biasing mosfet for large signal amplification
biasing mosfet for large signal amplification
(OP)
hello
Im trying to build an ultrasound transducer driver.
The load is a RLC circuit that resonates around 1.5mhtz, this is 20ohms in series wtih 1.8uh and then 9.9nf. The C is the transducer.
The load in either in the common source or drain configuration of the power mosfet in not amplfying the signal.
the mosfet is
http://ww w.irf.com/ product-in fo/datashe ets/data/i rfpg50.pdf
In the common drain configuration, the voltage gain is roughly proportinonal to the input signal and VGS is approximatly equal, which should be correct. With a resistor from gate to drain (vcc), a DC offset is given to the load rather than amplify the signal
I need the signal to be amplifed to around 100-500volts peak to peak. So AC voltage gain is needed. In the common source configuation, I am getting a large DC offset on top of the signal rather than having the signal amplified. The DC offset is roughly proportial to VCC wtih the peak to peak roughly equal to the input signal.
What kind of configuation should i be using.
what abuot modeling an audio amplifer but to run at 1.5mhtz?
any other ideas?
Im trying to build an ultrasound transducer driver.
The load is a RLC circuit that resonates around 1.5mhtz, this is 20ohms in series wtih 1.8uh and then 9.9nf. The C is the transducer.
The load in either in the common source or drain configuration of the power mosfet in not amplfying the signal.
the mosfet is
http://ww
In the common drain configuration, the voltage gain is roughly proportinonal to the input signal and VGS is approximatly equal, which should be correct. With a resistor from gate to drain (vcc), a DC offset is given to the load rather than amplify the signal
I need the signal to be amplifed to around 100-500volts peak to peak. So AC voltage gain is needed. In the common source configuation, I am getting a large DC offset on top of the signal rather than having the signal amplified. The DC offset is roughly proportial to VCC wtih the peak to peak roughly equal to the input signal.
What kind of configuation should i be using.
what abuot modeling an audio amplifer but to run at 1.5mhtz?
any other ideas?





RE: biasing mosfet for large signal amplification
vcc is 300volts
RE: biasing mosfet for large signal amplification
Off/0V
On/500V
TTFN
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RE: biasing mosfet for large signal amplification
then swictching at 1.5hmtz
Im sure there will be lots of frequency dependent losses.
i think i need to use a push pull configuration.
RE: biasing mosfet for large signal amplification
Will it take a digital supply square wave instead of a sine wave?
Or develop two 500V supplies from your 300V one and then use a class A or class B push-pull.
I'd be really surprised if the transducer manufacturer didn't have an app note with a recommended drive circuit.
Keith Cress
kcress - http://www.flaminsystems.com
RE: biasing mosfet for large signal amplification
its from
http://www.eblproducts.com/transducer.html
These are more specality transducers
For this applicatition, we are driving them at much more power than the average application would use.
It seems very similar to a audio amplifier, just a higher frequency responce and wtih a little added resonation at the center frequency.
Apparently only n type mosfets go to 1000v, the p type only go to -400.. These are the high voltage mosfets orignally made by irf and sold to vishay. Ill figure something out i guess. thanks for the help.
RE: biasing mosfet for large signal amplification
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RE: biasing mosfet for large signal amplification
i didnt know there were driver chips for switching mos in push pull configuration.
that saves a lot of trouble, thanks man!!
the even have high volt ones too.
https
Ill give the tank- paraell circuit a try too.
I know series works for now..
RE: biasing mosfet for large signal amplification
Pleased you like the IR driver chips - they take away a lot of headaches.
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If we learn from our mistakes I'm getting a great education!
RE: biasing mosfet for large signal amplification
the those 2xxx series would be perfect except that they dont really run over 1mhtz.
I need to have a 1.5htz driver.
interstil has 40 mhtz drivers, but they dont really have the output or voltage. pin drivers i believe
RE: biasing mosfet for large signal amplification
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: biasing mosfet for large signal amplification
ht
TTFN
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RE: biasing mosfet for large signal amplification
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: biasing mosfet for large signal amplification
First thing to do is plot the transducer impedance with frequency. It will have both a series resonance, and a parallel resonance, fairly close together in frequency. Phase change around these close frequencies will be very dramatic.
It is always easier to feed significant power into the transducer at series resonance, where the output load current is highest.
The trick to driving these transducers to very high power is to make the transducer itself the frequency determining part of a power oscillator. If this is not done, transducer energy and efficiency will fall away very rapidly as the drive frequency moves slightly outside the exact high Q series resonant frequency.
I suggest that you use a current transformer in series with the transducer to provide the feedback voltage for the oscillator. With sufficient power gain, and correct phasing, it will oscillate away quite happily and produce some serious ultrasonic power.
Loading on the transducer will change the oscillation frequency, but it will always run at it's own most efficient self resonant frequency.
Think of this in concept as a very high power crystal (or tuning fork) oscillator with current feedback.
The loaded Q of the transducer at resonance, will still be high enough that driving it with a square wave will work fine.
RE: biasing mosfet for large signal amplification
yup, i think i got the transducer loading down. I think.
Its just a series RLC and i calculated the L to make the trsanducer (C) resonsate.
RE: biasing mosfet for large signal amplification
A crude analogy here might be the speaker cone resonance of a loudspeaker. You dont need to tune the voice coil inductance to 35 Hz with a capacitor to see this low frequency resonant effect.
Likewise, a quartz or ceramic ultrasonic transducer may be cut to mechanically resonate around 1.5 Mhz, but that has very little to do with directly measured electrical capacitance or inductance. The effect of rapidly changing impedance and phase change with frequency, are purely mechanical in nature, reflected back into the electrical circuit.
Do not attempt to tune the transducer by adding external reactance. Just drive it with a plain square wave and pick off the sine wave drive current. At series resonance, current will be maximum and in phase with the drive voltage.
The oscillator will naturally seek out and rapidly increase amplitude at the point of maximum in phase feedback.
RE: biasing mosfet for large signal amplification
Any noise "pings" the resonant transducer, and it rings at its self resonant frequency. Amplitude rapidly builds up, and away it goes. The over driven amplifier will quickly saturate into a square wave output.
Using MOSFETS in the power stage will be fine, but they will need to be biased in the linear region somehow, for effective start up.
Do not use schmidt triggers, flip flops, or anything strictly digital. It needs to be able to directly amplify small amplitude noise signals, or it will not easily be able to start up.