Bench Talk

Operational amplifiers (op amps for short) are one of the workhorse components of circuit design. They can be used in wonderfully simple but also incredibly complex ways, including audio pre-amplifiers, small signal sensor amplification, filters, and digital-to-analog converters (DAC) to name a few. Notice that these are all analog signal examples, not digital signals (i.e., not a stream of 0s and 1s.) analog signals are real-world, continuous signals that have, theoretically an infinite resolution.

 Op amps come in variety of integrated circuit (IC) packages and typically feature two input pins [an inverting pin (-) and noninverting pin (+) ] and an output pin. Though they are often left off of schematic symbols, op amps also have pins for the positive and negative voltage supply rails. These are required to properly bias the internal components of the op amp which commonly includes transistors, resistors, diodes, and capacitors. Part of the appeal of using op amps is that you can eliminate these discrete components in favor of a single IC. Some ICs have multiple op amps in one package.

 What does an op amp do? In the simplest terms, an op amp produces an output voltage that is several orders of magnitude larger than the difference between the voltages that are present at the inverting and non-inverting input pins. In short, an op amp can amplify an input signal, i.e., make a signal larger in amplitude. The amount of amplification (also called “gain”) along with other performance characteristics such as bandwidth (BW) performance and output impedance can be tweaked by placing external components (e.g. capacitors, resistors, etc.) between the output and input pins in various ways.

 Before we look at some of the common op amp configurations, let’s first explore the characteristics of an analog signal which includes amplitude, frequency and phase. Amplitude relates to the signal strength (y-axis), frequency relates to the rate at which the signal repeats (related to the x-axis), and phase is how much a signal is shifted along the x-axis with respect to another signal or to an ideal signal.

Figure 1: Characteristics of an analog signal 

Two basic op amp configurations are known as “non-inverting amplifier” and “inverting amplifier”. The output of the non-inverting amplifier will remain in phase with the input signal, while the output of the inverting amplifier is 180-degrees out of phase. Both configurations take advantage of a design technique known as “negative feedback” which take the output signal and through a resistor network feeds it back into the inverting input. This is what allows amplification to occur. The non-inverting configuration feeds the input signal into the non-inverting (+) input of the op amp.  Conversely, with an inverting configuration, the signal is fed in through the inverting (-) terminal.

Figure 2: Non-inverting Op Amp Configuration 

Figure 3: Inverting Op Amp Configuration 

There are many ways to adjust basic op amp designs including using a variable resistor (a.k.a. trimpot or potentiometer) for the feedback resistor to help control the amount of amplification. This is useful when doing the final installation of a circuit out in the field. Additionally, you will sometimes place a capacitor across the feedback resistor to help roll off any positive feedback that might develop if the op amp does contain the necessary protections inside the IC. Did you know that there are almost 400,000 capacitors listed for purchase at Mouser Electronics?

Lastly, there are many more configurations that can perform special functions including differentiators, integrators, and summing amplifiers which are useful when trying to combine multiple input signals into one output.