Why does a balanced amplifier have good input/output match?

The 90-degree hybrid coupler splits the input signal into two equal paths with 90 degrees phase difference. Each path feeds one amplifier. The reflected signals from the two amplifier inputs travel back through the coupler. Because of the 90-degree phase relationship, these reflections cancel at the input port and add at the isolated (terminated) port. The result: the balanced amplifier's input return loss depends on the coupler's directivity and balance, not on the individual amplifier match. Even if each amplifier has only 5 dB return loss, the balanced pair can achieve 15-20 dB return loss.

What are the disadvantages of a balanced amplifier?

Three main drawbacks: (1) Requires two amplifiers for the same gain as one, doubling the DC power consumption and cost. You get 3 dB more output power, but at double the DC power. (2) The 90-degree hybrid couplers add insertion loss (typically 0.3-0.5 dB each), so the total added loss is 0.6-1.0 dB. This degrades noise figure by the same amount. (3) Size: you need two amplifiers plus two couplers, roughly 3x the board area of a single amplifier. For these reasons, balanced amplifiers are used where broadband match is critical (test equipment, EW receivers, fiber-optic links) rather than in cost/size-sensitive consumer products.

What happens if one amplifier in a balanced pair fails?

If one amplifier fails open (infinite impedance), the signal through that path is fully reflected back to the hybrid. The coupler directs this reflection to the terminated load, so the output port sees only the signal from the surviving amplifier. The result: the gain drops by 6 dB (half the signal path is lost, plus 3 dB from the output coupler splitting), but the amplifier continues to function. This graceful degradation is valuable in military and space applications where replacement is impossible. Some systems use this property intentionally, designing N+1 redundant balanced amplifier chains.

Amplifier Topologies

Balanced Amplifier

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A Balanced Amplifier connects two identical amplifier stages between input and output 90-degree hybrid couplers (quadrature couplers). Reflections from the individual amplifiers cancel at the hybrid's isolated port, providing excellent input and output return loss regardless of the individual device match. The topology also delivers 3 dB more output power than a single stage and degrades gracefully if one amplifier fails.

Category: Amplifier Topologies

Return Loss: 15-20+ dB (typical)

Power: +3 dB vs. single stage

Understanding Balanced Amplifiers

Getting broadband impedance match from an RF transistor is difficult. The device's S11 and S22 vary with frequency, and matching networks that work well at one frequency may be poor at another. The balanced amplifier sidesteps this problem entirely: the 90-degree hybrid coupler absorbs the mismatch reflections into its terminated port, presenting a well-matched impedance at the external ports across the entire coupler bandwidth. This is why nearly every broadband test instrument amplifier, EW receiver amplifier, and fiber-optic link amplifier uses a balanced topology.

Balanced Amplifier Performance

Balanced Amplifier:
A Balanced Amplifier connects two identical amplifier stages between input and output 90-degree hybrid couplers (quadrature couplers). Reflections from the individual amplifiers cancel at the...

Key specifications:
3 dB | 11 a | 22 v

Friis: NF_sys = NF₁+(NF₂−1)/G₁

Balanced vs. Single-Ended Amplifier Comparison

Parameter	Single-Ended	Balanced	Advantage
Return loss	5-12 dB (device-limited)	15-25 dB	Balanced
Bandwidth	Matching-limited	Coupler-limited (octave+)	Balanced
Output power	P_1dB	2×P_1dB (+3 dB)	Balanced
Noise figure	NF_device	NF + 0.3-0.5 dB	Single
DC power	P_DC	2×P_DC	Single
Size	1x	~3x	Single
Graceful degradation	No	Yes (-6 dB)	Balanced

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Aspect	Balanced Amplifier Spec	Typical Range	Impact	Design Note
Primary function	A Balanced Amplifier connects two identi...	Application-dep.	Critical	Verify in sim
Operating range	Reflections from the individual amplifie...	Application-dep.	Critical	Verify in sim
Performance	The topology also delivers 3 dB more out...	Application-dep.	Critical	Verify in sim
Integration	Understanding Balanced Amplifiers Gettin...	Application-dep.	Critical	Verify in sim
Trade-off	The device's S11 and S22 vary with frequ...	Application-dep.	Critical	Verify in sim

Common Questions

Frequently Asked Questions

Why does it have good match?

The 90-degree hybrid splits the input with 90-degree phase offset. Reflections from the two amps travel back through the coupler and cancel at the input port, adding at the isolated (terminated) port. Even with 5 dB individual amp return loss, the balanced pair achieves 15-20 dB. Match depends on coupler quality, not device impedance.

What are the disadvantages?

Double the DC power for same gain (you get +3 dB power, not +3 dB gain). Couplers add 0.6-1.0 dB total loss, degrading NF. ~3x board area. Used where broadband match is critical (test equipment, EW, fiber) rather than cost/size-sensitive consumer products.

What if one amp fails?

The signal through the failed path reflects to the terminated load. Output sees only the surviving amp: -6 dB gain drop but continued operation. Valuable in military/space applications. Some systems use N+1 redundant balanced chains intentionally.

Related Terms

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