Our assignment for this project consisted of designing & building a Signal Generator from scratch. This Generator must create Sine, Square and Saw Tooth wave forms.

Using our proto-board we assembled a generator using three staged 741 Operational Amplifiers. The first stage is a Wien-bridge Oscillator to create our Sine Wave shape. It's output is a 1.5 khz signal @ 2 Volts peak-to-peak.

The second stage consists of a Schmitt Trigger to take the Sine Wave & convert it to a Square Wave. The output signal for this stage is 25 Volts peak-to-peak with a 41.66 % Duty Cycle.

The final stage takes the Square Wave & convert it to a Triangle wave. This Wave shape is still running at 1.5 kHz at 2.6 Volts peak-to-peak.

The following is a description of each stage. Also included are graphical illustrations for each said stage.

Wien-Bridge Oscillator circuit...


The Wien-Bridge oscillator is used to convert DC voltages to an AC Sine Wave. In order for oscillation to take place, you must have 360 deg. of phase shift and meet 3 criteria.

The three criteria, or requirements, that must be met for oscillation are as follows...

1: Positive Feedback (or Regenerative Feedback) Using a ratio of RÄ and Ri a feedback voltage gain is applied to the input to help stabilize the output signal.
2: Trigger This Trigger causes the DC voltage to go to positive and negative Vsat condition.
3: The Barchcausian criteria This states that your attenuation factor multiplied by your voltage gain must equal 1.

The Wien-Bridge oscillator fulfills these requirements because there is a low pass filter, a high pass filter and a 180 deg. phase shift from the feedback networks from the input to output. This totals 360 deg. to start the circuit oscillating.

If all three of the criteria are met and you have a resonant Ärequency under 1 MHz, then you have a circuit that produces a Sine Wave from a DC power source.

Schmitt Trigger circuit...


The second stage of our Function Generator is this Schmitt Trigger. This circuit is used to convert a Sine Wave into a Square Wave. This happens because the input signal is large enough to pass through both the lower and the upper voltage trip points.

This is possible when the upper part of the Sine Wave hits the output voltage. The voltage will go directly to +Vsat and stay there until the lower peak causes it to go to -Vsat add infinitum.

This was all made possible from the negative feedback loops.

Intergrator circuit...


This stage of our frequency generator is known as the Integrator. So named because of the mathematical process that it does.

This circuit produces a "ramp" voltage. Normally you would have just a capacitor in the negative feedback loop. But because we want no output offset voltage, we had to put a resistor in parallel with the cap. Besides limiting the output offset, it produces our "Virtual Ground."

This virtual ground is created by the 741's very high input impedance. About 150 M*'s of it. What happens is the input resistor is connected to the positive input and a resistor coming from the output signal is connected also to the +in. Total current will go through the first resistor and split towards the second resistor because it's an easier path to ground.

When this is set up like this, as long as both resistors are less than 150 Megs, a ratio situation occurs between them. Depending on the ratio, or Gain created, you can control how much amplification it has. Now throe in the cap and a Saw Tooth wave shape is created.

Schematic of full circuit...


In conclusion, the 741 Op Amp can be used in a variety of ways to produce many different outputs. It does this by having many diverse methods of installation. By playing with component values and connections, you can create and manipulate any desired AC output signals.

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