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In this laboratory we continue our discussion on operational amplifiers (see the previous lab here: Activity 1. Simple Op Amps) focusing on variable gain / voltage-controlled amplifiers.
Most operational amplifier or op-amp circuits have a fixed level of gain. However it is often useful to be able to vary the gain. This can be done simply by using a potentiometer on the output of a fixed gain op-amp circuit, but sometimes it may be more useful to vary the actual gain of the amplifier circuit itself.
A variable-gain or voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage. This type of circuit has many applications, including audio level compression, synthesizers and amplitude modulation. It can be realized by first creating a voltage-controlled resistor, which is used to set the amplifier gain. The voltage-controlled resistor is one of the numerous interesting circuit elements that can be produced by using a transistor with simple biasing. Another approach is to use potentiometers to vary the value of the resistors that set the gain of the amplifier.
ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
2 1 kΩ resistor
1 4.7 kΩ resistors
3 10 kΩ resistors
1 10 kΩ potentiometer
1 OP97 operational amplifier
1 2N3904 npn transistor