A function generator is usually a piece of electronic test
equipment or software used to generate different types of
electrical waveforms over a wide range of frequencies.
Some of the most common waveforms produced by the function
generator are the sine, square, triangular and saw tooth shapes. These
waveforms can be either repetitive or single-shot.
Produces sine, square, triangular, saw tooth (ramp), and pulse output.
It can generate a wide range of frequencies.
Frequency stability of 0.1 % per hour for analog generators.
AM or FM modulation may be supported.
Output amplitude up to 10V peak-to-peak.
Simple function generators usually generate triangular waveform whose
frequency can be controlled smoothly as well as in steps.
This triangular wave is used as the basis for all of its other outputs. The
triangular wave is generated by repeatedly charging and discharging
a capacitor from a constant current source.
This produces a linearly ascending or descending voltage ramp. As the
output voltage reaches upper and lower limits, the charging and
discharging is reversed using a comparator, producing the linear triangle
By varying the current and the size of the capacitor, different
frequencies may be obtained.
Saw tooth waves can be produced by charging the capacitor slowly, using
a current, but using a diode over the current source to discharge quickly the polarity of the diode changes the polarity of the resulting saw tooth,
i.e. slow rise and fast fall, or fast rise and slow fall.
A 50% duty cycle square wave is easily obtained by noting whether the
capacitor is being charged or discharged, which is reflected in the current
switching comparator output.
Other duty cycles (theoretically from 0% to 100%) can be obtained by
using a comparator and the saw tooth or triangle signal.
Most function generators also contain a non-linear diode shaping
circuit that can convert the triangle wave into a reasonably accurate sine
wave by rounding off the corners of the triangle wave in a process similar
to clipping in audio systems.
SQUARE WAVE GENERATOR
First the inverting terminal (2) is at zero potential and the input at the noninverting terminal (3) has some potential V1. This occurs due to the power
supply of the operational amplifier. The potential difference between the
two input terminals is Vi= V1-0 = V1.
This ‘+ ve’ voltage drives the output of operational amplifier into ‘+ ve’
saturation voltage (+Vsat).
When the + Vsat is fed back to the inverting terminal (2) through the
resistor R, the capacitor C gets charged and the potential of the right side
plate of the capacitor gradually rises (or) the V2 value rises.
When V2 becomes slightly more than V1, the input becomes ‘–ve' and
immediately this ‘–ve’ voltage drives the output of the operational
amplifier in to ‘–ve’ saturation voltage (- Vsat).
Now the capacitor discharges gradually. When V2 becomes less than V1 and
(V1 – V2) becomes ‘+ve’ and the output drives to +Vsat. The same process
is repeated and the output of the operational amplifier swings between two
saturation voltages i.e. between + Vsat and - Vsat. The output Eo of the
operational amplifier is square wave.
The integrator acts like a storage element that "produces a voltage output
which is proportional to the integral of its input voltage with respect to
In other words the magnitude of the output signal is determined by the
length of time a voltage is present at its input as the current through the
feedback loop charges or discharges the capacitor as the required negative
feedback occurs through the capacitor.
SINE WAVE GENERATOR
Generation of sine wave can be accomplished by employing a positivefeedback loop. It consists of an amplifier and RC or LC frequency
A positive feedback loop is formed by an amplifier with gain A and a
frequency-selective network with gain β. In the figure an input signal of Xs is
shown but in actual oscillator no signal input is present. The amplifier,
however, needs a DC supply for proper operation. The loop gain of this circuit
in figure is given by –A(s)β(s).
For a sustained stable oscillation, the loop gain should be one; i.e.
1=A(s)β(s) = A(jω0)β(jω0). At the frequency ω0 the phase of the loop gain
should be zero and the magnitude of the loop gain should be unity. This is
known as Barkhausen criterion. The frequency ω0 should be unique
otherwise we wouldn’t have a pure sine wave.