In an instrumentation amplifier a differential input is provided, one side of which is connected to an instrument ground that is remote from the amplifier power supply ground. The remote ground can, under some conditions, operate at a potential that is outside the potential span of the amplifier power supply. Such common mode potentials are difficult to cope with when the difference exceeds about 0.6 volt. The circuit employs a pair of transistors biased to equal current densities. The emitters constitute the circuit input terminals. Means are provided to adjust the transistor collector to base voltage to establish a constant predetermined current. The transistor that is to be connected to the remotely grounded input has a resistor coupled between emitter and base to produce a reference current. A resistor connected between the transistor bases will then assume a potential equal to the emitter potential difference. A current mirror reproduces the base resistor current at the amplifier output terminal. Since the transistor emitters can operate at several volts below ground, an extended common mode range is available.
A differential shunt feedback amplifier includes dual shunt feedback paths, with an emitter follower in each feedback path to provide a low impedance output with a well-defined gain. A common-mode current generator responsive to only the common-mode component of an input signal generates a current which offsets base current changes in the emitter followers caused by the common-mode component. The circuit is well suited for either push-pull or single-ended input operation over a wide range of frequencies.
A high common mode rejection differential amplifier wherein two serially arranged Darlington amplifier stages (28, 44) are employed and any common mode voltage is divided between them by a resistance network (53). The input to the first Darlington amplifier stage (28) is coupled to a signal input resistor (8) via an amplifier (16) which isolates the input and presents a high impedance across this resistor. The output of the second Darlington stage (44) is transposed in scale via an amplifier stage (66) which has in its input a biasing circuit which effects a finite biasing of the two Darlington amplifier stages (28, 44).
A differential input stage for an operational amplifier includes a transistor pair differentially connected and supplied with tail current through a series resistor. The tail current is supplied by a pair of current amplifiers having their outputs coupled to the tail current resistor. The current amplifier inputs are coupled to the bases of the input transistor pair so that they are differentially driven. If the tail current resistor is properly selected the differential output current is a linear function of the differential input voltage. A clamp is provided for the differential input at some relatively large input signal voltage.
A differential amplifier includes first and second current sources, first and second pnp transistors having bases connected to first and second input terminals and emitters connected to the first current source, an npn transistor having a base, collector and emitter respectively connected to the collector of the first pnp transistor, the first current source and a ground terminal, and an npn transistor having a base, collector and emitter respectively connected to the collector of the second pnp transistor, the second current source and the ground terminal.
A differential transimpedance amplifier with reduced input impedance and increased bandwidth having a pair of input contacts, a pair of summing transistors, a pair of feedback transistors, a pair of output resistors, a pair of feedback resistors, a first and a second node and a pair of output contacts. The input contacts each being connected to one or more sources of current that are to be summed and are connected to the pair of summing transistors. The pair of feedback transistors are connected to the pair of input contacts, the pair of feedback resistors and to the first node for differentially reducing the input impedance of the pair of summing transistors and to overcome voltage excursions at the pair of input contacts to increase the operational bandwidth of the transimpedance amplifier. The pair of output resistors are connected to the pair of summing transistors for converting the current sum for each input contact into a voltage level that is transmitted differentially by the pair of output contacts that are connected to the pair of summing transistors.
