Amplifiers and Oscillators

This is the shallowest topic in Electronics Information. The subtest asks what these two circuits are for and what distinguishes them, and nothing about how they are designed.

Amplifiers

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An amplifier increases the amplitude of a signal.

The output is a larger copy of the input, and the shape should be unchanged - same frequency, same waveform, bigger. A change in shape is distortion, and it is a fault rather than a feature.

The energy comes from the power supply. A microphone signal is tiny; a loudspeaker needs watts. The amplifier does not manufacture that difference, it controls the supply so the supply delivers it in the shape of the input signal.

That is the same point the transistor lesson makes, which is unsurprising, because a transistor is what most amplifiers are built from.

Gain

Gain = output / input.

An amplifier turning a 0.1-volt input into a 5-volt output has a voltage gain of 50.

Gain is a ratio, so it has no units.

Gain can be voltage gain, current gain or power gain, and a question may specify which.

Decibels are the other way gain is reported. A decibel is a logarithmic measure, which means a large gain is a manageable number: each 10 dB is a factor of ten in power. The subtest asks you to recognize the unit rather than to calculate with it, with one exception worth remembering: a change of 3 decibels is a doubling or a halving of power, so a 3-decibel loss means the power has fallen to about half. Two such steps, 6 decibels, is a factor of four.

More gain is not automatically better. Too much and the amplifier clips - the peaks of the waveform flatten because the output cannot exceed the supply voltage - and clipping is distortion.

Feedback

Feedback is sending part of the output back to the input, which is what the loop in the figure shows.

Negative feedback trades gain for quality. It is used in nearly every practical amplifier, because an amplifier with too much raw gain is unstable and distorts.

Positive feedback is how an oscillator is made. That is the connection between the two halves of this lesson.

The squeal from a public address system is positive feedback: the microphone picks up the loudspeaker, the amplifier amplifies it, the loudspeaker plays it louder, and the loop runs away. That is the everyday example, and it is exactly the mechanism.

Oscillators

An oscillator produces a repeating output signal with no input signal.

It is an amplifier with positive feedback, arranged so that the loop sustains itself at one frequency. Some noise is always present; the loop amplifies it, feeds it back in phase, and it grows until the circuit settles into steady oscillation.

An amplifier needs a signal to work on. An oscillator makes its own. That is the distinction the subtest asks for.

What sets the frequency: a tuned circuit of a capacitor and an inductor, or a crystal. A quartz crystal is very stable, which is why it sets the timekeeping in a watch and the clock in a computer.

Uses: radio transmitters, clock signals in digital circuits, tone generators, timing circuits.

Other signal circuits

Worth recognizing by name.

An attenuator is worth noting because it is the one that is not obvious: there are real reasons to make a signal smaller, and a question offering it among amplifier choices is checking that you read the direction. Attenuation is the word for that reduction in signal strength.

An operational amplifier (op amp) is an integrated circuit that amplifies the difference between two inputs. It is the general-purpose amplifier chip, and feedback sets its gain.

What you can skip

Across all 1,358 Electronics Information questions in our bank:

• Amplifier classes - Class A, B, AB and push-pull - never appear.
• Biasing networks. Bias comes up as a word; no question asks you to design or calculate one.
• Timer chips and specific oscillator circuits by name never appear. Know that an oscillator makes a repeating signal and that a quartz crystal keeps it stable.

Where people lose points

Saying an amplifier creates energy. The supply provides it.

Saying gain is measured in volts. It is a ratio, with no units.

Swapping positive and negative feedback. Negative stabilizes; positive oscillates.

Saying an oscillator needs an input signal. It does not, which is the point of it.

Thinking distortion means the signal got bigger. It means the shape changed.

Saying more gain is always better. Too much causes clipping.

Work one in under a minute

A public address system squeals whenever the microphone is turned up. What is happening, and what kind of circuit is the system temporarily behaving as?

The microphone is picking up its own loudspeaker, so part of the output is returning to the input in phase - that is positive feedback.

The loop amplifies its own output until it runs away, which means the system has become an oscillator.

The fix follows from the diagnosis: break the loop, by moving the microphone, lowering the gain, or pointing the speaker elsewhere.

Where this leads

Oscillators generate the carrier waves that radio transmission uses, and amplifiers are what make a received signal usable.