PHYS 381/504: Op. amps., quartz resonators, and noise

Operational Amplifier Setup

This circuit will use an operational amplifier ("op amp") for two reasons.

First, an op amp can magnify the output signal coming from the circuit, allowing analysis of the low levels of noise generated by thermal fluctuations. By amplifying the output signal, we can increase the sensitivity of our measurements (the computer only has a −10V to 10V 16 bit data acquisition "DAQ" board, and the noise is rather small).

Also, an op amp can be used as a signal inverter. This allows us to split the signal that will be processed, and add circuit elements to mitigate the effects of the other half of the circuit. (We will return to this later.)

We may use an op amp without worrying about affecting our input signals because of the high input impedance and low output impedance on the chip.

The op amp DIP (dual in-line package) we use in this experiment is OPA-404KP. Specifications can be found here.

Power the chip with +9V and −9V at the two V_CC pins.

[NOTE: It is important to give the op amp references both for +9V and −9V with respect to ground. If you wish to see why, try putting +18V into the +V_CC terminal when the tuning fork is in place later.]

Now, test the amplifier by attaching two resistors in the configuration below for a standard amplifier

Install a function generator as the input signal at V_in. Pick two resistors R₁ and R₂, and verify that the output signal voltage is approximately related to the input signal voltage by the relation

V_out/V_in = 1 + (R₂/R₁)

Using an oscilloscope, verify that the signal holds over a wide range of frequencies. 

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