Amplifiers and Operational Amplifiers Lecture

Vocabulary flashcards covering key terms and definitions related to power amplifiers, differential and instrumentation amplifiers, operational-amplifier parameters, coupling methods, and multistage amplifier concepts.

Power Amplifier

A stage that delivers significant output power to a load, typically the final stage in an electronic circuit.

Conduction Angle

Portion of the input signal cycle during which a power-amplifier device conducts current, expressed in degrees (0–360°).

Class A Power Amplifier

Conducts for the full 360° of the input cycle; highest linearity and lowest distortion but only ~25-50 % efficiency and high heat.

Class B Power Amplifier

Uses complementary devices, each conducting 180°; ~75-78.5 % efficiency but suffers crossover distortion and needs ≥0.7 V to turn on.

Class AB Power Amplifier

Hybrid of Class A and B; each device conducts >180° but <360° (≈270°), giving 50–60 % efficiency with minimal crossover distortion—common in audio.

Class C Power Amplifier

Heavily biased device conducts <180° (≈90°); 80–90 % efficient with high distortion, suited to RF, not audio.

Class D Power Amplifier

Switch-mode (PWM or PDM) amplifier whose transistors act as switches; theoretical efficiency up to 100 % with low heat and precise output.

Differential Amplifier

Op-amp configuration that outputs a voltage proportional to the difference between two input voltages; acts as a subtractor.

Unity-Gain Differential Amplifier

Differential amplifier whose four resistors are equal, giving a gain of 1 so Vout = V2 – V1.

Instrumentation Amplifier

High-CMRR, high-input-impedance differential amplifier (often three-op-amp topology) used to amplify small sensor signals.

Wheatstone Bridge

Four-resistor network that converts a change in resistance (e.g., sensor) into a differential voltage for amplification.

Light Dependent Resistor (LDR)

CdS photo-resistive cell whose resistance decreases with light intensity; often used in bridge circuits for light sensing.

Thermistor

Temperature-sensitive resistor (NTC or PTC) used in bridge networks to create temperature-dependent differential signals.

Input Capacitor (Cin)

Coupling capacitor placed at an amplifier’s input to pass AC signal while blocking source DC from disturbing bias.

Emitter Bypass Capacitor (Ce)

Capacitor placed across an emitter resistor to provide a low-reactance AC path, boosting gain by avoiding negative feedback.

Coupling Capacitor (Cc)

Capacitor connecting two amplifier stages, passing AC while blocking DC so each stage keeps its own bias.

RC Coupling

Most common stage-to-stage connection using a resistor–capacitor network to pass AC and block DC.

Transformer Coupling

Stage coupling method using a transformer for impedance matching and efficient AC transfer without capacitors.

Impedance Coupling

Coupling that uses an inductive coil as the load; rarely used because gain depends on frequency-dependent inductive reactance.

Direct Coupling

Connects amplifier stages without a coupling element; DC biasing is designed so stages share correct operating points.

Multi-Stage Amplifier

Circuit with two or more cascaded amplifier stages to obtain higher overall gain or special frequency response.

Cascading (Amplifiers)

Connecting amplifier stages in series; overall voltage gain equals the product of individual stage gains.

Gain-Bandwidth Product (GBWP)

Constant equal to op-amp open-loop gain times frequency where that gain is measured; GBWP = A×BW, sets gain-vs-bandwidth trade-off.

Open-Loop Gain (Ao)

Very high raw gain of an op-amp with no feedback—often 20 000–200 000 (≈100 dB) at DC.

Slew Rate

Maximum rate at which an op-amp output can change (dV/dt), specified in V/µs; limits high-frequency, large-amplitude accuracy.

Input Offset Voltage

Small DC voltage that must be applied between op-amp inputs to force zero output; arises from internal mismatches.

Input Offset Current

Difference between bias currents entering inverting and non-inverting inputs of an op-amp, causing extra output error.

Common-Mode Rejection Ratio (CMRR)

Ratio of differential gain to common-mode gain; high CMRR means the op-amp rejects signals common to both inputs.

Differential Input Voltage

Voltage difference V2 – V1 applied between op-amp inputs that the amplifier truly responds to.

Ideal Op-Amp Characteristics

Infinite gain, infinite input resistance, zero output resistance, infinite bandwidth, zero offset, and infinite CMRR & slew rate.

Voltage Follower

Op-amp circuit with unity gain (buffer) that provides high input impedance and low output impedance.

Inverting Amplifier

Op-amp configuration where output equals –(Rf/Rin)×Vin; provides phase inversion and precise gain set by resistor ratio.

Non-Inverting Amplifier

Op-amp circuit with gain 1 + (Rf/Rg); input applied to non-inverting terminal, preserving signal phase.

Summing Amplifier

Inverting op-amp that adds several input voltages, each scaled by its input resistor.

Instrumentation Amplifier CMRR

Typically >100 dB at DC, allowing accurate amplification of tiny differential signals in noisy environments.

Op-Amp Input Stage

Dual-input, balanced-output differential pair that sets input resistance and provides most voltage gain.

Intermediate Stage

Second differential amplifier stage that increases gain and converts to single-ended output.

Level-Shifting Stage

Transistor stage that shifts DC level of intermediate output toward ground to center output swing.

Push-Pull Output Stage

Complementary emitter-follower stage providing large current drive and low output resistance.

μA741

Industry-standard general-purpose op-amp IC used in countless educational and practical circuits.

Common-Mode Gain

Amplifier gain when identical signals are applied to both inputs; ideally zero but finite in real devices.

Unity-Gain Bandwidth

Frequency at which the op-amp’s open-loop gain falls to one (0 dB); numerically equal to GBWP for voltage-feedback op-amps.

Common-Mode Operation

Condition when identical voltages appear at both inputs of a differential amplifier; ideally produces no output change.

Servo Motor Controller

Application of a power amplifier that drives and regulates servo motors.

Audio Power Amplifier

Amplifier designed to drive loudspeakers with significant power while maintaining audio fidelity.

Push-Pull Amplifier

Topology using paired devices that alternately source and sink current, reducing even-order distortion.

RF Power Amplifier

Power amplifier optimized for radio-frequency signals, often Class C or switching for high efficiency.

Slew-Induced Distortion

Waveform error occurring when input’s required dV/dt exceeds op-amp slew rate, causing output flattening or ramping.

Bandwidth (Amplifier)

Frequency range over which amplifier gain stays within –3 dB of its mid-band value.

Common-Collector (Emitter Follower) Gain

Voltage gain slightly less than 1, making CC unsuitable as a voltage-gain stage in cascaded amplifiers.

Common-Base Amplifier Gain

Voltage gain also <1, hence rarely used for cascaded voltage amplification stages.

Common-Emitter Amplifier Gain

Voltage gain >1; most suitable BJT configuration for multistage amplifiers due to high gain and ease of coupling.

Gain-Bandwidth Trade-Off

Inverse relationship wherein reducing closed-loop gain increases amplifier bandwidth and vice versa.