Lab 0: The Solex Agitator
In this mini lab, we created a circuit to produce signals varying in both amplitude and frequency.
The goal was to warm up our circuit prototyping skills and get comfortable working with the lab equipment again. To do this, we used four OP344 (the schematic has the OpAmps incorrectly labeled), three capacitors, resistors, and potentiometers. The configurations are shown below for each stage of the device as well as locations/types for outputs to the oscilloscope.
Identify the function of all four stages and explain how each works.
The first stage is the Astable Multivibrator. The Mutlivibrator is able to produce a square wave as output, with no input needed, by continuously switching between its two unstable states without the need for any external triggering. The width of each pulse is determined by the time spent in each state, which is determined by the charging or discharging of the capacitor through the resistor.
The second stage is the amplifier. This stage takes in the square wave from the Multivibrator as input and inverts and amplifies the signal; the potentiometer is used to control the amplitude of the pulse.
The third stage is the Integrator. This stage takes in the square wave of desired amplitude from the amplifier and integrates the signal. The output of this stage is a triangle wave, which is intuitive because the integral of the constant, straight line of each pulse in the square wave is a line of non-zero slope.
The final stage is also an Integrator. This stage takes in the triangle wave from the previous Integrator and integrates the signal one more time to produce a sinusoidal signal with the desired frequency and amplitude. Like the previous Integrator, this stage takes the integral of the signal. Since the integral of a linear function is quadratic, it is clear that the final output is a sine wave.
How would you convert the final sine wave to a square wave?
In order for the Agitator to output a square wave, a differentiator will need to be added after the second integrator.
How would you increase the maximum frequency of the circuit?
There are two main sections of the Astable Multivibrator, which is the stage that is used to control the frequency of the signal. The first section is the Feedback section, comprised of the feedback resistor and the capacitor; it provides frequency dependent feedback and controls the rate at which the capacitor charges and discharges. Since the width of the square pulse is determined by the time spent in each state, which is in turn determined by the charging/discharging of the capacitor, the values of the capacitor and the feedback resistor should be altered in order to increase the frequency of the circuit. The second section is the Hysteresis section, which is formed by the two resistors connected to Vp. These resistors enable the circuit to switch between the two states when the inverting input of the op-amp reaches at various voltages. When Vout feeds in a low voltage to the non-inverting input, it will take less time for the capacitor to charge/discharge to that value, shortening the width of the square pulse and thereby increasing the frequency.
The frequency is limited by the voltage rails. Because the voltage transfer of the differentiator is dependent on the resistance of the potentiometer, once the voltage rails of the OpAmps is reached, the frequency can not be increased. To increase the frequency, different resistor values for the differentiator would have to be calculated, and eventually, OpAmps that are designed for higher frequency output would be needed. Since OpAmps are internally transistors, there is a limited frequency range for each device.
What is the function of the Zener diodes?
A regular diode allows current to flow one way if the voltage is above a knee voltage and will have the same voltage drop independent of the amount of current flowing through. In reverse bias, no current will flow through the diode. A zener diode acts like a regular diode in the forward bias. However, when it is operated in reverse breakdown, the zener diode maintains the voltage across it at a constant level Vz, so long as the current passing through the diode remains between certain limits (above the knee current and below the maximum current the diode can handle). In the circuit, the zener diodes are used to make the edges of the square wave straighter.
The Zener diodes were omitted in the final circuit design by suggestion of the TAs. Theoretically, the zener diodes would have kept the “Square Wave” part of the circuit at ground voltage.