Last revised: October 18, 2024
Reviewed by: Scott Orlosky, consulting engineer

Signal transformers are used as a step-up/step-down, isolating, or impedance matching component in electronic devices. Power transformers, which carry current, are generally not suited for use as signal transformers and vice versa. Signal transformers are used in typical electronics applications, including telecommunications, radiofrequency (RF), audio, video, and general-purpose uses.                        

In general, a transformer consists of two wire coils wrapped around a core; the coils are termed primary and secondary and are separated by a medium such as air, water, or metal. The primary coil is typically connected to a source supplying a current or some other signal. When this signal is applied to the primary coil, it creates an electromagnetic field and induces a voltage in the secondary coil. Based on the number of windings per coil, a transformer can reduce or increase the applied signal, or simply isolate it, depending on the device's construction and intended application.

A basic transformer. Image credit: BillC

Applications

Signal transformers may be used in traditional power transformer applications, such as signal isolation and stepping signals up or down.

• Isolation prevents signal crossing and current leakage between two circuits.  
• Step-up/down — Transformers that step signals up or down are used to match signals between two different circuits or devices. This process is related to impedance matching, described below. Transformers that output a signal identical to the input are known as one-to-one devices. 
• Impedance matching is used to match impedances between two different devices, such as a speaker and an amplifier. If the impedance varies, the signal will become distorted and its quality will deteriorate.
• Lighting transformers are used to provide suitable power to lights and lighting systems.
• Inverters are used in inverter circuits, such as power conversion inverters.

Other signal transformer uses include medical applications, as switch mode devices in power supplies, and telecommunications use.

Types

Signal transformers can be classified primarily by the type of signal they carry and/or their function:

• Audio transformers are used for voice and data transmissions with receivers, amplifiers, speakers, and audio cables. They typically operate on frequencies from 300 Hz to 20 kHz.
• Baluns are used to convert a balanced signal — comprised of two signals working against each other — to an unbalanced one, or a single signal working against a ground. Like impedance matching transformers, baluns are used to gain compatibility between two different systems or devices. They are commonly found in radio, television, video, and audio applications; they may also match impedance in all of these uses.
• Digital (isolation) transformers carry signals comprised of discrete values, as opposed to analog signals of continuous functions. They are frequently used in audio applications to isolate signals or match impedance.
• Electronics transformers refer to current or voltage transformers used to test electronic devices.
• RF transformers carry radio frequency (RF) signals ranging from about 3 kHz to 300 GHz. RF transformers are typically low-power devices used for impedance matching.
• Telecom/LAN transformers are typically used for isolation and may feature high bandwidth and fast switching speeds.

Form Factor

Signal transformers may also be classified based on their mounting or form factor:

• Chassis mount transformers are screwed directly to an instrument chassis using tabs and fasteners.
• Chip transformers are integrated circuit (IC) chips manufactured using thin film technology. Signal transformers are far more likely to be found in chip form when compared to power transformers.
• Dish mount devices are secured to a chassis or other flat surface using a simple rubber washer and a metal disk. A hole in the middle of the disk allows the transformer to be screwed down.
• Modular jacks typically take the form of an RJ-45 form factor and are ideally suited to signal transformers.
• Printed circuit board (PCB) mounting is common in signal transformers.

Package Type

Many signal transformers are either semiconductor chips or are PCB-mounted modules. The main types of packaging are:

• Connectorized transformers attach to PCBs and other devices using electrical connectors.
• Flat pack (FPAK) transformers mount using leads - gull wing or flat - found on two or four of the device's sides. FPAK devices are often surface-mounted ceramic devices with excellent thermal and electrical performance.
• Surface mount (SMT) devices feature a flat pad surface, which is soldered onto a PCB. SMT is a newer technology that has largely replaced through-hole (THT) mounting, and has enabled smaller components, simpler and faster automated assembly, and better electrical performance and mechanical vibration resistance.
• Through hole technology (THT) devices have relatively long leads that are fed through holes in a PCB and soldered to the opposite side of the board. While THT products are capable of stronger mechanical connections when compared to SMT devices, THT drawbacks include increased cost of drilling holes and the inability to produce smaller components.

SMT leads (left) and the longer THT leads. Image credit: Custom Electronics | Shutterstock 

Signal Transformer FAQs

What is the role of signal transformers in impedance matching in signal transformers?

The primary goal of impedance matching is to ensure that the maximum amount of power is transferred from one device to another. This is particularly important in audio applications where mismatched impedances can lead to signal loss and reduced audio quality.

Audio transformers have primary and secondary windings with a specific number of turns. The ratio of these turns determines the impedance transformation ratio. For example, if the primary winding has 100 turns and the secondary winding has 10 turns, the impedance ratio will be 100:10 or 10:1.

