Temperature compensated amplifier and operating method

Abstract: An amplifier (10) is formed on an integrated circuit chip (12) and has a plurality of serially coupled amplifier stages (14 1 to 14 6 ). The stages (14 1 to 14 6 ) have an uncompensated gain which decreases with increasing temperature. Each one of a series (14 2 to 14 5 ) of these stages includes a feedback circuit (RF/10, RF,16) having a switchable feedback element (16), providing the stage with a first gain when the switchable element (16) is switched into one state and a second gain when switchable element (16) is switched into a second state. Temperature compensation circuitry (19, 20, 22) switches the switchable element (16) into one of the states selectively in accordance with the temperature of the chip (12), which is sensed by a thermometer (19) included in the temperature compensation circuitry. Each stage of the said series has a higher gain at a temperature above a predetermined threshold temperature associated with the stage and a lower gain at temperature below the predetermined threshold temperature associated with the stage, and each such stage is associated with a different predetermined threshold temperature (T 1 , T 2 , T 3 , T 4 ). In the feedback circuit of a stage, the switchable element (16) is operable to either shunt another feedback element (RF/10) or un-shunt such element. The stage provides a first gain when such element (RF/10) is shunted by the switchable element (16) and a second gain when the feedback (RF/10) is un-shunted by the switchable element.

Description: This invention relates generally to amplifiers and more particularly to temperature compensated amplifier. As is known in the art, amplifiers are used in a wide variety of applications. For example, modem wireless communication products such as cellular telephones, satellite receivers and pagers require amplifiers to boost very weak signals that enter from the antenna to levels useable in subsequent receiver processing circuits. Amplifiers are used in transmitters as well, raising the power level emanating from message modulation circuits to that suitable for long range transmission. As is known in the art, one problem with many amplifiers is that their electrical characteristics change, and in particular their gain level changes with ambient temperature. In many applications, some sort of temperature compensation is required to allow sensible operation without saturation. Several temperature compensation techniques have been suggested. One technique used is to change the operating current of the transistors used in the amplifier in accordance with the temperature. This technique, while inexpensive, is not very desirable when linearity and noise figure are important because, by the time the operating current is changed enough to change the gain the noise figure and linearity have been compromised. DE 19 718 109 A1 describes an amplifier comprising two serially coupled amplifier stages, each one of such stages having temperature compensation circuitry comprising a gain selector for providing such stage with a discrete change in gain. The gain selector constitutes a circuit supplying bias current to the respective stage, each stage being a transistor. Each gain selector is controlled by a microprocessor adapted to measure the temperature of the respective transistor and to set the value of the bias current in accordance with a stored value in a control table, the stored values being different for "cold" temperatures, normal temperature, and "hot" temperatures. Another complex technique is to install a variable attenuator in the amplifier chain of plurality of serially coupled amplifier stages and change its attenuation in accordance with temperature. While this technique is more costly than changing the operating current of the transistors, better results can be obtained since the amplifier stages maintain a more constant noise figure and saturation level. Further, more than one attenuator is sometimes needed to keep the operating power levels in the entire amplifier chain within reasonable limits over the operating temperature range. Besides the cost of the additional attenuators, more amplifier stages must be added to overcome the insertion loss of the attenuators. Further, tracking problems arise when an amplifier stage yields more or less gain than predicted due to manufacturing variations, which then means an attenuator in the same amplifier chain is set to the wrong level for proper system operation. A still more complex technique, used in expensive systems, is to use digitally programmed attenuators. The settings for these attenuators are individually determined at the time of manufacture by means of measuring the system performance over temperature and determining what settings are necessary for proper system operation. Sometimes even the frequency of operation is needed as well. All this information is stored in a digital programmable memory (PROM) and later used by a digital processor with its inputs of ambient temperature and frequency of operation and other system variables to determine the proper attenuator setting for system operation. In accordance with