Chapter 5 - Operational amplifiers

What does an amplifier do?

Increases the power or voltage of a signal

What is the general formula for voltage gain (G) of an amplifier?

G = VOUT / VIN

What is the output of a perfect amplifier compared to its input?

Enlarged and undistorted copy of the input

What is amplitude in a wave?

Maximum positive voltage from the 0V line (peak value)

What is an oscilloscope used for in amplifier work?

To observe signal voltage waveforms

What is a voltage amplifier?

An amplifier whose main job is to increase input voltage (not just current)

What is a power amplifier?

Amplifies both current and voltage to provide enough power to output devices like speakers

How do you calculate output voltage of an amplifier?

VOUT = G × VIN

How do you calculate required input voltage for a given output and gain?

VIN = VOUT / G

What is the frequency response of an amplifier?

How much the amplifier amplifies signals of different frequencies

What is ‘bandwidth’ of an amplifier?

Range of frequencies with gain greater than 70% of maximum (between -3dB points)

As gain increases, what happens to bandwidth?

Bandwidth decreases (inverse relationship; higher gain = lower bandwidth)

What equation relates gain and bandwidth for an amp?

Gain × Bandwidth = constant (gain-bandwidth product)

What is a block diagram of a simple audio amplifier system?

Microphone → Pre-amplifier → Power amplifier → Loudspeaker

What can a mixer block add to the system?

Adds together (and controls) signals from several sources, e.g. microphones, instruments, music

What is the symbol for an operational amplifier?

Triangle with inputs (-, +), output, powered by dual supply (sometimes not drawn)

What power supplies do standard op-amps use?

Dual rail: Positive and negative voltage (e.g. ±9 V)

What is the inverting input on an op-amp?

Labeled “–”, input that produces output 180° out of phase with input

What is the non-inverting input on an op-amp?

Labeled “+”, input that gives output in phase with input

What is the formula for voltage gain of a non-inverting op-amp?

G = 1 + RF / R1

What does the gain formula for non-inverting op-amp tell us?

All resistor values must be greater than 1kΩ, gain is always positive and >1

If R1 = 1kΩ and RF = 99kΩ, what’s the gain?

G = 1 + 99/1 = 100

Which op-amp input do you connect the signal to for non-inverting amplification?

Non-inverting input (+)

How does a non-inverting amplifier's output compare to input?

Same phase (positive in gives positive out)

What is ‘clipping distortion’ in amplifiers?

Output signal clipped when input is too large for supply, the amplifier cannot provide enough voltage—top/bottom of wave is cut off

How to avoid clipping in amplifiers?

Increase supply voltage, reduce gain, or reduce input amplitude

What is the voltage gain for an inverting op-amp?

G = -RF / RIN (negative sign for inverting)

What is the key feature of an inverting amplifier?

Positive input voltage produces negative output, and vice versa

If RF = 20kΩ, RIN = 2kΩ, what is the gain?

G = -20kΩ/2kΩ = -10

For inverting op-amp, if VIN = 0.1V and G = -10, what is VOUT?

VOUT = -1V (inverted and multiplied)

What is meant by an op-amp “saturating”?

Output cannot go beyond (is limited to) near the supply rails (e.g. ±13 V for ±15 V supply)

How can you display both input and output of an amplifier?

Use a dual-trace oscilloscope for accurate comparison

What is a ‘summing amplifier’?

Inverting op-amp circuit with two or more input resistors and input voltages added together

What is the formula for output of a two-input summing amplifier?

VOUT = -RF(R1×V1 + R2×V2)/(R1×R2)

Practical application for a summing amplifier?

Mixing signals in audio systems (e.g. DJ mixer, PA system)

What happens if you increase feedback resistor (RF) in a non-inverting amp?

Gain increases (output voltage increases for same input)

What’s the effect if the input amplitude exceeds what the supply rails allow?

Output is clipped/distorted—max/min value limited to supply rails

If a mixer needs three channels, how many input resistors does the summing amp need?

Three, one for each input. Each channel has its own resistor to the inverting input

What does a mixer allow you to do in a music system?

Independently fade in/out (control level of) each sound source before mixing

Why must all resistors in op-amp circuits be more than 1kΩ?

To limit current and avoid overloading op-amp or wasting power

What type of gain does a non-inverting amplifier have?

Positive gain (output and input in phase)

What type of gain does an inverting amplifier have?

Negative gain (output and input are 180° out of phase)

How does changing input resistor affect inverting op-amp gain?

Decrease RIN ⇒ gain increases (more negative); Increase RIN ⇒ gain decreases (less negative)

If gain is -50, RF is 100kΩ, what is RIN?

RIN = RF/50 = 2kΩ

What is “pre-amplifier” stage for?

Boosts quiet signals (e.g. microphones) before they’re mixed or sent to power amplifiers

If a PA speaker needs power, what must the power amplifier do?

Increase BOTH voltage and current

What determines where 70% gain (bandwidth) point is on a gain-frequency graph?

Frequency at which gain falls to 0.7 × max gain (find this on axis to determine bandwidth)

What happens if amplifier bandwidth is too small for signals used?

Some signal frequencies will be lost/attenuated—output distorted or sounds dull

What is the gain-bandwidth trade-off?

Higher gain means lower bandwidth: can’t get both high at once

What does “audio frequency amplifier” mean?

Amplifies signals in 20 Hz–20 kHz range (human hearing)

What does "wideband" or "video" amplifier mean?

Amplifies a range up to megahertz (for video signals)

If you apply two AC signals to a summing amp and both are max, what is the output?

Output combines both, may approach or reach supply rail and get clipped if not limited

What is the effect of having different resistor values for each input on a summing amp?

Weights how much each input contributes to output—controls balance/mix level

If a summing amplifier’s output is less than theoretical value at max input, why?

Op-amp is saturating—output can’t go beyond supply rail

What’s a public address system block diagram?

Microphone → Preamp → Mixer → Power amp → Loudspeaker(s), possibly with music source into mixer

How do you avoid distortion in a summing amplifier with two strong signals?

Reduce their amplitude or reduce RF value to lower total gain

Can you use op-amp amplifiers with DC and AC signals?

Yes; amplifies both voltage types, AC more common in audio applications

If a mixing circuit has input pots (variable resistors), what do they do?

Control the level ("fade in/out") of each channel’s contribution to the mix