High voltage, high frequency discrete opamp
in [Design]
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From: cassiope on 19 Apr 2010 13:43 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. 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 If you can tolerate a transformer-based solution, that might be good. Otherwise... Electrostatic deflection amplifiers (as were used in oscilloscopes) didn't generally use emitter followers since their load capacitances were typically below 10pF - EFs would add as much. Most MOSFETs have atrocious capacitances - it's for that reason, more than their gm, that makes HV bipolars more likely to succeed. Along that vein - with some effort you can get up to ~2.7x impedance-bandwidth improvement with judicious inductive coupling, more if you can tolerate some overshoot in the time domain response. See, for example http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01210924 -f (designed a few discrete deflection amps for Tek long ago)
From: JosephKK on 22 Apr 2010 08:41
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). > >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 Sounds like the vertical deflection amp from a hobby grade oscilloscope from 25+ years ago. DC to 10 MHz and a couple hundred volts differential output into a capacitive load. |