Amplifier Fundamentals

Section Overview

Chapter 4 taught you to identify the parts; this chapter returns to how analog circuits behave, starting with the most important analog function of all: amplification. An amplifier makes a signal larger — it uses a small input to control a larger output that is, ideally, a faithful scaled copy. You met amplification in the transistor and the op-amp; here you'll understand it properly: gain as the core measure (output divided by input), why gain is so often given in decibels, and the key specs that separate a good amplifier from a poor one — bandwidth, input and output impedance, and distortion, including clipping when a signal hits the supply rails. You'll also take a light tour of the amplifier classes (A, B, AB, D) and the difference between small-signal and power amplification. The math stays light: gain ratios and a couple of decibel anchors, no heavy algebra.

Why This Matters

Amplification is everywhere a weak signal must become a useful one: the microvolts from a microphone or sensor, the faint signal from an antenna, the line-level audio that must drive a speaker. Understanding gain, bandwidth, and distortion turns a vague "the sound is weak" or "the sound is distorted" complaint into something specific — is the gain wrong, is the bandwidth rolled off, is the stage clipping? For a repair technician, the amplifier vocabulary is the language of analog faults: a dead or weak amplifier stage, a distorted output, a channel that's lost its highs. And the concepts scale from a tiny preamp transistor to a hundred-watt power amplifier, so learning them once pays off across an enormous range of equipment. This section builds the framework the rest of Chapter 5 — filters, feedback, signal conditioning — rests on.

Required Prerequisites

Transistors — BJT and MOSFET Fundamentals — the transistor is the basic amplifying device; this section generalizes its amplifier role.
Op-Amps and Amplifier ICs — the op-amp gave you gain, input/output impedance, and clipping at the rails, which this section formalizes.
Capacitor Behavior in AC Circuits — reactance and frequency, the basis for an amplifier's bandwidth and frequency response.

Recommended Consumables

No consumables required. The exercises are pen-and-paper gain and decibel calculations, plus reasoning about amplifier behavior.

Recommended Practice Hardware

Optional: a small audio amplifier module or a powered speaker, a signal source (a phone works), and — if you have one — an oscilloscope to see gain and clipping directly
A calculator with a log function for the decibel conversions (or just use the ×10 = 20 dB and ×2 ≈ 6 dB anchors)
No special hardware is required; the section stands on the concepts and calculations

Real-World Applications

Every audio device is a chain of amplifiers: a microphone preamp lifts microvolts to line level, tone and volume stages shape it, and a power amplifier drives the speaker with watts. Radios amplify the tiny signal an antenna captures. Sensors — from a guitar pickup to a medical electrode — need amplification before their signals can be used. Instrumentation amplifies and measures small voltages precisely. In each case the same specifications decide whether the result is good: enough gain, adequate bandwidth for the signal, impedances that don't load things down, and low enough distortion. When any of these circuits fails — weak, distorted, dead, or noisy — the amplifier concepts here are exactly what let a technician localize and describe the fault.

Common Challenges

Confusing gain as a ratio with gain in decibels. A gain of 100 and a gain of 40 dB are the same thing expressed two ways. Decibels are a logarithmic restatement of the ratio, not a different quantity.
Forgetting the bandwidth limit. An amplifier's gain isn't the same at every frequency — it holds over a band and rolls off outside it. A "gain of 100" usually means within the amplifier's bandwidth.
Mistaking clipping for a broken amplifier. When an amplifier is driven so hard its output would exceed the supply rails, the peaks flatten — clipping. It's a distortion symptom (often just too much input or volume), not always a failed part.

Safety Notes

Risk Level: Low. The concepts and calculations here are low-risk. Two practical cautions apply to real amplifier hardware.

Professional Tips Before Starting

Keep gain and decibels as two views of one thing. If you memorize just two anchors — ×10 is 20 dB and ×2 is about 6 dB — you can estimate most voltage gains in your head, because decibels add where ratios multiply (×10 then ×2 is 20 + 6 = 26 dB).
Always ask "over what bandwidth?" A gain figure without a frequency range is only half the story; a stage can have plenty of gain at 1 kHz and almost none at 100 kHz.
When an output looks distorted, check for clipping first. Flat-topped peaks that reach the supply rails mean the stage is overdriven or the input is too large — a fast, common diagnosis before suspecting a failed component.

Understanding Amplification, Gain, and Amplifier Specs

What Amplification Is, and Gain

An amplifier's job is to make a signal larger while keeping its shape — to produce an output that is a faithful, scaled-up copy of the input. The signal carries the information (a voice, a measurement, a radio transmission); the amplifier makes it bigger or stronger so it can be used. The measure of how much bigger is the gain: simply the output divided by the input.

gain = output ÷ input

Gain comes in three flavors depending on what's being amplified: voltage gain (output voltage ÷ input voltage), current gain (the current version, which you met as a BJT's beta), and power gain (output power ÷ input power). Voltage gain is the one you'll use most. A worked example: a preamp with a voltage gain of 100 fed a 50 mV signal produces 50 mV × 100 = 5 V at its output — a small signal made a hundred times larger. A gain of 10 means the output is ten times the input amplitude; a gain of 1 (the voltage follower from Section 4.7) means the output equals the input, which is still useful for buffering.