The impedance seen at the primary winding is transformed to the secondary winding by the square of the turns ratio. For instance, if an audio transformer has a turns ratio of 10:1, an 8000-ohm impedance on the primary side will be transformed to an 8-ohm impedance on the secondary side.

Consider a push-pull vacuum tube amplifier with an output impedance of 8000 ohms that needs to connect to an 8-ohm speaker. An audio transformer with a turns ratio of 10:1 would be used. The 8000-ohm end of the transformer would connect to the amplifier, and the 8-ohm end would connect to the speaker. This ensures that the impedance is matched, allowing for efficient power transfer and optimal audio performance.

The core material of the transformer is chosen to optimize performance within the audio frequency range (20 Hz to 20,000 Hz). Materials with suitable magnetic properties are selected to minimize losses and distortion.

The winding configuration is designed to minimize leakage inductance and parasitic capacitance, which can affect the transformer's ability to accurately transfer audio signals.

How do pulse transformers differ in construction from audio transformers?

Pulse transformers are designed to handle very high-frequency pulses. They typically use core materials with high magnetic permeability and low core loss at high frequencies, such as ferrite and other specialized magnetic materials.

Meanwhile audio transformers are designed to operate within the audio frequency range (20 Hz to 20,000 Hz). Core materials are chosen to optimize performance within this frequency range, focusing on minimizing losses and distortion.

In pulse transformers, the objective is to minimize leakage inductance and parasitic capacitance, which can distort the pulse shape.

The number of turns and the arrangement of the windings are optimized for efficient transfer of high-frequency pulses. The primary and secondary windings are often closely coupled to minimize distortion.

In audio transformers, windings are configured to match the impedance between different audio devices to transfer the maximum amount of power.

Proper insulation between the windings in pulse transformers is essential to prevent electrical breakdown and ensure reliable operation at high voltages.

The insulation material must withstand high-frequency operation and the associated voltage stresses.

In audio transformers, insulation is also important but is optimized for the audio frequency range and the typical voltage levels encountered in audio applications.

Pulse transformers may include shielding to minimize electromagnetic interference (EMI) and ensure signal integrity.

Shielding helps protect the transformer from external noise and prevents the transformer from emitting noise that could affect nearby circuits.

In audio transformers, shielding is used to prevent ground loops and ensure high-quality sound transmission.

Pulse transformers can come in various form factors depending on their application. They may be designed as surface mount devices (SMD) for integration into printed circuit boards (PCBs) or as larger, chassis-mounted units for more robust applications.

Audio transformers are typically designed to be compatible with audio equipment, such as receivers, amplifiers, and speakers.

Form factors are chosen to fit within audio systems and ensure optimal performance within the audio frequency range.

How do baluns function in signal transformers?

Baluns are used to convert a balanced signal, which consists of two signals working against each other, to an unbalanced signal, which is a single signal working against a ground. This conversion is essential for ensuring compatibility between different systems or devices.

In a balanced signal, two conductors carry signals of equal magnitude but opposite polarity. A balun converts this balanced signal to an unbalanced signal by using a transformer with a specific winding configuration. The primary winding is connected to the balanced signal, and the secondary winding provides the unbalanced output.

Baluns also serve the purpose of impedance matching. For example, in a television antenna system, a balun can match the impedance of the antenna (typically 300 ohms) to the impedance of the coaxial cable (typically 75 ohms), ensuring efficient signal transfer.

The construction of baluns involves windings on a magnetic core. The number of turns in the windings and the core material are chosen to optimize performance for the specific frequency range and application. The design ensures that the balanced signal is effectively converted to an unbalanced signal while maintaining signal integrity and minimizing losses.

What are the benefits of using shielding in electronic devices?

Shielding uses conductive or magnetic materials to block or redirect electromagnetic fields away from sensitive components. Shielding provides a physical barrier against external electromagnetic fields, reducing the impact of EMI on the device's performance. By minimizing the impact of external noise sources, shielding helps maintain the integrity of the signals within the device. Shielding ensures that the transmitted signals remain clean and accurate, which is crucial for the reliable operation of various electronic devices.

Shielding helps protect the transformer and other components from external noise sources. It prevents the transformer from emitting noise that could interfere with nearby circuits, ensuring stable and reliable operation.

Devices that use shielding for noise reduction and EMI mitigation tend to perform better and are less susceptible to external interference leads to improved overall performance of the electronic device.

Shielding can be applied to various components and systems, including cables, enclosures, and electronic devices. It offers a flexible solution for reducing EMI across different applications and environments.

In applications like medical devices, shielding helps in protecting patients and sensitive equipment from potential electrical hazards It provides an additional layer of safety by reducing the risk of electrical faults and ensuring reliable operation.

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References

GlobalSpec—Toroidal Transformers

GlobalSpec—White Paper: An Electronics Refresher Guide from Triad Magnetics

GlobalSpec—Isolation Transformers

Bill Whitlock - Audio Transformers

N.H. Crowhurst - The Matching Transformer