Amplifiers Explained: Gain, BJT, MOSFET, Frequency Response, and Classes

An amplifier is an electronic circuit that increases the strength of a signal without changing its basic information or intended waveform shape.

The simplest way to understand amplification is signal scaling: a small input waveform enters the circuit and a larger output waveform appears at the load.

• Amplification should increase signal level, not create unwanted distortion.
• The amplifier uses DC supply power to make the output signal larger.
• Voltage gain is commonly written as Av = Vout / Vin.

Real-world signals from microphones, antennas, and sensors are often too weak to drive loads or processing circuits directly.

Amplifiers make these small signals usable in audio systems, communication receivers, measurement instruments, wireless devices, and control systems.

• Audio amplifiers drive speakers and headphones.
• RF and IF amplifiers strengthen communication signals.
• Instrumentation amplifiers help read small sensor outputs.

Every amplifier can be viewed as an input port, an output port, and a DC power supply. The input signal controls how supply energy is converted into output signal energy.

Gain tells how strongly the amplifier scales a signal. If Vin is 20 mV and Vout is 2 V, the voltage gain is 100.

• Input port receives the small signal.
• Power supply provides energy for the larger output.
• Output port delivers amplified signal to the load.

Amplifiers are classified by the quantity they amplify, the frequency range they work in, and the transistor configuration used.

For exam preparation, the most common categories are voltage amplifiers, current amplifiers, power amplifiers, audio amplifiers, RF amplifiers, common-emitter stages, and common-source stages.

• By signal quantity: voltage, current, and power amplifiers.
• By frequency: audio, IF, and RF amplifiers.
• By configuration: CE, CS, CB, CG, emitter follower, and source follower.

In a BJT amplifier, the input signal is applied at the base. A small change in base current causes a larger change in collector current, and the collector resistor converts that current change into output voltage.

A common-emitter amplifier gives significant voltage gain and usually produces a 180 degree phase shift between input and output.

• Input signal changes base current.
• Collector current changes more strongly.
• Output voltage develops across the collector load resistor.

In a MOSFET amplifier, the input signal is applied at the gate. Gate-source voltage controls channel strength, which changes drain current and output voltage.

A common-source MOSFET amplifier is the MOS counterpart of the common-emitter BJT amplifier. It can provide voltage gain and phase inversion with very high input impedance.

• Input voltage controls channel formation.
• Drain current changes with gate-source voltage.
• High input impedance makes MOSFET amplifiers useful in ICs and sensor interfaces.

Amplifier gain is not constant at every frequency. It usually falls at low frequencies due to coupling and bypass capacitors, remains nearly constant in the midband, and falls again at high frequencies due to internal capacitances.

The useful operating range is called bandwidth. It is commonly written as BW = fH - fL, where fL and fH are the lower and upper cutoff frequencies.

• Low frequency region: gain drops.
• Midband region: gain remains almost constant.
• High frequency region: gain drops due to capacitance and device limits.

Amplifier classes describe how much of the input cycle the active device conducts. This affects efficiency, distortion, and application area.

Class A gives high linearity but low efficiency. Class B improves efficiency but can create crossover distortion. Class AB is a practical compromise, while Class C is used mainly in tuned RF circuits.

• Class A: conducts for the full cycle.
• Class B: conducts for half cycle.
• Class AB: conducts slightly more than half cycle.
• Class C: conducts less than half cycle, useful in RF tuned circuits.

Distortion occurs when the output is not a faithful scaled version of the input. It can change waveform shape, frequency balance, or phase relation.

Common distortion types include harmonic distortion, frequency distortion, and phase distortion. Good amplifier design tries to keep signal scaling clean over the required bandwidth.

• Harmonic distortion changes waveform shape.
• Frequency distortion amplifies some frequencies more than others.
• Phase distortion shifts frequency components unevenly.

Practical amplifier questions often test gain, input impedance, output impedance, bandwidth, efficiency, and maximum undistorted output swing.

A good amplifier has the right gain for the job, enough bandwidth, acceptable distortion, suitable impedance levels, and safe power dissipation.

• Gain decides signal scaling.
• Input impedance decides loading on the source.
• Output impedance decides how well the amplifier drives the load.
• Efficiency matters strongly in power amplifiers.

BJT amplifiers are current-controlled and often provide strong transconductance for analog gain. MOSFET amplifiers are voltage-controlled and offer very high input impedance.

Both are important. BJT amplifiers are common in discrete analog learning, while MOSFET amplifiers dominate integrated circuits and many modern mixed-signal designs.

• BJT: current control, lower input impedance, strong analog gain examples.
• MOSFET: voltage control, high input impedance, common in IC design.
• Both require correct biasing before small-signal gain analysis.