[RFC] simple power amplifier

[jordens]

We have an AOM power amplifier application where Booster appears overkill (manufacturing/mechanics, complexity, number of channels, output power, bandwidth, gain, size, weight, cost). We are exploring alternative ideas and need:

• P1dB: 30 dBm min, 33 dBm target
• gain (over the bandwidth): 24 dB (10 dBm from Urukul with amp) or 40 dB (-7 dBm from Pounder, 0 dB ? from Phaser, Mirny)
• bandwidth: 40-350 MHz (10-450 MHz if possible, or talk about e.g. 80 MHz+-10 MHz narrowband if it dramatically simplifies the design)
• if this is a two-stage amplifier with a preamp, consider two preamp gain variants to cover both Urukul/Mirny/Pounder and Phaser input power.
• channel crosstalk: as good/bad as Urukul
• 3U form factor, maybe 4 HP, maybe 2 channels per board, SMA-to-SMA
• if possible no fan, no moving parts, convective cooling
• power 12 V or as convenient
• O/IIP2 doesn't matter
• OIP3 50 dBm target
• fast output reflection interlock (only if that design is risk-free), manual front-panel reset + LED and ~10 min automatic reset if that can be made safe and robust.
• LEDs for interlock, power good, over temp
• optional: a µC on board for calibration, monitoring, USB/UART/I2C

Potential USPs:

• no complicated sequencing
• no additional negative voltages for amplifier
• fewer amplification stages
• simpler integrated biasing
• smaller than a Booster channel
• lower minimal space usage
• Simpler mechanics, simpler shielding, simpler assembly and disassembly, no internal pigtails
• higher efficiency at its P1dB
• less gain, better matched to Urukul/Mirny
• no moving parts, no fans
• lower cost per channel
• fewer bugs/higher robustness from the start

Acceptable trade-offs:

• higher temp co
• 6 dB lower P1dB
• potentially less gain (important for Phaser)
• potentially lower bandwidth
• potentially higher crosstalk
• no monitoring, calibration, logging
• no remote control
• no remote interlock clear

Questions:

• Would this be a significant simplification w.r.t. Booster or would it meet the same challenges?
• Are there other interested users and use cases?
• Is there something missing or too much to make this both different from Booster and decidedly useful for applications?

[gkasprow]

The lower power mode (Ib=300mA) is useful for output power up to 24dBm. If someone wants more, 500mA current must be enabled. The output power higher than 33dB activates some thermal protection which causes the power to drop to 28dBm. After lowering the input power and waiting several seconds the amplifier works correctly again .

[gkasprow]

To conclude:

• we will use the preamp to ramp up the gain, so the S11 is not that important. We can use the same balun as at the output to limit the BOM. S11 is still much better than -10dB
• we will use a 1.5:1 output balun because it gives better S22 at the cost of a 1dB drop of gain. But we have preamp anyway.
• preamp assembly variants would be needed to cover 24dB and 40dB gain. We can also use two SPDT switches to disable part of preamp

[dnadlinger]

Clearly the only acceptable name is Boosty McBoostface.

[jordens]

@gkasprow you were worried about those catv amps squelching for low input powers. Is the gain the same at very low input powers?

Could you also check what pulse envelope response looks like with 100us pulses of low duty cycle?

[jordens]

@gkasprow What 1.5:1 output balun is that? And if you have the data: taking all together, what P1dB (or thermal shutdown power if that's earlier) did you get excluding cable losses?

[gkasprow]

This particular amp is linear even at very low power. It's foreseen for digital CATV when multiple tones need to be transmitted reliably. The ones squelching for low input power were designed for analog TV with AM modulation. It was P1dB that occurred earlier. I had to pump over 16dBm of input power to get a thermal shutdown. I used ADT1.5-122+. Right, they are specified for 0.5W power. I probably saturated one. However S parameters were taken at much lower power.

[gkasprow]

the internal LF generator in SMB100A has limited rise time and only a 50% duty cycle... I have to get some high-speed solid-state RF switch... Fin=150MHz at 10dBm. The scope is 50Ohm terminated and connected via a 20dB attenuator.

[gkasprow]

To measure the P1dB I will bring my Agilent power meter. SA is not good at that.

