SIGNAL GENERATORS

Sinusoidal Oscillators

- Positive feedback loop consisting of an amplifier and an RC or LC frequency selective network.

- Obtained by shaping a triangular waveform.

Circuits that generate square, triangular, pulse waveforms are called nonlinear oscillators and employ building blocks known as multivibrators.

- Bistable- Astable- Monostable

Depending on the disturbance present in the input get a finite output.Voltage divider will feed a fraction of output ( = (R1/(R1+R2)).

This voltage is amplified by the high gain of the opamp.Regenerative process continues until opamp saturates to output as L+ or L-

Bistable Operation.

How to make the bistable circuit change state?

Application of an external input to the inverting terminal

VTH = L+

VTL = L-

The circuit in the L+ state can be switched to L- state by applying an input vi of value greater than VTH.

vi merely initiates the trigger regeneration.

vi can be a pulse of short duration trigger signal.

Bistable circuit acts as a memory element. For input voltage in the range VTL

v+ = vi ( R2/(R1+R2)) + vo ( R1/(R1+R2))

To trigger the circuit from L+ to L- vi must be made negative and of such a value as to make v+ decrease below zero.

VTL = - L+ (R1/R2)

VTH = - L- (R1/R2)

L+ = VZ1 + VD and L- = -(VZ2+VD)

L+ = VZ + VD1 + VD2 and L- = -(VZ+VD1 + VD2)

A square wave generator can be made by making bistable to switch states periodically.

Done by connecting a bistable multivibrator with an RC network in a feedback loop.

ASTABLE MUTIVIBRATOR

ASTABLE MUTIVIBRATOR

V+ = L+

V+ = L-

Astable Multivibrator

During the charging interval T1 the voltage v- across the capacitor at any time t with t = 0 at the beginning of T1 is

v- = L+ - (L+ - L- )e (-t/) where = CRv- = L+ at t = T1

T1 = ln ( 1- (L-/L+)/(1- )

During the discharging interval T2 the voltage v- across the capacitor at any time t with t = 0 at the beginning of T2 is given by

v- = L- - (L- - L+) e(-t/)

v- = L- at t = T2

T2 = ln ((1- (L+/L-))/(1- )T = 2 ln (1+ )/(1- ) = 2 RC ln (1+ )/(1- )

= 2 RC ln (1+2R1/R2) = 2RC for R2 = 1.164R1.

Replacing the low pass RC section with an integrator.

(VTH VTL)/T1 = (L+/CR)

T1 = CR(VTH VTL)/L+

(VTH VTL)/T2 = = - L-/CR

T2 = CR ( VTH VTL)/(-L-)

Monostable Mutivibrator

Circuit has one stable state in which it can remain indefinitely. It also has one quasi stable state to which it can be triggered and in which it stays for a predetermined interval equal to the desired width of the output pulse.

When the interval expires it returns to its stable state and remains there.

In the stable state output of opamp is at L+, diode D1 conducting through R3 clamping vB to one diode drop.

Select R4 much larger than R1, D2 conduct a small current and vC is nearly equal to L+. > vD1. Stable state.Application of a negative going step at trigger input.

D2 conducts heavily pulling node C down.

When vC goes lower than vB opamp will switch at output to L-.

vC to go negative to L-.D2 cutoff

Negative voltage at A causes D1 to cutoff.

C1 begins to discharge exponentially towards L- with a time constant C1R3.

Monostable in quasi stable state. Prevail until vB goes below the node voltage at C which is L-.At this instant opamp will switch back to L+ and voltage at node C goes to L+.Capacitor C1 then charges toward L+ until D1 turns on and circuit returns to stable state.

T = C1 R3 ln ( 1/1- ).