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Piezoelectric hydrophone low noise amplifier design questions
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dmills:
Either is fine.

The traditional approach to a single transducer sonar Tx/RX switching is to just stick a pair of back to back diodes in series with the secondary of the transmit matching transformer and take the receive signal from across them, simple, robust and no need for active TR switching.

I always like the BF862 as a preamp input stage for low frequency sonar, not as nice as the Linear Systems Siliconix copies, but way cheaper, and above about sea state 3 or so you are usually noise limited by surface noise anyway.

Regards, Dan. 
Sparker:
Hi, Dan!
Thanks for your response!
Good to know that both designs are OK.
I understand what classic solution you are talking about. Just for reference, I've attached a schematic I took from some old soviet book.
Am I right if I say that this solution is worse than the previous ones because It's not symmetric and thus will provide less suppression for common mode noise?  :-//
I'm afraid I can't use it anyway because we have a single TX matching transformer driving two transducers. The transducers are isolated from each other with back-to-back diodes, there's also a schematic at page one of this thread.
dmills:
So you have some sort of push pull driven side scan affair? Weird, but whatever.

I would be looking at something with a pair of back to back diodes in each leg and some small transformers.

By 250KHz you are getting into the place where minicircuits have some interesting offerings for ready made small RF transformers, might be worth a look at something like a 16:1 impedance ratio jobbie (4:1 voltage gain).

Are you building the transmit transformer as a tuned job (to tune out the fixed capacitance of the transducer)?

On the variable gain amp front, there is something to be said for just using a log amp, 100dB of range is not that hard, AD have the good log amps, but it depends on what your waveform looks like. 

Regards, Dan.
Sparker:
The circuit you've attached looks like a nice solution. Previously we were discussing how to make a protection without noisy series resistors. I think this one is great, looks very symmetric  and without serial resistors!  :)

Yes, I was planning to use a push-pull driver for the transducers. Why do you think it's weird? We use this driver for several sonars here. Push-pull configuration seems like a traditional transformer configuration. See the attached schematic below to see how I'm going to attach the transducers.

No I didn't consider tuning out the capacitance of the transducer. But we have already a pair of transducers with a TX amplifier driving them, so I was going to reuse it.

I am going to use a chirp signal sythesized with an STM32 timer, so a demodulating logarithmic amplifier is not suitable here. Or are you talking about a non-demodulating one? I'm not really familiar with these devices. At the signal processign side, I am digitizing the signal right after the amplifier and do the rest in digital domain, the STM32 is very capable of doing that.
dmills:
I worked mainly on the transducer side of things, and typically our transducers did not have a centre tap on the ones with several series elements.
The output was typically unbalanced so we would just connect the screen to system ground and pick off single ended on the other side of the diode pair.

Primary side power stages are generally push pull into the transformer, we found that tuning things so the transducer ran on the inductive side of resonance was easier then running on the capacitive side as it makes for a simple active clamp to real with the reactive power flow.

Agree that a log amp is no use if you are chirping for range to target, but they are great for things like interferometric sidescan where relative phase is what you really care about.

Regards, Dan.
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