Differential amplifier multiplexer
FIELD OF THE INVENTION 
     This invention relates to differential amplifiers and more particularly to circuitry for selectively switching multiple input signals to the inputs of an analog differential signal stage. 
     BACKGROUND OF THE INVENTION 
     Certain known circuits for selectively switching analog signals to the inputs of a multiple-input signal processing stage, such as a differential amplifier or comparator, commonly rely upon switches in each signal input that include, for example, CMOS-type transistors that can be selectively triggered on or off to selectively pass or block the applied analog signals. However, circuits of this type commonly include significant resistance in the conduction mode of such transistor switches which, coupled with input capacitance of the following signal stage, significantly affects high-frequency signal transmission through the low-pass filter thus formed. 
     Alternatively, analog input signals may be selectively applied to the inputs of a multi-input signal processing stage via separate buffer amplifiers which can be selectively biased to enable and disable application of input signals to the inputs of the signal processing stage. However, such buffer amplifiers commonly introduce undesirable circuit complexity and offset voltages. 
     Still other known schemes for controlling the application of multiple analog signals to the inputs of an analog differential amplifier include cascaded pairs of differentially-connected transistors which are switched by transistors in the collector (or drain) circuits of each differential input pair of transistors. However, such circuits typically limit the range of signal levels over which high common-mode rejection can be achieved. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one embodiment of the present invention, pairs of gated transistors are serially coupled to form input stages of a differential amplifier, and control transistors are connected to the gates of input stages for controlling the application of the input signals and the conductivity of selected ones of the pairs of serially-coupled transistors. The conduction current through a pair of gated transistors of same conductivity type that form an input stage is compared with a reference current, and the difference of current levels in an input stage relative to the reference current is amplified and supplied as an output to following stages. The control transistors include pairs of gated transistors of opposite conductivity types forming connections to the gates of transistors in each input stage and commonly controlled in pairs by control signals applied to the gates of such serially-connected control transistors. In this way, switching of current sources to different stages with associated transient effects are substantially eliminated, and undesirable signal coupling through an `off` input stage is substantially eliminated. 
    
    
     DESCRIPTION OF THE DRAWING 
     The drawing is a block schematic diagram of the illustrated embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing, there is shown a block schematic diagram of one embodiment of the present invention in which a first pair of gated transistor devices 9, 11 of one conductivity type have source and drain output electrodes serially connected between a current source 13 and an input 15 of a current-difference amplifier 17. Second and third pairs of gated devices 19, 21 and 23, 25 have source and drain electrodes serially connected between the current source 13 and another input 27 of the current-difference amplifier 17. Gate (or input) electrodes of the second pair of devices 19, 21 are coupled together through the source and drain electrodes of gated transistor 29 of opposite conductivity type. One input signal Vs 1  30 is applied to the gate of device 19 in the second pair, and the source and drain electrodes of another gated transistor 31 of the one conductivity type connects the gate of the device 21 in the second pair to the node 20 in common connection with the current source 13. The gates of the devices 29 and 31 are connected in common to receive one control signal V a  33. 
     Similarly, gate electrodes of the third pair of devices 23, 25 are coupled together through the source and drain electrodes of gated transistor 35 of the opposite conductivity type. Another input signal Vs 2  36 is applied to the gate of device 23 in the third pair, and the source and drain electrodes of another gated transistor 37 of the one conductivity type connects the gate of device 25 in the third pair to the node 20 in common connection with the current source 13. The gates of the devices 35 and 37 are connected in common to receive another control signal V b  39. The gates of the devices 9, 11 in the first pair are connected to receive V REF  43 for bias to conduction, and the current-difference amplifier 17 produces an output current I 0  on line 41 proportional to the difference of the input currents I 2  and I 3  at the inputs 15,27. 
     In operation, the first pair of devices 9, 11 on one side and the second pair of devices 19, 21 (or, the third pair of devices 23, 25) on another side comprise a differential input stage for one input signal Vs 1  30 (or, for another input signal Vs 2  36) relative to V REF  43. With a control signal Va 33 high and control signal Vb 39 low for the connections of the conductivity type transistors as illustrated, then devices 23 and 25 form the differential input stage with devices 9, 11 for an input signal Vs 2  36 which thus compares the magnitudes and polarity of V REF  43 and the input signal Vs 2  36 to establish at the inputs 15, 27 of the amplifier 17 the ratio of currents I 3  and I 2  that flow from the source 13 of current I 1 . With control voltage Vb 39 in the low state while control voltage Va 33 is in the high state, then device 35 is biased conductive, or ON, and device 37 is biased non-conductive, or OFF. Thus, input signal Vs 2  36 is applied through device 35 to both of the devices 23, 25 in the third pair to drive, i.e., control the conductivity of such devices in the same way as the V REF  43 drives or controls the conductivity of the devices 9, 11 in the first pair. At the same time for the condition of signals Va 33 and Vb 39 as previously specified, the device 29 is biased non-conductive, or OFF, and device 31 is biased conductive, or ON, effectively biasing device 21 to very low conductance, or OFF, compared with the devices 23, 25 and 9, 11 biased to the conductive condition. In this bias condition of devices 9, 11, device 9 operates in the `saturated`, or voltage-controlled current source region, and device 11 operates in the linear resistive region of a typical MOSFET. The value of I 2  will thus be dependent upon the gate-source voltage between nodes 20 and 43 across device 9 and also across device 11 (where device 11 actually operates as a source degeneration resistor for device 9). 
     Similarly, if control signal Va 33 is low and control signal Vb 39 is high for the connections of the conductivity type transistors, as illustrated, then the device 29 is biased ON to connect together the gates of devices 29, 31 in the second pair, and to bias the device 31 OFF, while at the same time biasing device 37 ON which effectively biases device 25 OFF to isolate the input signal Vs 2  36 from affecting current in the input 27 of the amplifier 17. The amplifier 17 thus compares the magnitudes and polarity of V REF  43 and the input signal Vs 1  30 to establish the ratio of current I 3  and I 2  at the inputs 15, 27 of the amplifier 17. 
     The circuit of the illustrated embodiment and the associated operation thereof as previously described provides the advantage that the input signals Vs 1  30 and Vs 2  36 are directly applied to the input of the differential amplifier (formed by the first pair of devices 9, 11 and either the second pair of devices 19, 21 or the third pair of devices 23, 25) without loss or distortion of the applied signal. In addition, there is no switching required of current sources since current source 13 remains active with only minimum disturbances in I 2  and I 3 , and with only minimum output transients 41 upon switching of control signals Va 33 and Vb 39. The amplifier 17 of conventional design translates the difference of currents I 2  15 and I 3  27 to a proportional output current I o  41 for connection to following gain stages or other signal-processing stages in conventional manner. The amplifier 17 as an active load can have a number of different conventional configurations. For example, one common form of amplifier 17 converts differential current inputs to a single-ended output current to drive a succeeding stage. 
     In addition, since the input node 27 of amplifier 17 is typically low impedance for a current amplifier, then the only coupling of input signal is through the tiny gate capacitance of the device 19 or 23 (that is in the serial pair of devices which is biased OFF by the received input signal Vs 1  30 or Vs 2  36, respectively) to such input node 27, with only negligible effect. 
     Of course, it should be understood that the circuit of the present invention may be formed with the conductivity types of the gated devices 9, 11 19, 21, 29, 31, 23, 25, 35 and 37 reversed, and with associated reversals of current direction of source 13 and reversals of polarities of V REF  and Va and Vs for similar operation as previously described.