All you will need to carry out a few simple experiments is a breadboard, a couple of very inexpensive op amps - I'd recommend the MPC; a few resistors, maybe an LED, phototransistor or photodiode, and any other odd bits you have lying around. What is a signal? In our Arduino world its a value - usually a voltage or current - that carries information.

Your Arduino will have signals going in - Analog or digital values; and signals out, to control motors, lights etc. This tutorial focusses mainly on issues in dealing with Analog input signals. Measuring signals The ANALOG inputs found on most Arduinos is well suited to measuring signals of a few volts from a low resistance source, but there ARE signals that dont match well to those analog inputs.

For example many high quality sensors produce only very small currents or voltages, and have a very high resistance, so they arent suited to direct measurement with a conventional Analog to Digital Converter ADC. This implies that the voltage drop across R1 will be zero. As a result, the current flowing through R1 and R2 must be zero. Thus, there are zero voltage drops across R2, and therefore the output voltage is equal to the input voltage, which is 0V.

When a positive-going input signal is applied to the non-inverting input terminal, the output voltage will shift to keep the inverting input terminal equal to that of the input voltage applied. The closed-loop voltage gain of a non-inverting amplifier is determined by the ratio of the resistors R1 and R2 used in the circuit.

Practically, non-inverting amplifiers will have a resistor in series with the input voltage source, to keep the input current the same at both input terminals. Virtual Short In a non-inverting amplifier, there exists a virtual short between the two input terminals. A virtual short is a short circuit for voltage, but an open-circuit for current. The virtual short uses two properties of an ideal op-amp: Since RIN is infinite, the input current at both the terminals is zero.

Although virtual short is an ideal approximation, it gives accurate values when used with heavy negative feedback. As long as the op-amp is operating in the linear region not saturated, positively or negatively , the open-loop voltage gain approaches infinity and a virtual short exists between two input terminals.

Because of the virtual short, the inverting input voltage follows the non-inverting input voltage. If the non-inverting input voltage increases or decreases, the inverting input voltage immediately increases or decreases to the same value. In other words, the gain of a voltage follower circuit is unity. The output of the op-amp is directly connected to the inverting input terminal, and the input voltage is applied at the non-inverting input terminal. The voltage follower, like a non-inverting amplifier, has very high input impedance and very low output impedance.

The circuit diagram of a voltage follower is shown in the figure below. It can be seen that the above configuration is the same as the non-inverting amplifier circuit, with the exception that there are no resistors used.

So, the gain of the voltage follower will be equal to 1.

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