[gkasprow]

@jordens I did some observation and I'm not that sure about P1dB I wrote above. That was probably tested with a cold heatsink. I don't see noticeable power drop up to 32dBm. At 33dBm I see a 0.2dB drop. At 33.5dBM I see 1dB of drop. At 34...35dBm it enters power limit depending on how hot the heatsink is. So it looks like P1dB is just before the thermal shutdown.

[gkasprow]

I observed only the first harmonic using SSA.

[jordens]

@gkasprow Thanks. I'm happy. That looks really good. IMO no need to look at faster rise times. I was mostly worried about the transient behavior on timescales like on Booster. And P0.2dB of 33 dBm is fine.

[jordens]

Even the balun in the datasheet is only rated for 0.5 W...

[gkasprow]

Even the balun in the datasheet is only rated for 0.5 W...

Yep, I wrote about it above It does not influence the S measurement. I think we can stay with 75Ohm balun. S22 is not that much worse.

[gkasprow]

I ordered an RF switch module, will need it to play with Booster anyway.

[gkasprow]

We can use TRS1.5-182+ which works up to 1W

[gkasprow]

I think we don't need high interlock precision and can use ADL5904 with the internal comparator and flip-flop to drive the RF switch directly. I don't remember why we switched to an external comparator and flip-flop in Booster. Was it because we wanted to shape the time constant or because the comparator was not precise enough?

[hartytp]

I don't remember why we switched to an external comparator and flip-flop in Booster. Was it because we wanted to shape the time constant or because the comparator was not precise enough?

IIRC (but the issues will give a better record) it was two issues:

• (1) avoiding the interlock tripping on very fast transients (switch glitches). IIRC this was mainly an issue for the input interlock since the transients were pretty well filtered by Booster's gain curve. Since we're not really using an input any more this may not be an issue
• (2) IIRC the threshold pin needed an annoying amount of current pretty near GND and it wasn't totally trivial to drive it without a negative rail
• (3) I can't remember if there were issues with the linearity of the set point response, which made calibration over a wide range of inputs tricky without a more complex calibration

These may not be big issues for a new amp design, but I think that's what lead us in a different direction for booster.

[jordens]

Maybe the speed was an issue also due to the bias transients on Booster. Let's go for the same switch as on Urukul then, those aren't that fast and don't overshoot. In fact I'd prefer a very fast interlock plus the internal thermal shutdown. There should also be a nice transient response knob with the RFIN coupling capacitor value. 20 µA at > 80 mV for < -10 dBm detector threshold bias should be doable. Trimmer is fine IMO. A few things for discussion:

• Maybe it's ok to do without the couplers and just tap off a bit for the detectors. For protection it shouldn't matter whether it's forward or reverse power.
• Let's make sure the TVS don't hurt the OIP3
• If it's no trouble, maybe group the bias setting trimmers, power and temperature monitoring, interlock reset, switch, power enable signals so that we could in principle slap on some µC later.

[gkasprow]

Is the STM32F072CBU6 supported by RUST? It's low cost, has two DACs and two ADCs. It has also a USB. We would avoid using any ADCs and DACs.

[gkasprow]

Since the amplifier can handle the full reflection indefinitely, why we need an output interlock? Just to protect the AOM? We can use a uni-directional coupler with 26dB coupling like ADC-26-52+

[gkasprow]

If we skip the coupler, the protection will work only in the case of an open circuit, the reflected power will increase the voltage.

[jordens]

Pretty much all the stm32s are supported including that one. I'm not sure the output interlock is really needed in practice, especially if there is an input interlock. It might be useful for monitoring output power. If there is no coupler it seems to me that the interlock would conveniently both work as a output power and reflection power interlock, though somewhat uncalibrated for reflected power.

[gkasprow]

I assumed we won't make an input interlock here. Do we really need them all? They would need calibration. The idea was to keep it simple :)

[jordens]

Ok. In my eyes, an input interlock would protect the amplifier. But it's reasonable to expect a user to be able to limit the input power. Let's dispense with it. The output power interlock could be useful to protect downstream devices. But it's hard to do that accurately when looking at the possible SWR, transients, and thermal behavior. Output reflected would be similarly difficult. On top of the difficulty it's not needed since the amplifier is robust. Summarizing, I think an output power monitor and integrated fast interlock from a 20 dB tap without coupler would be sufficient. And even that can be made optional in many cases.

[gkasprow]

This is the initial component placement. My student will make an MWO model, write initial SW and perform tests.

[gkasprow]

Next iteration. The board is equipped with a low-cost, 2-part shield which also works as a heatsink