Oscillator

An oscillator is a circuit that produces a repetitive waveform on its output with only the dc supply voltage as an input.A repetitive input signal is not required.The output voltage can be either sinusoidal or nonsinusoidal depending on the type of oscillator.

The basic concept of an oscillator is illustrated in figure(a).Essentially an oscillator converts electrical energy in the form of dc to electrical energy in the form of ac.A basics sinusoidal oscillator consists of an amplifier for gain( either discrete transistor or op-amp ) and a positive feedback circuit that produces phase shift and provides attenuation,as shown in figure(b).

The basic oscillator concept showing three common types of output waveforms sine wave,square wave and sawtooth.

Oscillator Principles:

Positive Feedback:

Positive feedback is characterized by the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift,resulting in a reinforcement of the output signal.This basic idea is illustrated in figure.As you can see the in phase feedback voltage Vf is amplified to produce the output voltage which in turn produces the feedback voltage.That is,a loop is created in which the signal sustains itself and a continous sinusoidal output is produced.This phenomenon is called oscillation.

Conditions for Oscillation:

Two conditions are required for a sustained state of oscillation.

1.The phase shift around the feedback loop must be zero degree.

2.The voltage gain Acl,around the closed feedback loop (loop gain) must equal 1 (unity).

The voltage gain around the closed feedback loop(Acl) ia the product of the amplifier gain (Av) and the attenuation (B) of the feedback circuit.

Acl=Av.b

For example,if the amplifier has a gain of 100,the feedback circuit must have a attenuation of 0.01 to make the loop gain equal to 1( that is Av.B=100*0.01=1).These conditions of oscillation are illustrated in figure.

Start-Up Conditions:

So far,you have seen what it takes for an oscillator to produce a continous sine wave output.Now let's examine the requirements for the oscillation to start when the dc supply voltage is turned on.As you know,the unity-gain condition must be met for oscillation to be sustained.For oscillation to begin,the voltage gain around the positive feesback loop must be greater than 1 ,so that the amplitude of the output can be build up to a desired level.The gain must then decrease to 1 so that the output stays at the desired level

Read More!

Operational Amplifier

The standard operational amplifier (op-amp) symbol is shown in figure(a).It has two input terminals,the inverting input(-) and the noninverting input(+) and an output terminal.The typical op-amp operates with two dc supply voltages,one positive and the other negative as shown in figure(b).Usually these dcvoltage terminals are left off the schematic symbol for simplicity but are always understood to be there.Some typical op-amp IC packages are shown in figure(c).

The Ideal Op-Amp:

To illustrate what an op-amp is.Let's consider its ideal characteristics.A practical op-amp of course,falls short of these ideal standards,but it is much easier to understand and analyze the device from an ideal point of view.

First,the ideal op-amp has infinite voltage gain and infinite bandwidth.Also it has an infinite input impedence(open),so that it does not load the driving source.Finally,it has a zero output impedence.These characteristics are illustrated in figure.The input voltage Vin appears between the two input terminals and the output voltage is AvVin as indicated by the internal voltage source symbol.The concept of infinite input impedence is a particularly valuable analysis tool for the various op-amp configuration,which will be discussed.

The practical Op-Amp:

Although modern integrated circuit (IC) op-amps approach parameter values that can be treated as ideal in many cases,the ideal device can never be made.

Any device has limitations,peak-to-peak output voltage,for example,is usually limited to slightly lass than the two supply voltages.Output current is also limited by internal restrictions such as power dissipation and component ratings.

Characteristics of a practical op-amp are very high voltage gain,very high input impedence,very low output impedence and wide bandwidth.Three of these are lebelled in figure:

.

Read More!