Introduction to Operational Amplifiers

In this Instructable, I will give an introduction to the Operational Amplifier, one of the most useful of analog devices. This device can be configured as a non-inverting or inverting amplifier, a comparator, voltage amplifier, summing amplifier, instrumentation amplifier, buffer, active filter, Wien bridge oscillator plus many other applications. The opamp comes in different configurations such as the single LM741 8 pin DIP or the LM324 14 pin quad op amp shown above. There are also types which come in surface mount variants.

The operational amplifier which is also known for short as an op-amp, is a DC coupled high gain voltage amplifier, incorporated into an IC chip. They have two inputs (differential input) and one output. They have been used as building blocks in analog electronics since the first devices came out in the late 1960's. One of the beauties of these devices is that they greatly simplified electronic design by the nature of their standardization. Designing amplifiers with discrete components involved a lot of tweaking because of the differences between active devices. If amplifiers are all built from the same silicon die, they can all be made the same and have the same characteristics. When designing with operational amplifiers, a specific gain for the device can be obtained by installing two external resistors with a specific resistance ratio. For example, if a voltage gain of 100 is desired, a 100k resistor and a 1k resistor could be put in the circuit to obtain a ratio of 100. Using this strategy, the gain is the same every time. The most popular op-amp of all time is the 741 which has been around since the early 70's and has been used by generations of hobbyists in everything from audio amplifiers to power supplies. The 741 hasn't been used by industry for many years because better op-amps have been developed, but they still have a following among hobbyists and are easy to obtain. The first devices came out in either an 8 pin dual inline package style or a circular metal can. Later, surface mount devices became available. The 741 and other op-amps of its vintage used bipolar transistors with devices using field effect transistor inputs coming out later. Field effect transistor inputs started to be used because of the need to have greater input impedance and lower current drain.

The non- inverting amplifier is the first circuit we will cover. In the above diagram, the op amp is wired with the input going to the positive input with the feedback resistor going to the negative input. The ratio of Rf to Rg determines the gain. In the case of the above circuit, the voltage gain is 10. The diagram in the middle the "real world" limitations of the 741 op amp when a 10 kHz square wave is fed into the input but comes out as a triangular waveform due to the limited switching speed of the device. When the input is lowered to a 1 kHz square wave the output improves and looks more like a real square wave. The measurement of the op amp's ability to follow the input signal's changes in amplitude is called the "Slew Rate" and is measured in volts-per-microsecond. The 741 has a very tame rating at .5 volts-per-microsecond. High-speed op amps have ratings as high as 5000 volts-per-microsecond although a typical one like the TL081 will have an average rating of 13 volts-per-microsecond.

The opamp can be configured in such a way as a 1-volt negative going waveform can be inverted and amplified to give a 10-volt positive going waveform. Uses for this configuration could be anywhere a phase change is needed such as in the driver stages of discrete transistor amplifiers.

The above circuit will change a 1000 Hz square wave to a 1000 Hz sine wave. It does this by filtering out all frequency components (harmonics) above and below the fundamental, which is a sine wave. Instead of using resistors in the feedback circuit, we use frequency selective components (capacitors) that provide negative feedback to cancel out the unwanted frequencies. The middle diagram shows the actual circuit simulated and the waveform produced. The third diagram shows the frequency response of the circuit. The technical name of this type of circuit is an active bandpass filter. It allows only a very narrow band of frequencies to pass through without being attenuated.

There are dedicated chips that are better comparators, but sometimes you might not have one on hand, so it's always useful to know how to make a comparator out of an opamp. A quick review of what a comparator is, it's basically an opamp set up as an amplifier with no feedback, allowing the amplifier to operate at its maximum gain. When one input is tied to a specific voltage such as the 3 volts shown on the diagram the circuit will give an output that is almost the maximum rail voltage when the two inputs are at the same voltage. In the case of the above circuit, a sine wave of 1 kHz gives an output when it rises above 3 volts and switches again when the sine wave goes below 3 volts. Comparators are commonly used in (ADCs) and relaxation oscillators.

The summing amplifier above takes in two 1 kHz signals, one of 10 mV peak to peak and another of 20 mV peak to peak. The resultant output is 60 mV peak to peak. Since it's an inverting amplifier, it puts out a signal of opposite phase.

Summing amplifiers are used in audio mixers where different inputs need to be added together. By feeding signals into potentiometers, the signals can be varied to give the desired output.

This circuit could be used to mix together two instruments and a vocal track, more inputs could be added on as were needed. Each input level can be adjusted independently with the potentiometers.