An oscillator produces an alternating output voltageof fixed or variable frequency.The waveform of many types of oscillator is a sinusoid(p. 44) but other waveforms may be generatedsuch as triangular, sawtooth, and square waves. Circuitsthat produce square waves, often called astable multivibrators,are described on p. 43.Oscillators may be based on a single transistor butan op amp is preferred for greater reliability. Each ofthe three oscillators described in this topic operateson a different principle but are all op amp oscillators.The main requirement is that the op amp shouldhave a full-power bandwidth that includes theintended operating frequency.PHASE SHIFT OSCILLATORAll three oscillators rely on positive feedback tokeep them oscillating. Part of the output of the opamp is fed back to its non-inverting input to maintainthe oscillations.The voltage at the positive terminal of the op ampis held constant at half the supply, and stabilised bythe capacitor.A rising voltage at the output is fed back to theinverting input (2), causing a fall of output. This isnegative feedback and the op amp would have astable output. There is positive feedback too, throughthe phase shift network.The network consists of three high-pass filters inseries. On p. 48, it is explained that the output from alow-pass filter lags behind the input by up to 90_.Conversely the output of a high-pass filter leads inputby up to 90_.Three filters could produce a phase lead of up to270_. At a particular frequency the lead will be 180_.The signal passing through the phase-shift network isfully out of phase with the op amp output. This signalis now amplified and inverted by the op amp, so it isin phase with the negative feedback signal. The outputfrom the circuit is unstable and oscillatesstrongly.The output waveform is a sinusoid with frequencyf 5 1/15.39RC.The circuit is a simple one that is best used as afixed-frequency oscillator. It would be too difficult tohave variable resistors or capacitors in the networkand to tune them all at the same time.

Power AmplifiersA power amplifier is used to produce a major effecton the surroundings. Examples are power audioamplifiers, producing sound at high volume, or motor

control circuits actuating the arm of an industrialrobot or aligning the dish of a radio telescope. Otherexamples include the circuits that produce the dramaticeffects of disco lighting. The power of thesedevices is rated in tens or hundreds of watts, sometimesmore. Even a pocket-sized CD player can havea 700 mW output, yet the laser light signal that itshead is picking up from the disc is far less powerfulthan that.The electrical signals that initiate any of theseactions may be of extremely low power. For example,the power output of a microphone or many othertypes of sensor, or the control outputs from a microcontroller,are usually rated at a few milliwatts. Theaim is to amplify the power of the signals from thesedevices so that they can drive powerful speakers,motors or lamps.The power at which a device is operating dependson only two quantities, the voltage across the deviceand the current flowing through it. The relationship issimple:P5IVThe amplifiers that we have described in Topics 8to 12 have mainly been voltage amplifiers. Some,such as the FET amplifiers, actually produce a currentthat is proportional to their input voltage but, eventhen, we usually convert this current to an outputvoltage by passing it through a resistor. Thus theearly stages of amplification are usually voltageamplification. Since P is proportional to V, amplifyingthe voltage amplitude of a signal amplifies itspower by the same amount.Descriptions of voltage amplifiers often refer tothe fact that the currents in amplifiers are small. Inmany instances the collector or drain current is only1 mA. There is good reason for this.As explained in Topic 27, large currents through asemiconductor device generate noise. This type ofnoise is a random signal, which shows up as backgoundhissing in an audio circuit. If the audio signalis weak, it may not be possible to pick it out againsta noisy background. In other kinds of circuit it maybecome evident as unpredictable behaviour, makingthe circuit unreliable.When a signal has been through several stages ofamplification it may have reached an amplitude of afew volts or perhaps a few tens of volts. But, for thereason given above, its current is rated as no morethan a few milliamps, perhaps less. Consequently, itspower level is low. The next and final stage is toamplify the current.CURRENT AMPLIFIERS

The two current amplifiers most often used are thecommon-drain amplifier and the common-collectoramplifier. Both of these are voltage follower amplifiers.Their voltage gain is just less than 1 and theyboth have high current gain. In addition, they havelow output resistance, a useful feature when drivinghigh-power devices.Self TestWhat are the alternative names for CD and CC amplifiers ?The voltage output of these amplifiers is generatedby passing a variable current through a resistor,either the drain resistor or the collector resistor. Thevalue of the resistor is chosen so that, when there isno signal, the output voltage sits at half waybetween the 0 V line and the supply voltage. Thisgives the output voltage room to swing to maximumextent in either direction without clipping orbottoming.An amplifier of this type is known as a Class Aamplifier. We examine this type in more detail in thenext section.