The Noninverting Op Amp

The noninverting op amp has the input signal connected to its noninverting input (Figure 3–2), thus its input source sees an infinite impedance. There is no input offset voltage because V OS = V E = 0, hence the negative input must be at the same voltage as the positive input. The op amp output drives current into R F until the negative input is at the voltage, V IN . This action causes V IN to appear across R G.

Figure 3–2. The Noninverting Op Amp

The voltage divider rule is used to calculate VIN; VOUT is the input to the voltage divider, and VIN is the output of the voltage divider. Since no current can flow into either op amp lead, use of the voltage divider rule is allowed. Equation 3–1 is written with the aid of the voltage divider rule, and algebraic manipulation yields Equation 3–2 in the form of again parameter.

When RG becomes very large with respect to RF, (RF/RG)⇒0 and Equation 3–2 reduces to Equation 3–3.

Under these conditions VOUT = 1 and the circuit becomes a unity gain buffer. RG is usually deleted to achieve the same results, and when RG is deleted, RF can also be deleted (RF must be shorted when it is deleted). When RF and RG are deleted, the op amp output is connected to its inverting input with a wire. Some op amps are self-destructive when RF is left out of the circuit, so RF is used in many buffer designs. When RF is included in a buffer circuit, its function is to protect the inverting input from an over voltage to limit the current through the input ESD (electro-static discharge) structure (typically < 1 mA), and it can have almost any value (20 k is often used). RF can never be left out of the circuit in a current feedback amplifier design because RF determines stability in current feedback amplifiers.

Notice that the gain is only a function of the feedback and gain resistors; therefore the feedback has accomplished its function of making the gain independent of the op amp parameters. The gain is adjusted by varying the ratio of the resistors. The actual resistor values are determined by the impedance levels that the designer wants to establish.

If RF = 10 k and RG = 10 k the gain is two as shown in Equation 2, and if RF = 100 k and RG = 100 k the gain is still two. The impedance levels of 10 k or 100 k determine the current drain, the effect of stray capacitance, and a few other points. The impedance level does not set the gain; the ratio of RF/RG does.