the present invention, an amplifier is provided as defined by claim 1 hereinafter, to which reference should now be made. Preferred embodiments of the invention are defined by the dependent claims hereinafter. With the distributed nature of the temperature compensation scheme, performance of the overall amplifier chain will not change much from room temperature since the changes that occur on a stage to stage basis are small and are distributed over the entire amplifier chain. The invention will now be described by way of example with reference to the accompanying drawings, in which:- FIG. 1 is a block diagram of a temperature compensated amplifier according to the invention; FIG. 1A is a schematic diagram of a thermometer adapted for use in the amplifier of FIG. 1; FIG. 1B is a schematic diagram of another thermometer adapted for use in the amplifier of FIG. 1; FIG. 2 is a plot of gain variation vs. temperature of the amplifier of FIG. 1 without gain compensation as a function of temperature according to the invention; FIGS. 3A through 3D are curves showing the relation between the gain change in each of a plurality of gain stages in the amplifier of FIG. 1 as a function of temperature measured by the thermometer of FIG. 1A or FIG. 1B; FIG. 4 is a curve showing gain variation of the amplifier of FIG. 1 as a function of temperature measured by the thermometer of FIG. 1A or FIG. 1B; FIG. 5 is a schematic diagram of an exemplary of on the gain stages of the amplifier of FIG. 1; FIG. 6 is a schematic diagram of another embodiment of a temperature compensated amplifier according to the invention; FIG. 7A-7D are curves showing the relation between the gain change in each of a plurality of gain stages in the amplifier of FIG. 6 as a function of temperature measured by the thermometer of FIG. 1A or FIG. 1B; FIG. 8 is a curve showing gain variation of the amplifier of FIG. 6 as a function of temperature measured by the thermometer of FIG. 1A or FIG. 1B. Description of the Preferred Embodiments Referring now to FIG. 1, a temperature compensated amplifier 10 is shown. The amplifier 10 is formed on a semiconductor, here silicon or gallium arsenide, for example, integrated circuit chip 12 and includes a plurality of, here for example, six, serially coupled amplifier stages 14 1 -14 6 . Here, the amplifier 10 includes a first stage 14 1 (stage 1) optimized for noise figure and a last stage 14 6 (stage 6) optimized for power output and efficiency. Each one of the remaining portion of such stages 14 2 -14 5 (i.e., stages 2, 3, 4 and 5) has a gain selected in accordance with a control signal fed to such stage on lines 16 1 -16 4 , respectively, as shown. A temperature sensing circuit 17, i.e. a thermometer such as serially connected diodes, as shown in FIG. 1A, is provided for measuring temperature of the chip 12 over a range of temperatures, here from -10EC to +60EC and for producing a temperature signal, T, on line 20 representative of the measured temperature. The amplifier 10 includes a control section 22 comprising a plurality of comparators 24 1 -24 4 . Each one of the comparators 24 1 -24 4 , for example, a Schmitt trigger, is fed by the fed temperature signal, T, on line 20, and a different temperature threshold signal, T 1 -T 4 , respectively, as shown. Each one of the temperature threshold signals, T 1 -T 4 , represents a different temperature in the range of temperatures. Each one of the comparators 24 1 -24 4 produces the control signal for a corresponding one of the portion of amplifier stages 14 2 -14 5 on a corresponding one of the lines 16 1 -16 4 , respectively, as shown. More particularly, here, if left uncompensated by the control section 22, the gain variation of the amplifier 10 would decease with temperature as, for example, shown in FIG. 2. Thus, it is noted that the gain of the amplifier 10 here changes -4.5 db over an operating range of temperatures from -10EC to +60EC. In order to compensate for this gain variation, the gain of each one of the four stages 14 2 -14 5 (FIG. 1) is changed by -0.9 db over uniformly over the operating temperature range. Referring to FIGS. 3A through 3D, here the gain of stage 14 2 (stage 2) is changed by -0.9 db at a threshold temperature, T 1 , of +3EC, the gain of stage 14 3 (stage 3) is changed by -0.9 db at a threshold temperature, T 2 , of +17EC, the gain of stage 14 4 (stage 4) is changed by -0.9 db at a threshold temperature, T 3 , of +32EC and the gain of stage 14 5 (stage 5) is changed at a threshold temperature, T 4 , of +46EC. More particularly, these gains are increased as the temperature increases above the threshold temperatures T 1 -T 4 and the gain is reduced as the temperature is reduced below the threshold temperatures T 1 -T 4 , as shown in FIGS. 3A-3D. Referring to FIG. 4, the overall gain variation from an average value over the operating temperature range is shown with a total variation within " 0.5 db. It is noted that the gain variation in the compensated amplifier is inversely proportional to the number of stages switched according to the following relationship: Per Stage Compensated Gain Variation = Total Gain Variation/(Number of Stages +1) In the example