Similarities and Differences between Power Amplifiers and Voltage Amplifiers

 

 

 

Similarities between Power Amplifiers and Voltage Amplifiers

 

Most of the time, power amplifiers and voltage amplifiers differ only in their emphasis on output quantity. Voltage amplifiers emphasize voltage, while power amplifiers emphasize power. Both voltage and power are output without distortion through the amplifier. Amplification circuits are generally used to increase voltage or power amplitude, so different electronic engineers use different terms for them. Regardless of the type of circuit amplified by a power amplifier, the load simultaneously presents voltage, current, and power.

 

Core Commonality: Signal Amplification

Despite their different focuses, voltage amplifiers and power amplifiers share a fundamental, core goal: increasing the amplitude of the input signal.

 

Same Basic Principle:

Both operate based on active devices (such as bipolar junction transistors (BJTs), field-effect transistors (FETs), or operational amplifiers. Energy is supplied by an external power source and manipulated by the input signal, resulting in a signal at the output that is identical to the input waveform but with a significantly increased amplitude.

 

Both are linear amplifiers:

Under ideal conditions and within a specific operating range, they are designed to perform linear amplification. This means the output signal should be a perfect reproduction of the input signal, without distortion, only with increased amplitude. Of course, all real amplifiers exhibit some nonlinear distortion, but minimizing distortion is one of their design goals.

 

Shared Core Performance Parameters:

They both focus on parameters such as gain, bandwidth, input/output impedance, and distortion, but the priorities and value ranges of these parameters differ significantly.

 

Differences between Power Amplifiers and Voltage Amplifiers:

 

The main difference between power amplifiers and voltage amplifiers is that power amplifiers have low output impedance, while voltage amplifiers have higher output impedance.

 

Besides this primary difference, power amplifiers and voltage amplifiers also differ in other ways:

1. Voltage amplifiers have very low collector currents, reaching up to 1mA, while power amplifiers have very high collector currents, exceeding 100mA;

2. Voltage amplifiers can be used for small voltage signals, while power amplifiers are often used for higher voltage signals;

3. Voltage amplifiers are used to amplify extremely low voltage signals in circuits, while power amplifiers are used to drive speakers.

A voltage amplifier increases the voltage level of a signal, while a power amplifier increases the power of a signal. This is a key difference between power amplifiers and voltage amplifiers. Voltage amplifiers have a relatively high output impedance, so they cannot be connected to low-impedance loads because they cannot deliver high current. Furthermore, power amplifiers have a relatively low output impedance and can deliver relatively high current.

 

Voltage amplifier circuits and power amplifier circuits also perform different tasks. The primary requirement for a voltage amplifier is to deliver an undistorted voltage signal to the load. Key specifications include voltage gain, input and output impedance, and so on. High output power is not necessarily required. A power amplifier circuit's primary requirement is to achieve a certain level of output power with no or minimal distortion, and generally operates with large signals. Therefore, power amplifier circuits will involve special problems that do not occur in voltage amplifier circuits.

 

Characteristic dimensions	Voltage Amplifier	Power Amplifier
Core objectives	Achieve high voltage gain. Amplify weak input voltage signals to a sufficiently high amplitude to prepare for subsequent circuitry (usually a power amplifier stage).	Get high output power. Provide the largest possible, undistorted AC power to the load.
Output	Output voltage. Output is typically measured in voltage (Vrms, Vpp).	Output power. The output is measured in power (W, mW). $P = V^2 / R_L$ or $P = I^2 \times R_L$.
Load impedance	High impedance load (usually several kiloohms to megaohms). The design goal is voltage amplification, and high impedance loads help reduce current demand, thereby lowering internal power consumption.	Low impedance load (usually 4Ω, 8Ω, 16Ω, etc., such as speakers). In order to deliver high power, a large current must be driven across a low impedance.
Transistor operating conditions	Usually operates in Class A or small-signal mode. The transistor remains on throughout the entire cycle of the input signal, resulting in optimal linearity but extremely low efficiency.	There are various operating categories, including Class A, Class B (Class B), Class AB (Class A and B), Class D (Class D), etc. In order to balance efficiency and linearity, Class B or Class AB push-pull structures are often used. Class D uses a switching mode and is extremely efficient.
Performance priorities	High voltage gain, high input impedance (to reduce impact on preceding stages), low output impedance (to enhance load handling capability), low noise, and a wide bandwidth.	High efficiency, low distortion, high power gain, and good heat dissipation management. The voltage gain may be less than 1 or even unity (1).
Signal amplitude	Small-signal operation. Input and output voltage and current amplitudes are small, and the transistor operates near the center of its linear region.	Large signal operation. The input and output voltage and current amplitudes are large, and the transistors operate in an area close to their limit parameters.
Heat dissipation considerations	Usually, no large heat sink is required, as the transistor itself consumes little power.	A heat sink is required. Because efficiency is not 100%, a significant amount of energy is dissipated as heat in the transistor.
Equivalent circuit model	Commonly used is the controlled voltage source model (voltage-controlled voltage source).	A controlled current source model (voltage-controlled current source) is often used because it must supply high current.