Gain in Decibels

Amplifier gains span an enormous range — from a few times to millions — and they stack as signals pass through stages, so engineers usually express gain in decibels (dB), a logarithmic scale on which multiplying gains becomes adding decibels. For voltage gain the conversion is:

dB = 20 × log10(output ÷ input)

You don't need to compute logs by hand if you memorize a couple of anchors:

×10 = 20 dB (and ×100 = 40 dB, ×1000 = 60 dB — each factor of ten adds 20 dB).
×2 ≈ 6 dB (doubling adds about 6 dB).

So a gain of 100 is 40 dB, and a gain of 200 (×100 then ×2) is about 40 + 6 = 46 dB. The value of decibels is exactly this additivity: three stages of ×10, ×10, and ×2 give 20 + 20 + 6 = 46 dB total, no multiplication needed. (One caution: for power gain the formula uses 10 × log10 instead of 20×, because power depends on voltage squared — so a ×10 power gain is 10 dB. Voltage gain uses 20×; keep the two straight.)

Key Specifications: Bandwidth, Impedance, Distortion

Gain alone doesn't describe an amplifier. A few more specs matter:

Bandwidth — an amplifier doesn't amplify all frequencies equally. It holds its gain over a range of frequencies — its bandwidth — and the gain rolls off above (and often below) that range. (By convention the band edges are taken where the gain has fallen to about 70% of its peak — the −3 dB points.) An audio amplifier needs roughly a 20 Hz to 20 kHz bandwidth; a radio-frequency amplifier, much higher. There's a general gain-bandwidth tradeoff: for a given device, pushing for more gain tends to reduce the bandwidth, and vice versa.
Input and output impedance — from the op-amp section: a good amplifier usually has a high input impedance so it barely loads the source it reads, and a low output impedance so it can drive its load without sagging. Mismatched impedances lose signal.
Distortion and linearity — ideally the output is a perfectly faithful scaled copy (linear). Real amplifiers add some distortion; a good one keeps it low. The most dramatic distortion is clipping: drive the amplifier so hard that its output would need to exceed the supply rails, and the peaks are chopped flat at the rails instead — the same rail limit you saw with the op-amp. Clipping badly distorts the signal and, in audio, sounds harsh.

Amplifier Classes

Amplifiers are grouped into amplifier classes by how their output devices conduct — a light tour is enough here:

Class A — the output device conducts all the time, over the whole signal. This is the most linear (lowest distortion) but the least efficient: it wastes a lot of power as heat even with no signal, so Class A amps run hot. Common in high-quality small-signal and audio stages where fidelity matters most.
Class B — two devices split the work, each handling one half of the signal (one the positive half, one the negative). This is far more efficient, but the handover between the two halves creates crossover distortion at the zero-crossing.
Class AB — a practical compromise: biased so both devices conduct a little around the crossover, nearly eliminating crossover distortion while keeping most of Class B's efficiency. This is the workhorse of audio power amplifiers.
Class D — a switching amplifier: the output devices rapidly switch fully on and off (pulse-width modulation) and a filter reconstructs the signal, much like a switching regulator. Very efficient (little heat), so it dominates modern compact and battery-powered audio.

Small-Signal versus Power Amplification

Finally, keep the two big roles distinct. Small-signal amplification boosts a weak signal's voltage while handling very little power — a preamp lifting a microphone or sensor to a usable level. Power amplification takes a line-level signal and delivers the current and power to drive a real load, like a speaker or motor. A full audio chain does both: small-signal stages first for gain and shaping, then a power stage to drive the speaker. The same gain and specification ideas apply to both, but the power stage adds the concerns of heat, current, and efficiency that make the amplifier classes matter.

Common Mistakes

Treating a dB figure as a different quantity than the ratio. 40 dB is a gain of 100 — same thing, logarithmic units. Convert with the anchors.
Using 20× log for power gain. Voltage gain uses 20 × log10; power gain uses 10 × log10. Mixing them doubles or halves your dB.
Quoting gain without a bandwidth. Gain applies within the amplifier's bandwidth; outside it, the gain rolls off. State the frequency range.
Calling clipping a component failure. Flat-topped, rail-limited peaks usually mean the stage is overdriven (too much input or volume), not necessarily a broken part.

Troubleshooting Guidance

Amplifier faults become tractable once you speak the specs. When a signal is weak, think gain: has a stage lost its gain (a failed transistor or op-amp, a wrong-valued or open resistor setting the gain, a bad bias) so the output ratio is too low? When the signal is distorted, look first for clipping — flat-topped peaks pinned near the supply rails point to an overdriven stage or too much input, while distortion that isn't clipping suggests a biasing problem (a Class AB output stage drifting toward crossover distortion, say) or a failing device. When highs or lows are missing, think bandwidth: a stage whose frequency response has narrowed (often a changed capacitor value, tying back to Chapter 3's failure modes) rolls off part of the band. When a stage seems to load down the one before it, suspect an impedance problem. And in a multi-stage chain, use the additive nature of decibels to reason about where signal is being lost — trace the signal stage by stage and find where the level drops unexpectedly. Throughout, remember the power-amplifier cautions: its output stage runs hot and delivers real current, so treat it like the power circuit it is.

