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Hello everyone,

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a while ago there was a discussion in a german forum where forum member ArnoR proposed modifications to the original LZ1AQ circuit.

The modifications seemed plausible to me but as an electronic hobbyist with little knowledge (but a lot of passion!) I had difficulties understanding them.

A circuit (and later an updated version) was presented as an improvement over the original LZ1AQ design that i know has been in use for years (and has highly regarded updated versions published in this forum).

Since I don’t have the experience to evaluate the design myself, I thought I’d bring it here, where the level of expertise is much higher. My goal is not to argue for or against the changes but simply to understand whether and why they might be beneficial—or if they introduce new drawbacks. Even if there is no benefit, the proposed changes are still interesting from a circuit design perspective.

I would appreciate any insights you can share!

(I created some LTSpice files for the proposed circuits, they do run but are not refined enough to do a proper circuit simulation. They should be seen as a starting point for experimentation. They are called Arno_V1.asc and Arno_V2.asc in the files section.)

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Here is a summary of the changes:

1. Output Transformer Issues

• Output transformer has too low an inductance (18?H, actually likely ~10.2?H).
• Results in too high a lower cutoff frequency (~500kHz or even 800kHz).
• Incorrect dimensioning limits low-frequency performance rather than the input resistance or loop inductance.

2. Base Circuit Design Issues

• Separate base voltage supplies used instead of a proper differential amplifier.
• Better approach: Connect bases directly for true base-coupled differential amplifier.
• Eliminates unwanted differential voltages.
• Improves symmetry, reduces component count, lowers noise.
• No issues with biasing due to resistor tolerances.
• Enhancement: Replace lower divider resistor with an LED for thermal compensation and power indication.

3. Emitter Circuit Design Issues

• Same as above: Should be a true differential amplifier.
• Converting saves components, improves performance, no downside.
• Output drive capability remains unchanged.

4. Signal Tapping at Collector Resistors

• Output should be collector current/voltage across resistors, not collector voltage to ground.
• Incorrect method introduces power supply noise into the signal.
• Fix: Use PNP emitter circuits or PNP differential amplifier for:
• Massively better power supply rejection.
• Eliminating the need for regulated supply voltage.
• Improved output drive capability and fewer components.

5. Frequency Response & Component Choice

• Original upper cutoff frequency: ~10MHz, due to capacitances in base/emitter circuits.
• Author claims flat response up to 40MHz—but only due to input VHF filter resonance.
• Better solution: Use 2SA1015/2SC1815 instead of 2N2222A for:
• Lower noise, better linearity, smaller capacitances.
• Higher cutoff frequency, higher slew rate.
• Lower input resistance → Better low-frequency response.

6. Final Circuit Comparison

• New design vs. original tested with 1m, 3.4mm AL loop, no VHF filter.
• Same input & bias currents, measured max output level.
• Conclusion: Much better performance with significantly less complexity.

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I’d love to hear your thoughts. Could they offer any advantages, or might they introduce unintended issues?

Looking forward to learning from your insights!

Best regards,

Nils

Just to be super clear: none of this is my original work, all work was done by ArnoR. I just translated his post to post it here and condensed it for clarity.

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For reference i include the original forum post in translated form:

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Source:

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Since I previously criticized LZ1AQ’s circuit without providing specifics, I now want to briefly address this to avoid the impression of baseless complaining.

Let us start at the very end. The output transformer supposedly has a winding inductance of 18?H. With the output resistances of the emitter circuits (220Ω), which generate the output voltage, and the load resistor, the resulting lower cutoff frequency is around 500kHz. This means the inductance is far too low if one intends to amplify cleanly down to the longwave (LW) range.

*) The core is specified with ?=1000 and core size R10. According to the Epcos catalog, this core has an AL value of 407, which, with 5 turns, results in an inductance of only 10.2?H. This would place the lower cutoff frequency at about 800kHz.

Thus, the lower cutoff frequency is not limited by the input resistance of the circuit and the loop inductance but by the incorrect dimensioning of the output transformer.

The two base circuits are powered by separate base voltage supplies. However, the circuit is actually supposed to amplify the differential signal between the emitters. This only works properly if there is no differential voltage between the base connections. Here, this is achieved for AC signals using bypass capacitors. A much more natural approach would be to connect the base terminals directly, thereby constructing a true base-coupled differential amplifier.

