droop compensation circuit. After the end of the transmit gate on state, the regulator output voltage  930  falls off during its recovery stage, although the value remains above nominal for a period of time after time T 1 . 
     FIG. 10 is similar to FIG. 9, except that the pulsed resistive load has been applied at time T 0  (start of the trigger input on state) in order to simulate the transistor loading. Accordingly, the load current of graph  1055  is non-zero (peaking at about 23 Amps) over time  1002  during which graph  1045  reflects that the trigger input waveform is in the on state. Graph  1050  reflects that the regulator output voltage  1005  is ramping up or increasing due to the effect of the droop compensation circuit, although there is some oscillation (minor instability) due to the RC loading of the simulated transistor. The gate pulse period ends at time T 1  and the regulator output voltage  1030  slowly recovers to nominal. The load current returns to zero. 
     FIG. 11 is a graph illustrating a variation whereby the pulsed resistive loading representing an RF transistor is not imposed until the passage of a time period after the trigger input waveform is in the on state. In FIG. 11, graph  1145  shows that the trigger input waveform begins the on state at time T 0  for a period of time  1102  until time T 1 . However, the load is not applied until time TO′, as reflected by graph  1155  of the load current, which does not begin to rise until just about time TO′. Graph  1050  shows that the voltage output  1105  increases during time  1102 . Additionally, it can be seen that the oscillations (minor instability) exhibited in output voltage  1005  of FIG. 10 are nearly eliminated. It is believed that the delay in applying the load (TO′-TO) allows the voltage regulator to stabilize before the regulator output voltage is supplied to the pulsed resistive load (e.g., the transistor-based circuit). It can also be seen that the recovery time for the regulator output voltage  1130  seems to have improved relative to that of FIG.  10 . 
     Those of skill in the art will recognize that the duration of the delay in applying the load can be varied, depending on the overall length of the gate pulse, the particular configuration of the circuitry, and so forth. In a preferred embodiment, the delay is about 5-25% of the length of the transmit pulse. For a 30 microsecond transmit pulse, the delay may be about 5 microseconds. 
     The droop compensation circuit used for FIGS. 9-11 did not include a discharge circuit (e.g., see discharge circuit  485  of FIG.  4 ). It can be expected that the recovery time of the regulator output voltage would improve with the addition of a discharge circuit. 
     FIG. 12 is a flow diagram according to an embodiment of the invention for compensating for droop in a preamplifier circuit or amplifier circuit. 
     The method starts at step  1200 . At step  1205 , a delay (in applying a load) may be applied. If “no,” the method proceeds to step  1210  where the (regulator output) voltage is provided to a preamplifier/amplifier. If “yes,” step  1210  is skipped. At step  1220 , compensation is provided for a variable output. This variable output may be associated with a voltage droop of a voltage regulator, a transistor droop, or any other device, component, or system exhibiting an undesirable time-variable response. 
     According to an embodiment, step  1220  may include the step  1225  of selecting or computing an offset. According to one or more of steps  1230 - 1240 , the offset may be intended to compensate for one or more of a regulator droop  1230 , a transistor droop  1235 , and a regulator droop and a transistor droop  1240 . At step  1245 , the offset is applied. For example, referring to FIG. 4, the offset may be applied to a nominal reference voltage in order to supply an adjusted reference voltage (e.g., V REFERENCE ′). 
     It will be understood by those of skill in the art that step  1220  for compensating for a variable voltage droop could be implemented as a relatively complex prediction algorithm, a feedback-based prediction algorithm, a set of stored values reflecting expected droops, or as a simple linear ramp algorithm. Other variations of the aforementioned could be added or substituted without departing from the true spirit and scope of the present invention. 
     At step  1250 , the end of a transmit cycle (gate pulse) may have arrived. If “no,” the method returns to step  1205 . If “yes,” the method proceeds to step  1255 , where the regulator output voltage may be discharged. The method ends at step  1260 . 
     Having described circuitry and methods for voltage droop compensation, it is apparent that a number of beneficial applications and advantages may flow therefrom, including, but not limited to: compensating for the droop of an RF transistor and compensating for the droop of a linear regulator; compensating for the droop of the next stage in a cascaded RF design; improving overall system performance by increasing the average power of each pulse; allowing the use of low cost RF transistors; providing high fidelity inputs to successive stages; providing feed forward input to a linear regulator; providing larger effective bandwidth due to a lower required output capacitance; allowing the use of smaller energy recovery circuits; providing limited energy availability in the event of a continuous wave (CW) fault; reducing criticality of the equivalent series resistance (ESR) of capacitors due to ramp compensation; allowing a reduction in output capacitor size and an increase of input capacitor size in order to provide a local source of high energy; allowing smaller size wire to be used for bus interconnects; reducing input bus electromagnetic interference (EMI) because high inductive filtering can be applied without compromising transient response; and allowing a reduction of front-end power supply bandwidth characteristics, thereby also possibly reducing cost of the input power supplies. Other beneficial applications and advantages may exist. 
     Other embodiments and uses of this invention will be apparent to those having ordinary skill in the art upon consideration of the specification and practice of the invention disclosed herein. The specification and examples given should be considered exemplary only, and it is contemplated that the appended claims will cover any other such embodiments or modifications as fall within the true scope of the invention.