Verification & Testing Methods

Check your understanding before moving on:

[ ] Calculate a voltage gain as output ÷ input, and find the output for a given input and gain.
[ ] Convert a gain to decibels using the ×10 = 20 dB and ×2 ≈ 6 dB anchors (and note power gain uses 10× log).
[ ] Explain bandwidth, the gain-bandwidth tradeoff, and why high input / low output impedance is wanted.
[ ] Describe clipping and distinguish the amplifier classes A, B, AB, and D.

Then try the practice exercises below — pen-and-paper gain and decibel work, plus reasoning.

Practice Exercises

Gain arithmetic (5 minutes, pen and paper). (a) An amplifier with a voltage gain of 200 is fed 10 mV — what is the output? (b) A stage outputs 3 V from a 30 mV input — what is its voltage gain? Show output ÷ input.
Decibels by anchors (5 minutes, pen and paper). Using ×10 = 20 dB and ×2 ≈ 6 dB, give the dB value of a voltage gain of: (a) 40; (b) 20; (c) 4. Explain how you combined the anchors.
Spec reasoning (10 minutes, reasoning). An audio amplifier is described as "gain 30 dB, bandwidth 20 Hz to 20 kHz, high input impedance, low output impedance." Explain in plain words what each of those four specs means for how it behaves and why each is desirable.
Classes and clipping (10 minutes, reasoning). (a) Match each to its trait: Class A, Class B, Class AB, Class D — with "most linear but hot," "efficient but crossover distortion," "practical audio compromise," and "switching, very efficient." (b) A speaker output shows flat-topped waveform peaks. What is happening, and what would you check first?

These core ideas — gain and its decibel form, the key specs (bandwidth, impedance, distortion/clipping), and the amplifier classes — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

An amplifier makes a signal a larger, faithful copy; its gain is output ÷ input, in voltage, current, or power form (50 mV × gain 100 = 5 V).
Gain is often given in decibels: for voltage, dB = 20 × log10(out ÷ in), so ×10 = 20 dB, ×100 = 40 dB, ×2 ≈ 6 dB — and decibels add where gains multiply. (Power gain uses 10 × log10.)
Bandwidth is the frequency range over which the gain holds; gain rolls off outside it, and there's a general gain-bandwidth tradeoff.
A good amplifier has high input impedance (barely loads the source) and low output impedance (drives the load), and keeps distortion low; clipping is the flat-topping that occurs when the output is driven into the supply rails.
The amplifier classes: Class A (most linear, inefficient, hot), Class B (efficient, crossover distortion), Class AB (the practical audio compromise), Class D (switching, very efficient).
Small-signal amplification boosts a weak signal's voltage; power amplification delivers the current and power to drive a load like a speaker — a full chain does both.

Skills Learned

You can now calculate voltage gain as output ÷ input and apply a gain to find an output.
You can now convert simple gains to decibels using the ×10 = 20 dB and ×2 ≈ 6 dB anchors.
You can now name the key amplifier specs and explain bandwidth, impedance, and clipping.
You can now distinguish the amplifier classes and tell small-signal from power amplification.
You can now describe an amplifier fault — weak, distorted, clipping, or band-limited — in specific, diagnostic terms.

Glossary Additions

gain — the measure of how much an amplifier enlarges a signal: output divided by input. It comes as voltage gain (output ÷ input voltage), current gain, or power gain (output ÷ input power), and can be expressed as a plain ratio or in decibels.
decibel — a logarithmic unit (dB) for expressing gain (and other ratios): for voltage gain, dB = 20 × log10(output ÷ input), so ×10 is 20 dB and ×2 is about 6 dB; decibels add where gains multiply. Power gain uses 10 × log10 instead.
bandwidth — the range of frequencies over which an amplifier maintains its gain; outside this range the gain rolls off. There is a general gain-bandwidth tradeoff, where more gain tends to come at the cost of less bandwidth.
amplifier class — a classification (A, B, AB, D, and others) describing how an amplifier's output devices conduct, trading off linearity, efficiency, and heat: Class A is most linear but inefficient, Class B efficient but with crossover distortion, Class AB a practical compromise, and Class D a highly efficient switching design.
clipping — the distortion that occurs when an amplifier is driven so hard that its output would exceed the supply rails, flattening the peaks of the waveform at the rail voltage; a sign of an overdriven stage rather than always a failed part.

Suggested Next Sections

Must read next:

Filter Circuits — Low-Pass, High-Pass, Band-Pass — having made signals bigger, the next step is shaping them by frequency: the filters that pass some frequencies and block others, built from the resistors, capacitors, and inductors you know.

Recommended:

Op-Amps and Amplifier ICs — the op-amp gain, impedance, and rail-clipping this section generalizes.
Transistors — BJT and MOSFET Fundamentals — the transistor as the basic amplifying device behind the classes.

What You Will Learn

What You Will Be Able To Do

Required Tools