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High voltage, high frequency discrete opamp

H

Hans Wolfenstein

Jan 1, 1970
0
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
 
T

Tim Williams

Jan 1, 1970
0
Hmm, 6MHz on 30pF is 884 ohms reactance, so the peak current will be 170mA.

The average CRT driver parts are 1GHz, 100V and 100mA, which falls a little
short. Might be able to find some in the 300V, 200mA range.

I've heard of fair bandwidths from fairly pedestrian parts, e.g. MJE350. If
you drive the piss out of them (possibly more drive than cascode), you might
get something.

Tim
 
J

JosephKK

Jan 1, 1970
0
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.
 
C

Carl Ijames

Jan 1, 1970
0
Hans,

If you are still out there, there are two used high voltage excite amps on
Ebay in the US. Look at item 290360425190 and item 350159383389, each
listed for $999. These are from the Nicolet-designed FTMS 2000 system
(Nicolet, Extrel, Finnigan, Thermo, whatever). They have two independent
channels with differential output, gain of 100, max output of +/- 100 Vpp,
and frequency response starting at about 5 kHz and flat past about 6 MHz,
with response going up to at least 13-14 MHz at somewhat reduced output.
Not quite the output voltage you wanted but having used a few of these over
many years they are great pieces and at that price having something up and
working almost immediately has a lot of attraction no matter what you may
wind up building :). The second vendor, craig4076, had an almost
mechanically complete (no data system) FTMS 2000 on Ebay a couple of years
ago, with excite amp, preamp, vacuum chamber with dual cells, and 3 tesla
magnet. It looks like they are parting it out (they have a couple of
chamber pieces and the old vacuum control computer listed now along with the
excite amp). Might be worth asking if they still have the dual preamp if
you need one of those, as well. (JL, if you are reading this close your
eyes :).)
 
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