Relaxation Oscillator
The circuit in Fig.1
operates as an unstable multivibrator producing at the output rectangular
pulses. It consists of a capacitor in parallel with a negative impedance
converter (NIC) shown within the dot lines. The circuit is known as Relaxation oscillator or Schmitt-trigger
oscillator
[1]
.
Fig.1 a) Relaxation oscillator
b)
I-V characteristic of the Negative Impedance Converter
The I-V characteristic of the NIC is shown in
Fig.1b. Due to the fact that there is a part in the I-V characteristic with a
negative slope (P2P4), it is possible to have
instability. This is achieved by placing a capacitor in parallel with the NIC.
As a result, rectangular pulses are
produced at the output of the amplifier. The OFF
state of these pulses is defined by,
while, the ON
state is defined by,
where, VCC and VEE are the DC power
supplies and,
The period of oscillation is given
by,
or,
In the case where, VCC=-VEE the expression could be rewritten as follows
that is, at the output we have square pulses.
For R1=R2
, it is, β=0.5, and
then,