above, as noted from FIG. 4, each stage 14 2 -14 5 provides 0.9 db peak to peak or 0.45 db peak gain variation. Referring in more detail to FIG. 1, each one of the stages 14 2 -14 5 is identical in construction. An exemplary one thereof, here stage 14 2 being shown in detail in FIG. 5. It is first noted that such gain stage 14 2 includes a gain element, here a transistor Q1 and biasing resistors R1-R4, dc blocking capacitors C1 and C2, and a pair of serially connected feedback resistors Rf/10 and Rf. As is known, the gain of the gain element is proportional to the feedback resistance. It is noted that here one of the resistors in the feedback, here the resistor Rf/10 has a switch 31 connected in shunt across its terminals. The switch 31 is here a field effect transistor (FET) Q2. The gate of the FET Q2 is fed by the control signal on line 16 1 . When the temperature T is move than T 1 , the switch 31 is open and the resistor Rf/10 is not shunted so that the total resistance in the feedback is Rf+(Rf/10). When the temperature T is less than T, the switch is closed so that the total resistance in the feedback is Rf or 10 percent less. Thus, the gain of stage 14 2 is 0.9 db lower at temperatures less than T 1 than at temperatures move than T 1 . Referring now to FIG. 6, a temperature compensated amplifier 10' is shown wherein each stage 14' 2 -14' 5 of the amplifier 10 has more than two gain states, here three gain states, to be described. The control section 22' includes four pairs of comparators: 24' 1 , 24" 1 ; 24' 2 , 24" 2 ;24' 3 , 24" 3 ; and 24' 4 , 24" 4 , as shown. Each of the four pairs of comparators operates in a similar manner. Thus, considering comparator 24' 1 , such comparator 24' 1 is fed by the signal on line 20 representative of the temperature T and a signal representative of a first threshold temperature, T' 1 . The output of comparator 24 1 ' produces a control signal on line 16' 1 , as shown. Comparator 24" 1 is fed by the signal on line 20 representative of the temperature T and a signal representative of a second threshold temperature, T" 1 . The output of comparator 24" 1 produces a control signal on line 16" 1 , as shown. More particularly, referring to FIGS. 7A-7D, as the ambient temperature increases, the gain of stage 14 2 (stage 2) as shown in FIG. 7A is changed by -0.5 db at a threshold temperature T" 1 of +5.2EC. Thus, stage 14 2 (stage 2) has either a 0 db gain state, (no gain change), a +.5 db gain compensated state, or a +1.0 db gain compensated state, as indicated. In like manner, the gain of stage 14' 3 (stage 3) is changed by +.5 db at a threshold temperature, T' 2 , of +13.3EC, and again by +0.5 db at a threshold temperature, T" 2 , of +21.1EC, as shown in FIG. 7B; the gain of stage 14' 4 (stage 4) is changed by +.5 db at a threshold temperature, T' 3 , of +28.9EC and again by +0.5 db at a threshold temperature, T" 3 of +36.7EC, as shown in FIG. 7C; and, the gain of stage 14' 5 (stage 5) is changed by +0.5 db at a threshold temperature, T' 4 , of +44.4EC and again by +0.5 db at a threshold temperature, T" 4 of +52.5EC, as shown in FIG. 7D. More particularly, these gains are increased as the temperature increases above the threshold temperatures T' 1 -T" 4 and the gain is decreased as the temperature is reduced below the threshold temperatures T' 1 -T" 4 , as shown in FIGS. 7A-7D. The overall gain error is shown in FIG. 8 where less than 0.5 db peak to peak is demonstrated. It is noted that under these conditions, the compensated gain variation is given by: Compensated Gain Variation=Total Gain Variation/(Number of Gain States) It is noted that while the two switches per stage used in the amplifier 10 (FIG. 6) could provide four gain states per stage with the use of a digital coder, here such digital coder is not used and, as noted above, the switching points use different temperatures for each level sensor (i.e., comparator) as before, as shown below: Switch State Temperature Gain Level None "On" Hot Highest One "On" Warm Normal Both "On" Cold Lowest If further reduction in the compensated gain variation is required, it is only necessary to increase the number of gain states, by increasing the number of gain stages with switches, or increasing the number of switches in a gain stage, or some combination thereof. In other embodiments, for example, a digital coder may be used to obtain more gain states at a moderate increase in complexity. Further, the chip may have on it different types of amplifier, so that a radio frequency (RF) amplifier, an intermediate frequency (IF) amplifier and baseband amplifiers on the same chip as would be found in a receiver, for example, may be compensated together with a common thermometer. Thus, only one wire representing the temperature being sensed need be threaded through the receiver stage by stage. The switching points of the comparators should be chosen carefully and controlled in production to ensure the operation of the entire scheme. The thermometer should be very repeatable on a unit to unit basis. The system using this temperature compensation scheme should be tolerant of small gain changes occurring over a relatively small temperature range.

