Electronics > Beginners
Piezoelectric hydrophone low noise amplifier design questions
Marco:
--- Quote from: Sparker on April 30, 2018, 04:51:22 pm ---The problem with your diode based protection is that, maybe I didn't mention it good enough, the antena works both in TX and RX modes, so when we are transmitting the impulse, about 50...100 volts will be applied to the protection circuit, thus it should have some kind of series protection.
--- End quote ---
Just put something like a VO1400AEF in there to block the receiver during transmission. As for MOSFET current limiting, no resistor with low noise (ie. ~10 Ohm) is going to turn off MOSFETs at reasonable currents. You'll need something slightly more complex if you go that way.
You simply can't have it both ways, 1k range series input resistance and low noise don't mix.
The opamp doesn't do the biasing in the schematic, you should see the circuit as a composite opamp where the gate is the positive input and the source is the negative input (with a very high offset voltage). The JFET sets the noise, the opamp provides the loop gain and low output impedance. You can do much the same thing with a PNP transistor, but the opamp allows more flexibility (the PDF is good reading, his cheap circuit and hydrophone beats the much more expensive ones on the market ... probably for the reason mentioned above, input series resistance is poison to low noise).
David Hess:
--- Quote from: Marco on April 30, 2018, 05:18:59 pm ---Just put something like a VO1400AEF in there to block the receiver during transmission. As for MOSFET current limiting, no resistor with low noise (ie. ~10 Ohm) is going to turn off MOSFETs at reasonable currents. You'll need something slightly more complex if you go that way.
--- End quote ---
I was thinking something like that but wondered if it would be too slow. 500 microseconds comes out to a range of about 1 foot so it would actually be quick enough.
Sparker:
Do I understand your depletion MOSFET idea correctly, David? The picture is in the attachment.
--- Quote ---As for MOSFET current limiting, no resistor with low noise (ie. ~10 Ohm) is going to turn off MOSFETs at reasonable currents.
--- End quote ---
It looks like I can only buy a BSP129 depleted MOSFET here. With Rs=20 Ohms and Rds=7 Ohms (typical), the total equivalent resistance of the whole protection (2x20 Ohm resistors and 4x transistors, all in series) is 50 Ohms, or ~0.9 nV/sqrt(Hz) of noise, not so bad compared to 1k resistors.
500us for an externally controlled switch(like the optical relay suggested) is probably not bad. The pulse length will be about 600us, but extra 500us of 'off time' won't matter much for a side scan sonar if it's located several meters above the ground.
I'm quite constrained in space so I can't afford a nice & expensive toroid 40mm in diameter, so I stopped at this one: 10x6x4 mm toroid, T38 material. Probably I can fit about 60 turns total one layer. Is it a good choise?
I am thinking if I could just move the protection to the "op-amp" side of the transformer, so its noise voltage is not amplified by the transformer, but probably it won't work well with the 10mm transformer mentioned above.
Marco:
For a 100V RMS signal you're passing 100 mA and burning 10 Watt in the MOSFET (which it can't take for very long).
The problem with the transformer is getting sufficient self inductance, the primary impedance does need to be significantly higher than the piezo to not make a mess.
David Hess:
--- Quote from: Sparker on April 30, 2018, 11:28:59 pm ---Do I understand your depletion MOSFET idea correctly, David? The picture is in the attachment.
--- Quote ---As for MOSFET current limiting, no resistor with low noise (ie. ~10 Ohm) is going to turn off MOSFETs at reasonable currents.
--- End quote ---
--- End quote ---
That is exact it.
--- Quote ---It looks like I can only buy a BSP129 depleted MOSFET here. With Rs=20 Ohms and Rds=7 Ohms (typical), the total equivalent resistance of the whole protection (2x20 Ohm resistors and 4x transistors, all in series) is 50 Ohms, or ~0.9 nV/sqrt(Hz) of noise, not so bad compared to 1k resistors.
--- End quote ---
Depletion mode MOSFETs are kind of expensive also.
--- Quote ---500us for an externally controlled switch(like the optical relay suggested) is probably not bad. The pulse length will be about 600us, but extra 500us of 'off time' won't matter much for a side scan sonar if it's located several meters above the ground.
--- End quote ---
I was thinking that it might be a problem if the sonar was up against something or if there was another sonar in the water.
--- Quote ---I'm quite constrained in space so I can't afford a nice & expensive toroid 40mm in diameter, so I stopped at this one: 10x6x4 mm toroid, T38 material. Probably I can fit about 60 turns total one layer. Is it a good choise?
--- End quote ---
Inductor for what? For the transformer? I would probably use a pot core for easier winding. What transformer are you using for the transmit side? I might use the same transformer or at least the same core for both.
--- Quote ---I am thinking if I could just move the protection to the "op-amp" side of the transformer, so its noise voltage is not amplified by the transformer, but probably it won't work well with the 10mm transformer mentioned above.
--- End quote ---
I was thinking the same thing but the transformer will increase the applied voltage from the transmit chirp making protection more difficult. Applying the pulse to the transformer itself could be a problem if it saturates and the protection is on the secondary side so a larger than necessary transformer is needed for this.
--- Quote from: Marco on May 01, 2018, 12:22:27 am ---For a 100V RMS signal you're passing 100 mA and burning 10 Watt in the MOSFET (which it can't take for very long).
--- End quote ---
This needs to be considered but the chirps are pretty short.
--- Quote ---The problem with the transformer is getting sufficient self inductance, the primary impedance does need to be significantly higher than the piezo to not make a mess.
--- End quote ---
I was going to say getting a sufficiently high frequency response but obviously this is not a large problem given the existence of the transmit transformer which must be larger to prevent saturation. I wonder if a tuned transformer would be better in this case.
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