Advanced Physics Lab / Physics Instruments -  

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The Power/Audio Amplifier has a continuously adjustable input attenuator, both ac-coupled and dc-coupled inputs on separate BNC connectors, and output banana terminals. The input and output ground are common. On the rear panel is a 4 volt, 100 mA, regulated DC power source from two female banana jacks. This supply was designed to power the diode laser used in the Faraday Rotation apparatus but can be used for any other application. The unit is powered by a universal power supply with a universal AC power entry connector that can be used with AC power from 10 - 230 VAC and 50 - 60 Hz frequency. Thus, this unit can be used with AC power sources anywhere in the world.

As we have mentioned, the PAA1-A is used when measuring a small Verdet constant in a Faraday Rotation experiment. Since the angle of rotation of the polarization depends linearly on the solenoidal magnetic field, the larger the solenoid current, the greater the angle of rotation. Typical lock-in amplifiers produce only milliamperes of modulation current (TeachSpin's Signal Processor/Lock-In Amplifier SPLIA-1 produces 35mA) so the amplitude of the modulating magnetic field it can produce is very small. With its one ampere output current, the PAA1-A can produce a significant modulated magnetic field. For the solenoid used in TeachSpin's Faraday Rotation apparatus, the peak-to-peak magnetic field is about 22 mT. For some large inductive loads, the maximum output voltage may be somewhat reduced at the higher frequencies.

Large modulating currents can also be useful in other experiments that use lock-in detection. Measurements of very small Hall effect voltage can be enhanced by using large Hall currents. The measurement of extremely small resistance using four terminal connection is another.

Suppose the student is asked to measure the resistance of a once centimeter diameter, ten centimeter long rod of pure copper. The resistance of such a bar is about 13 micro ohms. In even an expensive multimeter, the ohmmeter will not make such a measurement; it will read zero. But a four terminal AC lock-in amplifier system, with one ampere modulating current easily measures the 13 microvolt ac signal.

The low output impedance of the PAA1-A makes it ideal for driving acoustic speakers. Of course, TeachSpin is not recommending students use this in place of their "boom boxes" but it is appropriate for various acoustic experiments in the lab. The Power/Audio Amplifier also finds a useful application in the temperature control circuit to operate a Peltier cooler, which typically needs a bipolar supply to be able either to heat or to cool the sample.

We've listed some of the jobs we've been having our "utility infielder" doing. We have no doubt that you will be extending this list. Let us know what you use it for and we'll pass the ideas on to the rest of the TeachSpin team.