This would require only a single bias voltage divider, with changes in base currents perfectly canceling each other out. No interference voltage could be coupled between the bases, and there would be no difference in operating points due to resistor tolerances. This results in better symmetry, saves some components and power, and delivers improved performance without any drawbacks compared to separate base circuits. The frequency response remains identical to that of the separate base circuits.

At the same time, the lower resistor of the divider could be replaced with an LED, which provides good thermal compensation of the operating point while also serving as an operating indicator.

The same mistake as in point 2 has also been made in the emitter circuits. These, too, can be converted into a true differential amplifier without any drawbacks. This again saves several components and improves performance—without even affecting the output drive capability (large-signal behavior).

The most serious error, however, is the incorrect signal tapping at the 220Ω collector resistors. The output signal of the base circuits is their collector current, or the voltage across the collector resistors—not the voltage at the collector relative to ground. If, as in LZ1AQ's circuit, the emitter circuits are driven against ground, then the supply voltage and any noise on it appear directly in the signal. Additionally, the operating point becomes highly dependent on the power supply. For these reasons, the circuit can only be operated with a stabilized supply voltage (the 10V regulator).

This issue can be easily avoided by using PNP emitter circuits or a PNP differential amplifier. This improves power supply rejection by orders of magnitude compared to the original circuit, eliminating the need for supply voltage stabilization, reducing component count, and increasing output drive capability.

The upper cutoff frequency of the original circuit is about 10MHz. It is determined by the effective capacitances at the collector resistance of the base circuits: the Miller capacitance of the emitter circuit, feedback and output capacitance of both the base circuit and the emitter circuit itself. The transistors are no longer suitable for these frequencies or dimensions because their capacitances are too large.

According to the author, the circuit is supposed to maintain a flat frequency response up to about 40MHz. However, this is due to the input-side VHF filter introducing a resonance peak in the 10MHz–40MHz range, which compensates for the amplifier’s frequency response roll-off. I consider this an unclean approach.

Much better performance can be achieved with more suitable transistors, such as the 2SA1015/2SC1815. These transistors are extremely low-noise, highly linear, have much smaller capacitances, and are very inexpensive. With these transistors, one achieves a significantly higher upper cutoff frequency, a higher slew rate, and a lower input resistance than with 2N2222A, leading to a lower cutoff frequency at the loop.

Finally, I present the circuit resulting from the above considerations in the attached images and compare its characteristics with the original circuit. In both cases, the same 1m, 3.4mm AL loop was used, and the VHF input filter was omitted to focus solely on the amplifier’s characteristics. The input signal was kept the same in both setups, slightly into overdrive, to show the maximum output level. The operating currents of all stages are identical in both circuits.

Conclusion: Significantly better performance despite much less effort.

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Source:

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Transistors T1/T2 and T5/T6 each form a composite transistor (Sziklai pair) configured as a common-base circuit. The base is clamped (via D1/C1), and the input signal is fed into the composite emitter. This composite transistor has a significantly lower emitter input resistance compared to a single transistor, which is essential since the lower cutoff frequency of the circuit is determined by the relationship:

fu=re2πLf_u = \frac{r_e}{2\pi L}fu?=2πLre??

The two composite transistors operate as a base-coupled differential amplifier for the floating magnetic loop, which is connected to the two blocks at the bottom. The output signal from this input differential stage is amplified by the emitter-coupled differential stage T3/T4 and then fed via C8/C9 into a balun, which sums the output signals and provides high common-mode rejection. The resulting 50Ω output is floating (potential-free).

C11 and C12 limit the upper frequency response and must be selected according to the desired bandwidth. I have used only C12 with a few picofarads.

The input impedance of the circuit is 0.4Ω differentially (i.e., 0.2Ω per side), which allows for a lower cutoff frequency of approximately 20kHz (-3dB) with a 1m loop.

The balun is simply a bifilar winding on a toroidal core and is connected like a standard common-mode choke, meaning both winding starts are connected to C8/C9, and the winding ends go to the output. The core must have low losses in the desired frequency range, and the winding inductance should be at least 100?H.

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