What balanced amplifiers could be used on a VLF loop antenna?

For a VLF loop antenna (typically covering a few kHz up to perhaps 30–300 kHz), a balanced amplifier should ideally have:

• Very high input impedance.
• Low noise at low frequencies.
• Excellent common-mode rejection.
• Differential (balanced) inputs directly connected to the loop.
• Good immunity to local electric-field interference.

Several approaches work well:

Instrumentation Amplifiers

These are often the simplest solution for small VLF loops.

• INA128
• INA163
• AD620 instrumentation amplifier”]
• AD8429

The loop connects directly across the differential inputs, and the amplifier output can then feed filters or an SDR.

For frequencies below about 100 kHz, instrumentation amplifiers can provide excellent balance and CMRR.

JFET Differential Pair Preamplifiers

Many serious VLF experimenters use discrete JFETs.

Common devices include:

• 2SK170
• LSK170
• LSK389

A long-tailed differential pair built around matched JFETs gives:

• Very low noise.
• Extremely high input impedance.
• Excellent balance.

This approach is often used in professional magnetic-field VLF receivers.

Operational Amplifier Differential Amplifiers

Good op-amps for VLF work include:

• OPA827
• OPA1642
• LT1028

A two-op-amp or three-op-amp differential front end can be built with matched resistors.

Transformer-Coupled Balanced Inputs

Many VLF stations use:

1. Loop antenna.
2. High-permeability ferrite transformer.
3. Single-ended amplifier.

Suitable cores include:

• Ferrite toroids with high μ material.
• Nanocrystalline cores.

The transformer provides:

• Galvanic isolation.
• Common-mode rejection.
• Lightning/static protection.

This is particularly useful if the loop is outdoors and the receiver is some distance away.

Fully Differential Amplifiers

If you are feeding an ADC directly:

• THS4521
• ADA4940

These preserve the balanced signal all the way into a differential ADC.

For Radio Astronomy and Weak-Signal VLF

Given your interest in radio astronomy and low-noise receiving systems, a particularly effective arrangement would be:

Balanced loop
      │
JFET differential pair
      │
Band-pass filter
      │
Instrumentation amplifier
      │
SDR / sound card

or

Balanced loop
      │
1:1 ferrite transformer
      │
INA128 / AD8429
      │
SDR

The JFET differential pair using matched LSK170s is probably the lowest-noise practical balanced amplifier for a high-Q VLF loop, while an AD8429-based front end is the easiest high-performance solution to construct.