Balanced Amplifier
Understanding Balanced Amplifiers
The balanced amplifier is one of the most important amplifier topologies in RF engineering. It solves the fundamental problem of achieving good impedance match at amplifier ports, which is inherently difficult for maximum gain or minimum noise design. The quadrature couplers absorb reflections, presenting matched impedance at both ports.
How It Works
- Input hybrid splits signal with 90-degree phase shift. Each amplifier receives half the power.
- Output hybrid recombines amplified signals. The 90-degree phase shifts cancel, producing a combined output.
- Reflections from mismatched amplifier ports are routed to the termination resistors by the hybrid's isolation property.
Advantages
- Match: Input and output return loss limited only by hybrid balance, typically > 15-20 dB regardless of amplifier match.
- Bandwidth: Match improvement is broadband (limited by hybrid bandwidth).
- Graceful degradation: If one amplifier fails, the other continues at -6 dB (half power + hybrid loss).
- Even-order cancellation: Second harmonic and even-order IM products are partially cancelled.
Frequently Asked Questions
What is a balanced amplifier?
A balanced amplifier uses two identical amplifiers with 90-degree hybrid couplers at input and output. The hybrids provide excellent impedance match by absorbing reflections from the amplifiers, regardless of individual amplifier match quality.
What are the advantages of balanced amplifiers?
Excellent wideband input/output match (>15 dB return loss), graceful degradation if one amplifier fails, partial cancellation of even-order distortion products, and reduced sensitivity to component variations.
What is the disadvantage of balanced amplifiers?
Requires two amplifiers and two hybrids, doubling the parts count and size. The output power is the same as a single amplifier (2x amplifiers, but each gets half the input power). DC consumption doubles. The topology gains match, not power.