Abstract:
A circuit for reducing input offset error and improving gain switching speed in a programmable gain amplifier includes a level shifting buffer that senses a signal on a common mode conductor in a differential input stage of an operational amplifier, and shifts the level of that signal up to the level corresponding to a level of an input signal applied to a non-inverting input of the operational amplifier. If a gain select signal is at a first logic level, the voltage produced by the buffer is applied to a gate electrode of one of a plurality of gain switching JFETs coupling a gain network to the inverting input of the operational amplifier, turning that JFET on. If the gain select signal is at a second logic level, the output of the buffer is isolated from the gain switching JFET and a turn off voltage is applied to the gate of the gain switching JFET.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to circuitry biasing a field effect transistor having a source or drain coupled to an input of an amplifier so as to ensure that the field effect transistor is maintained in its triode region despite large changes in an input signal on an input of the amplifier, and more particularly to circuitry that greatly reduces input offset voltage due to parasitic leakage currents in JFET gain switches in a resistive gain network of a programmable gain amplifier and maintains selected JFETs in their triode regions irrespective of changes in the amplifier input signal. 
     Various programmable gain amplifiers (also referred to as switchable gain amplifiers are known in the art. For example, U.S. Pat. No. 4,855,685 discloses a typical programmable gain amplifier. Leakage currents from JFET switches commonly used in programmable gain amplifiers cause errors that degrade the gain accuracy. FIG. 4 shows a block diagram of a programmable gain amplifier including a high gain operational amplifier 50. Its non-inverting input receives the input voltage V IN . The inverting input of amplifier 50 is connected by conductor 22 to an input of each of P-channel JFET switch circuits 30-1, 30-2 . . . 30-N. Each such JFET switch circuit connects a different feedback resistor junction of feedback resistors 24-1, 24-2 . . . 24-N to the inverting input 22, to thereby allow accurate setting the gain of the programmable gain amplifier if JFETs of the JFET switch circuits are maintained in their triode regions. A decoder circuit 58 responds to gain setting inputs 59 to determine which of the various JFET switch circuits 30-1 . . . 30-N are open and which are closed. 
     The JFET switch circuits 30-1 . . . 30-N must be biased so that when each is turned off its gate-to-channel PN junction is reverse biased beyond its pinch-off voltage. The JFET switch circuits also must be capable of operating at suitably high speed. The parasitic junction leakage currents of the JFET switch circuits influence the voltage on conductor 22 and thereby cause an input offset error in amplifier 50. An example of a switching circuit that might be used to implement the JFET switching circuits 30-1 . . . 30-N is shown in FIG. 5, but that switching circuit has very high parasitic gate-to-substrate leakage current, and would cause a high input offset voltage error in amplifier 50. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a high speed programmable gain amplifier having low input offset errors. 
     It is another object of the invention to ensure that a turned on field effect transistor having a source or drain coupled to receive a common mode signal applied to an input of an amplifier is maintained in its triode region and is prevented from entering its current saturation region. 
     It is another object of the invention to provide circuitry useful in a programmable gain amplifier for greatly reducing or eliminating gain switch leakage currents flowing into the inverting input of the amplifier. 
     It is a general object of the invention to provide a circuit and technique for ensuring that a field effect transistor having a source or drain electrode coupled to an input of an amplifier remains in its triode region when the field effect transistor is in an on condition, despite large variations in a common mode voltage of the amplifier. 
     Briefly described, and in accordance with one embodiment thereof, the invention provides a circuit for reducing input offset error and improving gain switching accuracy and speed in a programmable gain amplifier. The circuit includes a level shifting buffer that senses a signal level on a &#34;common mode&#34; conductor in a differential input stage of a operational amplifier and shifts that signal level up to the level of an input signal applied to a non-inverting input of the operational amplifier. If a gain select signal is at a first logic level, the voltage produced by the buffer is applied to a gate electrode of one of a plurality of gain switching JFETs coupling a gain network to the inverting input of the operational amplifier, turning that gain switching JFET on. If the gain select input is at a second logic level, the output of the buffer is isolated from a gain switching JFET and a turnoff voltage is applied to the gate electrode of the gain switching JFET. In the described embodiment, first and second operational amplifiers receive first and second input signals at their non-inverting inputs. The outputs of the first and second operational amplifiers are coupled to the inputs of a difference amplifier, the output voltage of which is equal to the difference between the first and second input signals multiplied by the selected gain of the programmable gain amplifier. The gain network is coupled between the outputs of the first and second operational amplifiers. Separate buffers are connected to sense the signals and the common mode conductors in input stages of the first and second operational amplifiers, respectively. The various gain switching JFETs have their gate electrodes coupled to corresponding control circuits, a first group of which gate the output of the first buffer to gate electrodes of &#34;selected&#34; gain switching JFETs. Control circuits of a second group gate the output of the second buffer to gate electrodes of &#34;selected&#34; gain switching JFETs. The various control circuits apply a high voltage level to the gate electrodes of &#34;unselected&#34; gain switching JFETs connected thereto, and present high impedance to the gate electrodes of &#34;selected&#34; gain switching JFETs. The level shifting buffer circuits prevent forward biasing of gate-to-channel PN junctions of the various gain switching JFETs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is block diagram of a programmable gain amplifier of the present invention. 
