High voltage, high frequency discrete opamp
in [Design]
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From: Hans Wolfenstein on 17 Apr 2010 04:48 Hello, I am electronic technician employed at mass spectrometry laboratory. Current problematics which I am involved includes building a high voltage, high frequency discrete operational amplifier which can amplify AC sweeps from 100 kHz to 6 MHz with output voltage swing from -150 to +150V. Gain is not needed to be more than X10 (we've already build separate preamp). My idea was using the MOSFETs in the output as for the differential input I considered the "super match" wide band dual FETs. Signal is always sine wave which makes job a lot easier. Load on the output has practically infinite ohmic resistance; only load is capacitance of measuring cell (which works in high vacuum) of tipically 30pF (it is a system of metal plates that performs cyclotron moving of ions but has negligable interaction with ions, much like deflection electrodes in classic oscilloscope tube, so current should be very small). Basic topology of the circuit is standard, includes differential input (non-inverting and inverting inputs) stage and complementary output powered with symmetrical PSU. Before I got employed in this lab my colleagues have made an amplifier based on APEX opamp but it had significant roll-of at frequencies over 1,5Mhz (despite to nice looking computer simulations), not to mention the price of these opamps. Demands are looking "heavy" - there is a large AC voltage swing and wide freq. bandwidth. Does anybod have recommendation about construction and parts choice ? Thank you. Hans
From: Vladimir Vassilevsky on 17 Apr 2010 05:43 Hans Wolfenstein wrote: > > Hello, > > I am electronic technician employed at mass spectrometry laboratory. > > Current problematics which I am involved includes building a high > voltage, high frequency discrete operational amplifier which can > amplify AC sweeps from 100 kHz to 6 MHz with output voltage swing from > -150 to +150V. Gain is not needed to be more than X10 (we've already > build separate preamp). My idea was using the MOSFETs in the output as > for the differential input I considered the "super match" wide band > dual FETs. Signal is always sine wave which makes job a lot easier. > Load on the output has practically infinite ohmic resistance; only load > is capacitance of measuring cell (which works in high vacuum) of > tipically 30pF > > Demands are looking "heavy" - there is a large AC voltage swing and wide > freq. bandwidth. Does anybod have recommendation about construction and > parts choice ? Your requirements look a lot like those for CRT video amplifiers; consider topologies from there. For discrete solutions, typically, the output stage is cascode and pnp/npn follower after that. Perhaps, you can use a single video IC solution. If your signal is AC sine wave, you don't have to worry about level shifting. However who does CRT ICs in our days? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: John Larkin on 17 Apr 2010 10:50 On Sat, 17 Apr 2010 10:48:43 +0200, Hans Wolfenstein <hans(a)somewhere.com> wrote: > >Hello, > >I am electronic technician employed at mass spectrometry laboratory. > >Current problematics which I am involved includes building a high >voltage, high frequency discrete operational amplifier which can >amplify AC sweeps from 100 kHz to 6 MHz with output voltage swing from >-150 to +150V. Gain is not needed to be more than X10 (we've already >build separate preamp). My idea was using the MOSFETs in the output as >for the differential input I considered the "super match" wide band >dual FETs. Signal is always sine wave which makes job a lot easier. > >Load on the output has practically infinite ohmic resistance; only load >is capacitance of measuring cell (which works in high vacuum) of >tipically 30pF (it is a system of metal plates that performs cyclotron >moving of ions but has negligable interaction with ions, much like >deflection electrodes in classic oscilloscope tube, so current should >be very small). Sounds like some sort of mass spectroscopy thing. FTMS? FTMS is interesting, circuit-wise, sort of like radar as regards the t/r issues. > >Basic topology of the circuit is standard, includes differential input >(non-inverting and inverting inputs) stage and complementary output >powered with symmetrical PSU. > >Before I got employed in this lab my colleagues have made an amplifier >based on APEX opamp but it had significant roll-of at frequencies over >1,5Mhz (despite to nice looking computer simulations), not to mention >the price of these opamps. > >Demands are looking "heavy" - there is a large AC voltage swing and wide >freq. bandwidth. Does anybod have recommendation about construction and >parts choice ? > >Thank you. > >Hans It would be interesting to transformer couple into the gates of a pair of totem-pole nfets. Do any required dc bias with dc/dc converters or better yet PV couplers. That has some huge drive advantages... a grounded medium-power opamp becomes the low-impedance driver for each fet. You may be able to run almost open-loop, with overall DC feedback and little or no AC feedback. Or make the feedback local to the power fets themselves, with just a couple of resistors. A complementary output stage can also benefit hugely from transformer coupling the drive. You could even float some gate drive electronics. I'd scribble something but I've gotta hit the road. Maybe later. John
From: whit3rd on 17 Apr 2010 23:46 On Apr 17, 1:48 am, Hans Wolfenstein <h...(a)somewhere.com> wrote: > Hello, > > I am electronic technician employed at mass spectrometry laboratory. > > Current problematics which I am involved includes building a high > voltage, high frequency discrete operational amplifier which can > amplify AC sweeps from 100 kHz to 6 MHz with output voltage swing from > -150 to +150V > Demands are looking "heavy" - there is a large AC voltage swing and wide > freq. bandwidth. Does anybod have recommendation about construction and > parts choice ? For fast slewing, a cascode with the high voltage on a bipolar transistor is good (MOSFETs that take high voltage don't have as much transconductance). You might also consider using low-voltage power amp and a transformer to boost the output. You lose DC performance, of course, but there are ways to put a separate (slower) DC amp alongside; just drive the output winding end-number-two instead of grounding it. Vacuum tubes ARE still available, and might be useful here...
From: Robert Baer on 18 Apr 2010 05:06
Hans Wolfenstein wrote: > > Hello, > > I am electronic technician employed at mass spectrometry laboratory. > > Current problematics which I am involved includes building a high > voltage, high frequency discrete operational amplifier which can > amplify AC sweeps from 100 kHz to 6 MHz with output voltage swing from > -150 to +150V. Gain is not needed to be more than X10 (we've already > build separate preamp). My idea was using the MOSFETs in the output as > for the differential input I considered the "super match" wide band > dual FETs. Signal is always sine wave which makes job a lot easier. > > Load on the output has practically infinite ohmic resistance; only load > is capacitance of measuring cell (which works in high vacuum) of > tipically 30pF (it is a system of metal plates that performs cyclotron > moving of ions but has negligable interaction with ions, much like > deflection electrodes in classic oscilloscope tube, so current should > be very small). > > Basic topology of the circuit is standard, includes differential input > (non-inverting and inverting inputs) stage and complementary output > powered with symmetrical PSU. > > Before I got employed in this lab my colleagues have made an amplifier > based on APEX opamp but it had significant roll-of at frequencies over > 1,5Mhz (despite to nice looking computer simulations), not to mention > the price of these opamps. > > Demands are looking "heavy" - there is a large AC voltage swing and wide > freq. bandwidth. Does anybod have recommendation about construction and > parts choice ? > > Thank you. > > Hans Maybe another approach: wideband low(er) voltage amplification but more power to drive a step-up transformer which drives the load. |