unchanged, then the resistance of the wire increase byR: Subtracting the two equations give: Expression divided by : Hence the change in resistance is directly proportional to the increase in length (extension), R LSome crystals such as quartz crystals produce an electric field when a force is applied causing changes in the shape of the crystal, known as the piezo-electric effectA piezo-electric crystal consists of positive and negative ions in a regular arrangement, hence when it is stressed, a small voltage is produced between the faces of the crystal, hence it acts as a transducer In microphones, the crystal is made into a thin sheet with metal connections on opposite sides; when a sound wave hits one side of the sheet, the compressions and rarefactions cause the pressure to increase and decrease, hence the crystal
9 |P a g e h t t p s : / / w w w . c i e n o t e s . c o m / changes shape in response to these pressure changes and a small voltage is induced across the connections Acoustic guitars and other instruments often use a piezo-electric transducer to produce an electrical output; the microphone is stuck to the body of the guitar and the electrical output can be amplified and played back through loudspeakers Electronics (Chapter 21): The goal of an amplifier is to produce a constant amplification or gain (all frequencies), hence operational amplifier (op-amp)has a very high gain and then provide an external circuit which reduces the gain but ensures that the overall gain is the same for signals of a greater range of frequencies The op-amp has two inputs: Inverting input, marked (-) Non-inverting input, marked (+) The function of op-amp is to use the potential difference between the two inputs (potential at inverting input (V-) and potential at non-inverting input (V+) to produce as large an output voltage Voutas possible The open-loop voltage gain G0is given by: Hence for the op-amp in Fig 25.15, the open-loop voltage gain is given by: It is called an open-loop because there is no loop of resistors or other components linking the output back to the input –it is just the operational amplifier alone Unlike a transformer, an op-amp’s output power is much greater than its input power; achieved by having two power supplies (e.g. +9 V and -9 V connections), and a zero volt line, or earth (all voltages are measured relative to this potential); one power supply will be between the +9 V and 0 V line and the other between -9 V connection and the 0 V line
10 |P a g e h t t p s : / / w w w . c i e n o t e s . c o m / The actual voltage used for the power supplies can vary in different circuits (providing the power for the op-amp); the positive and negative supply voltages are equal in magnitude and may be written as +Vsand –Vs, and are often left out for clarity The largest voltage an op-amp can produce is a value close to the supply voltage, e.g.