Download A Triangular Wave Oscillator

Relaxation Oscillators
Use an RC timing circuit and a device that
changes states to generate a periodic
waveform
Non sinusoidal Waveform Generation
A Triangular Wave Oscillator
Positive and Negative going
ramps are generated when the
dual polarity switch is thrown
back and forth at fixed intervals.
This creates the shape of a
triangular waveform
Basic Triangular Wave Oscillator
Practical Triangular Wave Oscillator
• Output of Comparator circuit
is a square wave having
constant positive and
negative levels.
• Square wave at the input
(inverting) generates
triangular wave at the output
of Integrator circuit.
• UTP (Upper Trigger Point)
and LTP (Lower Trigger Point)
determine the time of
switching for comparator
• More than one output
periodic waveforms are
present in the circuit and
hence the name Function
Generator is also applicable
for this circuit
• The amplitude of the square wave is set by the
output swing of the comparator
• R2 and R3 set the amplitude of the triangular
wave by establishing UTP and LTP voltages
• +Vmax and –Vmax levels are equal
• The frequency of both waveforms depends on
R1 C time constant as well as the amplitude
setting resistors,R2 and R3
By varying R1 ,the frequency of oscillation can be adjusted without
changing the output amplitude
Sawtooth Voltage Controlled Oscillator (VCO)
• VCO is a relaxation oscillator whose frequency can be changed by a
variable dc control voltage.
• VCO can be either sinusoidal or non sinusoidal.
• PUT>>(Programmable Unijunction Transistor) terminates each ramp
at a prescribed level and effectively reset the circuit.
• PUT is a programmable uni junction transistor with an anode, a cathode,
and a gate terminal.
• The gate is always biased positively with respect to the cathode.
• When the anode voltage exceeds the gate voltage by approximately 0.7 V,
the PUT turns on and acts as a forward-biased diode.
• When the anode voltage falls below this level the PUT turns off.
Also, the current must be above the holding value to maintain conduction.
•The circuit operates as a regular integrator when the negative input dc
voltage –VIN is applied
•The PUT triggers on when the output ramp (at the anode)
exceeds the gate voltage by 0.7 V
•The gate is set to the approximate desired sawtooth peak voltage.
•When the PUT turns on, the capacitor rapidly discharges.
•The capacitor does not discharge completely to zero because of the PUT's
forward voltage. V F .
•Discharge continues until the PUT current falls below the holding value.
•At this point, the PUT turns off and the capacitor begins to charge again,
thus generating a new output ramp.
•The sawtooth amplitude and period can be adjusted by varying the PUT gate
voltage.
•The frequency of oscillation is determined by the RiC time constant of the
integrator and the peak voltage set by the PUT.
•The time it takes a capacitor to charge from VF to Vp is the period, T, of the
sawtooth waveform (neglecting the rapid discharge time).
A Square-Wave Oscillator
The Op-amp's inverting input is the
capacitor voltage VC
The non inverting input is a portion
of the output fed back through
resistors R2 and R3
Let initially the capacitor be
uncharged, and thus the inverting
input is at 0 V.
This makes the output a positive
maximum
The capacitor begins to charge
toward Vout through R1
When VC equals Vf , the op-amp
switches to the maximum negative
state.
The operation of this oscillator is
based on charging and
discharging of a capacitor, so it is
a relaxation oscillator
At this point, the capacitor begins to discharge from + Vf towards –Vf
When the capacitor voltage reaches – Vf , the op-amp switches back to the
maximum positive state.