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Chapter 9. Operational Amplifier


The picture above shows the symbol on an operational amplifier, or opamp. Opamps are differential amplifiers with a very large gain: VOUT=(VIN1-VIN2)∙A. Gain A is usually greater than 100000. This means that VIN1-VIN2 must be very small: even 1mV would result in an output voltage of more than 100V, which of course is impossible, because the source voltage is just 18V. In the next section we'll see that the gain can be reduced by using resistors.

The input resistance of both input terminals is very high: »1GΩ. This means that there will be no current flow in the input terminals of an opamp.

IN1 is called the non-invering input; IN2 is called the inverting input, because its signal is inverted: if VIN2 increases, VOUT decreases and vice versa. The voltage at the non-inverting input is called Vp; the voltage at the inverting input is called Vn. So in this case: Vp=VIN1 and Vn=VIN2.