     FIG. 2 is a detailed circuit schematic diagram of the decoder 40 of FIG. 1. 
     FIG. 3 is a detailed circuit schematic of the amplifier circuitry contained in blocks 13 and 25 of FIG. 1. 
     FIG. 4 is a diagram useful in describing the prior art. 
     FIG. 5 is another diagram useful in describing the prior art. 
     FIG. 6 is a schematic diagram of an alternative differential input stage that can be used in amplifiers 13 and 14 of FIG. 1. 
     FIG. 7 is a schematic diagram of another alternative differential input stage that can be used in amplifiers 13 and 14 in FIG. 1. 
     FIG. 8 is a schematic diagram of yet another alternative differential input stage that can be used in amplifier 13 and 14 of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, programmable gain amplifier 10 applies a differential input voltage equal to the difference between V IN   -   and V IN   +   across input terminals 11 and 12. Input terminal 11 is connected to the non-inverting input of operational amplifier 13, and input terminal 12 is connected to the non-inverting input of operational amplifier 14. The output of amplifier 13 is coupled by conductor 19 and resistor 20 to the inverting input of a differential amplifier 21, and the output of amplifier 14 is coupled by conductor 44 and resistor 45 to the non-inverting input of differential amplifier 21. An output voltage V PUT  is produced on the output of differential amplifier 21. 
     Resistors 24A-24G are series-connected between conductors 19 and 44. The inverting input of amplifier 13 is connected by conductor 22 to the drain electrodes of P-channel gain switch JFETs 23A-23D. The source electrodes of JFETs 23A-23D are connected to conductor 19, the junction between feedback resistors 24A and 24B, the junction between feedback resistors 24B and 24C, and the junction between feedback resistors 24C and 24D, respectively. Similarly, the inverting input of amplifier 14 is connected by conductor 42 to the drain electrodes of P-channel gain switch JFETs 23E-23H. The source electrodes of JFETs 23E-23H are connected to the junction between feedback resistors 24D and 24E, the junction between feedback resistors 24E and 24F, the junction between feedback resistors 24F and 24G, and conductor 44, respectively. 
     The gate electrodes of JFETs 23A-23H are connected by conductors 37A-37H to outputs of control circuits 30A-30H, respectively. Inputs of control circuits 30A-30H are connected by conductors 32A-32H, respectively, to outputs of a decoder circuit 40 which receives digital control input signals A0 and A1 to control which of the switch JFETs 23A-23H are opened and closed and thereby set the gain of programmable gain amplifier 10. 
     Amplifier 13 includes a differential input stage including an input NPN transistor 15 having its base connected to non-inverting input conductor 11. The emitter of transistor 15 is connected to the emitter of NPN transistor 16, which has its base connected to inverting input conductor 22. These two emitters are connected by conductor 18 to current source 17 and to the input of a level shifting buffer amplifier 25, the output of which is connected to conductor 26. Conductor 19 is sometimes referred to by those skilled in the art as a &#34;common mode conductor&#34; because it carries a voltage that is equal to or shifted by a constant voltage from the average of the voltage between the inverting and non-inverting inputs of the amplifier (assuming that the input transistors are matched). Conductor 26 is connected to a buffer input conductor of each of control circuits 30A-30D. Similarly, amplifier 14 contains two emitter-coupled NPN input transistors having their base electrodes connected to the non-inverting and inverting inputs, respectively, of amplifier 14. The common emitters of these two input transistors are coupled to the input of a level shifting buffer amplifier 46, the output of which is connected by conductor 43 to buffer inputs of control circuits 30E-30H. 
     Each of control circuits 30A-30H is essentially identical to the others, so only control circuit 30A will be described in detail. It includes a resistor 31 connected between +V CC  and conductor 32A. Conductor 32A is coupled to the source electrode of P-channel JFET 33, the drain of which is connected by conductor 35 to the base of NPN transistor 34 and the anode of diode 36 (which is implemented as a diode-connected NPN transistor having an emitter area equal to one fifth of the emitter area of transistor 34). The cathode of diode 36 is connected by conductor 37A to the emitter of transistor 34 and to the source and gate electrodes of P-channel JFET 38. The drain of JFET 38 is connected to conductor 26. The gate electrode of JFET 33 is connected to the collector of transistor 34 and to +V CC . 
