Why do standard amplifiers fail at broadband frequencies?

Standard high-efficiency amplifiers (like Class F) achieve their efficiency by using precise resonant stubs to 'short circuit' the harmonic frequencies. Because a quarter-wave stub only acts as a short circuit at one specific frequency, the amplifier's high efficiency is strictly narrowband. If the operating frequency shifts by 10%, the stub no longer shorts the harmonic, the voltage and current waveforms overlap, and the amplifier burns up as heat. Continuous-mode amplifiers like Class BJ solve this by abandoning the need for perfect short circuits.

How does Class BJ achieve broadband efficiency?

Instead of demanding a strict 0-ohm short circuit for the second harmonic, Class BJ relies on a mathematical concept called 'continuous mode' space. It allows the fundamental impedance to be slightly reactive (imaginary) and allows the second harmonic impedance to be highly reactive. As long as the specific mathematical relationship between the fundamental reactance and the second harmonic reactance is maintained, the voltage waveform will naturally 'shape' itself to avoid overlapping with the current waveform. This reactive relationship can be easily maintained over an octave or more of bandwidth.

What is the difference between Class B, Class J, and Class BJ?

Class B is the traditional baseline, requiring a purely resistive fundamental load and a strict 0-ohm short at all harmonics. Class J was a breakthrough that proved you could achieve Class B efficiency by using a highly reactive fundamental load paired with a purely capacitive second harmonic. Class BJ is the 'continuous spectrum' connecting the two. It represents the entire family of mathematically valid tuning solutions that lie between pure Class B and pure Class J, giving designers a massive design space to synthesize broadband matching networks.

Active Components

Class BJ

An engineer needs to design an amplifier for a military software-defined radio that can instantly hop anywhere between 1 GHz and 2 GHz. A standard Class F amplifier is useless because its efficiency relies on narrow quarter-wave stubs that only work at one frequency. The engineer shifts to continuous-mode design, specifically Class BJ. Instead of trying to force the second harmonic to be a perfect 0-ohm short circuit (which is impossible across an octave of bandwidth), Class BJ allows the fundamental and harmonic impedances to vary wildly. As long as the reactive components of these impedances follow a specific mathematical ratio, the transistor's voltage and current waveforms will organically shape themselves to prevent overlap. By exploiting this "continuous design space" between standard Class B and Class J, the engineer builds a matching network that maintains a constant 70% efficiency across the entire 1-to-2 GHz band.

Category: Active Components

Architecture: Continuous-mode harmonic tuning

Primary Benefit: Broadband high-efficiency amplification

Harmonic Termination Comparison

Amplifier Class	Fundamental Load (f0)	2nd Harmonic Load (2f0)	Bandwidth
Class B	Purely Resistive (R_opt)	Strict Short Circuit (0Ω)	Narrowband (< 10%)
Class J	Resistive + Inductive	Purely Capacitive (-j)	Moderate (~ 30%)
Class BJ (Continuous)	Varies (Resistive ± Reactive)	Varies dynamically with f0	Broadband (> 100% / Octave)

The Continuous Design Space Math:
Z_f0(α) = R_opt + j · α · R_opt
Z_2f0(α) = -j · (3π/8) · α · R_opt
Where α is the continuous parameter (-1 ≤ α ≤ 1).

What this means:
When α = 0, the amplifier acts like standard Class B (resistive fundamental, shorted 2nd harmonic). When α = 1, it acts like pure Class J. As frequency changes across a broad band, α is allowed to slide continuously between -1 and 1. As long as the matching network provides impedances that satisfy the equations for *some* value of α, the theoretical efficiency remains locked at 78.5%.

Common Questions

Frequently Asked Questions

Why is overlapping voltage and current bad?

In a transistor, Power Dissipated (Heat) = Voltage × Current. If the transistor has high voltage across it at the same exact time that high current is flowing through it, it burns massive amounts of energy as heat, ruining efficiency. High-efficiency amplifiers try to shape the waveforms so that when Voltage is high, Current is zero (and vice versa). Class BJ proves this shaping can be achieved with reactive loads, not just resistive ones.

Why was Class J invented?

To overcome the parasitic output capacitance (Cds) of high-power transistors. In standard Class B, designers tried to tune out Cds with an inductor. But at the second harmonic, that inductor looks like a high impedance, ruining the required Class B short circuit. Class J mathematically proved that you could simply leave the Cds capacitance un-tuned at the second harmonic, and compensate for it by deliberately adding inductance at the fundamental frequency.

How do you design a Class BJ matching network?

You cannot use a Smith Chart and a simple stub. You must use a synthesis algorithm or a continuous-mode optimizer in a circuit simulator (like ADS or AWR). The optimizer is given a target: it must synthesize a network of inductors, capacitors, and transmission lines that keeps the fundamental and second harmonic impedances trapped inside the mathematically valid α space across the entire frequency sweep.

Related Terms

← Clapp Oscillator Coaxial Cable →

PA Design

Continuous Mode Trajectory Analyzer

Upload your fundamental and second-harmonic impedance sweeps. Visualize the trajectory of your matching network across the continuous Class BJ design space and predict broadband efficiency.

Analyze Continuous Space