With this circuit the output is in phase with the input. The input impedance for the overall amplifier will be simply R G:įigure 2.4 shows what is known as the non-inverting configuration.
We can thus consider the - input to be at what is referred to as virtual ground. The - input is at the same voltage as the + input which is ground. The input impedance, as always, is the impedance to ground for an input signal. If we suppose that a second resistor (R G2) were to be connected from a second input voltage ( V IN2) to the summing junction, then the current in R F would be equal to the sum of the two input currents in R Gand this new R G2. Since the inputs to the op amp have infinite impedance and thus no current will flow in them, the sum of the current from the input in R G and the current from the output in R F must be zero. In the figure the inverting (-) input terminal is referred to as the Summing Junction.
Note that the output responds to differential-mode voltage and not the common-mode input voltage.Ģ.4 Inverting and Non-inverting Configurations As long as the inputs and output stays in the operational range of the amplifier, typically bounded by the positive and negative power supply voltages, it will keep the differential input voltage at zero, and the output will be the input voltage multiplied by the gain determined by the feedback network. The infinite open loop gain of the amplifier along with the external negative feedback will attempt to force the differential input voltage to zero. If the magnitude of the differential voltage is more positive on the non-inverting (+) terminal than on the inverting (-) terminal, the output voltage will swing toward the positive supply. If the magnitude of this differential voltage is more positive on the inverting (-) terminal than on the non-inverting (+) terminal, the output will swing toward the negative supply. Any differential voltage across the input terminals of the op amp is multiplied by the amplifier's open loop gain which is infinite for the ideal op amp. First, we assume that there is a portion of the output that is fed back to the inverting terminal to establish the fixed gain for the amplifier. The basic operation of the ideal op amp can be easily summarized.
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