     Table 1 lists exemplary values for the various resisters shown in FIG. 1, although one skilled in the art can readily supply suitable resistances to suit his or her specific design objectives. 
     
                       TABLE 1______________________________________  Resistor         Kilohms______________________________________  20,45  25  24A    18  24B    1.8  24C    0.18  24D    0.04  24E    0.18  24F    1.8  24G    18  31     5  47     25  48     25______________________________________ 
    
     A presently preferred implementation of decoder circuit 40 is shown in FIG. 2, with the control conductors 32A-32H being identified and the two gain selection inputs A1 and A0 also being identified. The four logical combinations of A1 and A0 allow four different gain values to be selected for programmable gain amplifier 10. The implementation of decoder circuit 40 is straightforward, and one skilled in the art can easily supply device geometries and component values to meet his or her circuit design objectives. Or, various alternate decoder circuit configurations could be used instead. For completeness, FIG. 3 shows a detailed circuit schematic diagram of amplifier 13, which includes level shifting buffer amplifier 25. Again, the implementation of amplifier 13 is straightforward, and one skilled in the art can readily supply the various device geometries and component values (or alternate circuit configurations) to meet his or her particular circuit design objectives. Buffer amplifier 25 is implemented by means of P-channel JFET 27 and diode-connected NPN transistor 28 in FIG. 3. Conductors 11 and 22, input transistors 15 and 16, current source 17, and common emitter conductor 18 have been labeled in FIG. 3 to illustrate precisely how amplifier 13 is connected into the diagram shown in FIG. 1. The implementation of amplifier 14 and buffer 46 is essentially similar to what is shown in FIG. 3. 
     It should be appreciated that in the differential programmable gain amplifier circuit structure shown in FIG. 1, which includes two operational amplifiers 13 and 14, rather than a single operational amplifier as shown in FIG. 4, cancellation of equal parasitic input offset currents such as I LTOT  in FIG. 4 is achieved if the operational amplifiers 13 and 14 are identical, because the input offsets will be of a common mode nature. Therefore, gain errors are produced in programmable gain amplifier as a result of parasitic leakage offset current such as I LTOT  only to the extent that those parasitic input offset currents are unequal or mismatched. 
     One integrated circuit implementation of the above described circuit configuration has an input offset specification of approximately 50 microvolts, and the typical input offset voltage is only approximately 20 microvolts at 25 degrees Centigrade, and at the highest specification temperature of 125° Centigrade the input offset voltage typically is only 35 microvolts. 
     The operation of the programmable gain amplifier can be easily understood if the operation of control circuit 30A is understood. Control circuit 30 contains internal circuitry responsive to the digital gain control inputs A0 and A1 to open or close a switch 61, so that if switch 61 is closed a current source 60 pulls control conductor 32A to V CC  -V P , where V P  is the pinch-off voltage of JFET 33, turning off current source JFET 33. If switch 61 is open in response to the gain setting control inputs A0 and A1, resistor 31 and JFET 33 cooperate to bring conductor 37A close to +V CC . If P-Channel gain switch JFET 23A is to be turned on, so as to cause it to approximate an electrical short circuit between its source and drain electrodes, its gate electrode must be brought to the same voltage as its source and drain electrodes, i.e., it must be biased to its &#34;triode&#34; region (which is well understood by those skilled in the art). 
     In control circuit 30A, NPN transistor 34 and diode 36 act as a follower circuit that tends to cause the voltage on conductor 37A to &#34;follow&#34; a rising &#34;deselect&#34; voltage on conductor 32A. When the &#34;deselect&#34; voltage on conductor 32A is pulled up to approximately +V CC , a voltage level of V CC  -V BE34  is produced on conductor 37A and hence on the gate electrode of gain switch JFET 23A, thereby turning it off (V BE34  being the base-to-emitter voltage of transistor 34). In this case, the high voltage on conductor 32A biases JFET 38 in its current saturation region, producing a high impedance between conductors 37A and 26, thus isolating the output of buffer 25 from conductor 37A. Because of the slow response of operational amplifier 13, the gate-to-source junction of switching JFET 23A can become forward biased during transient response of the circuit to a fast negative-going input voltage applied to terminal 11. For the present embodiment of programmable gain amplifier 10, recovery from such forward biased condition is fast enough that no undesirable change is produced in V OUT . 
     If switch 61 of decoder 40 is closed in response to the control inputs A 0  and A 1  so that a &#34;select&#34; voltage of V CC  -V P  appears on control conductor 32A, then JFET 33, transistor 34, and diode 36 are off, producing a very high impedance at conductor 37A, allowing it to rapidly respond to changes in the voltage produced on conductor 26 by buffer 25. The voltage on common mode conductor 18, which is one V BE  below the highest of V IN   +   and V IN   - , is sensed by JFET 27 (FIG. 3) in buffer 25 and is level-shifted up by V BE  voltage by diode-connected NPN transistor 28 in FIG. 3 to the original common mode input voltage level and appears on conductor 26. (Recall that level-shifting buffer 25A is implemented by means of JFET 27 and diode-connected transistor 28 in FIG. 3.) 