Claims: An amplifier (10) comprising: a plurality of serially coupled amplifier stages (14), each one of such stages having temperature compensation circuitry comprising a gain selector (24 1 -24 4 ) for providing such stage with a discrete change in gain, characterised in that the amplifier is formed as an integrated circuit chip (12) provided with means (17) for measuring the temperature of the chip (12), and in that the chip temperature measuring means (17) is coupled to the temperature compensation circuitry of each stage, whereby the gain selectors overate in accordance with the temperature of the chip (12). An amplifieraccording to claim 1, characterised in that the integrated circuit chip (12) is adapted to operate over a range in temperature. An amplifier according to claim 2, characterised in that each amplifier stage has a switchable element (31) and provides a first gain when said switchable element (31) is switched into one state and a second gain when said switchable element (31) is switched into a second state, and in that the respective temperature compensation circuitry switches said switchable element (31) into one of the states selectively in accordance with the temperature of the chip (12). An amplifier according to claim 3, characterised in that each amplifier stage has a feedback element (RF/10) and a switch as said switchable element (31), the feedback element (RF/10) being in shunt with said switch (31), and said switch (31) being operable to either shunt said feedback element (RF/10) or un-shunt said feedback element (RF/10), said amplifier stage providing a first gain when said feedback element (RF/10) is shunted by said switch (31) and a second gain when said feedback element (RF/10) is un-shunted by said switch (31). An amplifier according to any preceding claim, characterised in that the amplifier has an uncompensated gain which decreases with increasing temperature of the chip (12), wherein each of said amplifier stages has a higher gain at a chip temperature above a predetermined threshold temperature associated with said stage and a lower gain at a chip temperature below said predetermined threshold temperature associated with said stage, and in that each one of the said plurality of stages is associated with a different predetermined threshold temperature. An amplifier according to any preceding claim, characterised in that the chip temperature measuring means (17) produces a temperature signal representative of the measured temperature and supplies the temperature signal to a control section (22) comprising said gain selectors (24 1 - 24 4 ), each gain selector being a comparator fed by the temperature signal and a different threshold signal, each one of the threshold signals representing a different temperature in a range of temperatures, each one of the comparators (24) producing a control signal for a corresponding one of the said amplifier stages (14). An amplifier according to claims 1 to 6, characterised in that each of the said amplifier stages (14 2 - 14 5 ) comprises feedback circuitry in the respective stage. An amplifier according to any one of claims 1 to 7, characterised in that each of the said amplifier stages (14 2 '- 14' 5 ) includes at least two switchable elements (31 1 , 31 2 ) for providing the respective amplifier stage with at least two discrete changes in gain in accordance with the temperature of the chip (12). A method for operating an amplifier according to claim 1, characterised by measuring the temperature of the chip; and changing the gain by at least one of the gain selectors in accordance with the temperature of the chip. A method according to claim 9, characterised in that each amplifier stage has a switchable element and provides a first gain when said switchable element is switched into one state and a second gain when said switchable element is switched into a second state, and in that the step of changing the gain comprises operating such temperature compensation circuitry switches selectively in accordance with the temperature of the chip. A method according to claim 10, characterised in that each amplifier stage has a feedback element and a switch as said switchable element said feedback element being in shunt with said switch; and the step of changing the gain comprises: operating said switch to either shunt said feedback element or un-shunt said feedback element, the respective stage providing a first gain when said feedback element is shunted by the switch and a second gain when said feedback element is un-shunted by said switch, the respective temperature compensation circuitry placing said switch in a condition to either shunt said feedback element or un-shunt said feedback element selectively in accordance with the temperature of the chip. A method according to claim 9, characterised in that the amplifier has an uncompensated gain which decreases with increasing temperature, and the step of changing the gain comprises: operating the at least one gain selector to provide the respective amplifier stage with a higher gain at a temperature above a predetermined threshold temperature associated with such stage and with a lower gain at a temperature below such predetermined threshold temperature associated with such stage, each one of such stages being associated with a different predetermined threshold temperature.

Inventors: GOFF MILES E, GOFF, MILES E.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1999
Main classification: H03F1
IPC codes: H03F1