     With transistor 34 and diode 36 both off, and with JFET 38 biased in its triode region so as to produce a short circuit between its source and drain electrodes, the voltage level produced on conductor 26 is applied by JFET 38 and conductor 37A to the gate electrode of gain switching JFET 23A. Since the source electrode of JFET 23A is connected by conductor 22 to the inverting input of operational amplifier 13, it also has a voltage corresponding to the input signal level applied thereto. JFET 23A therefore is turned on and maintained in its triode region, so its drain electrode and conductor 19 will be electrically coupled to the inverting input of operational amplifier, establishing one of the four possible values of gain for programmable gain amplifier 10. With switching JFET 23A being maintained in its triode region while conductor 32A is at its &#34;select&#34; voltage, the feedback voltage is precisely fed back through gain switching JFET 23A to conductor 22 for the selected gain determined by A0 and A1. If gain switching JFET 23A were to be not maintained in its triode region, and it were to enter into its current saturation region, voltage feedback through switching JFET 23A would not be precise, so the amplifier gain would be imprecise. 
     Note that operational amplifier 14 and buffer circuit 46 simultaneously function entirely similarly to operational amplifier 13 and buffer 25 in response to the present values of A0 and A1, producing a value of V OUT  on the output of differential amplifier 21 equal to the selected gain multiplied by V IN   -  -V IN   + . 
     The remaining control circuits 30B-30H function essentially the same as 30A in response to different configurations of control inputs A 0  and A 1  to electrically connect various junctions of gain setting resistors 24A-24G to the inverting inputs of operational amplifiers 13 and 14. The gain network nodes are switched to the inverting inputs of op amps 13 and 14 simultaneously in response to A0 and A1. 
     It should be appreciated that there are many other applications of the invention than the programmable gain amplifier wherein a source or drain of a field effect transistor is coupled to an input of an operational amplifier or the like, and in which the FET needs to be maintained in its triode region, despite large charges in the common mode component of the input voltage. It is intended that the present invention encompass any circuit in which a common mode voltage conductor in the operational amplifier is coupled to the gate electrode of the FET so as to translate the common mode voltage to the gate of the FET while it is supposed to be in its on condition. 
     For example, FIG. 6 shows an alternate JFET differential input stage that could be included in amplifiers 13 and 14 instead of the bipolar NPN transistor differential input stage shown in FIG. 1. In FIG. 6, the signal on common mode conductor 18 is at a level that is equal to the average between the voltage levels on the inverting and non-inverting inputs plus a level shift of one V BE  voltage. The voltage on common mode conductor 18 is level shifted down by the V BD  voltage of emitter follower 64 to conductor 18A, which is applied to the gate of input JFET 27 of buffer 25A. The voltage produced on the source of JFET 27 is level shifted up to conductor 26 by level shift circuit 28A, schematically represented by a constant voltage source. Alternatively, the JFET differential input stage in FIG. 7 or FIG. 8 could be used. In FIG. 8 the input of buffer 25A is connected directly to the inverting input of differential input stage 13C, rather than being coupled to and driven in response to common mode conductor 18. The embodiment of FIG. 8 might be especially suited to an integrated circuit programmable gain amplifier manufactured using a Dielectric Isolation (DI) process, because gate-to-substrate parasitics thereof would be much smaller than for more standard integrated circuit manufacturing processes. If the differential input stage of FIG. 6, FIG. 7 or FIG. 8 is used, buffer amplifiers 25 and 46 would have a different configuration than shown in FIG. 3 to level shift the voltage at the drain of JFET 27 in FIG. 3 by a suitable amount. The &#34;DC&#34; level shift produced by circuit 28A in each case is the amount of level shift needed to cause the signal on conductor 26 to be equal to the V IN   -   signal on conductor 11. It should be appreciated that the common node conductor 18 referred to herein can be any conductor that produces a voltage which is a shifted average of the two input signals applied to the inverting or non-inverting inputs of the amplifiers 13 and 14 respectively. For example, conductor 30 in FIG. 3 of commonly assigned U.S. Pat. No. 4,901,031 &#34;COMMON-BASE, SOURCE-DRIVEN DIFFERENTIAL AMPLIFIER&#34;  issued Feb. 13, 1990 to Kalthoff et al. is such a common node conductor. 
     While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make the various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. It is intended that all combinations of elements and steps which perform substantially the same function in substantially the same way to achieve the same result are within the scope of the invention.