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mmf Post subject: Driving low impedance Posted: Mon Nov 23, 2009
2:21 am Lieutenant Joined: Mon Nov 23, 2009 1:21 am
Posts: 1 Hello, I'm no RF expert and could use some help.
I need to drive a single-turn coil over the range of 5 to 200 MHz. I
am using a DDS to generate my sine wave and have used a PGA IC meant
for driving CATV cable to drive the coil. Since the amp is meant for
driving 75 ohm loads, I have quite a voltage drop when driving the low
impedance of my coil. I would like to put more power into the coil and
wondered how I might do this. I can get almost 1 VRMS across the coil
now but would like get about 2.5 VRMS. I want a fairly flat amplitude
versus frequency curve into this coil as it is used to excite a system
that has it's resonances sensed. It seems like RF amps are all designed
for the standard 50 or 75 ohm loads. Any help is appreciated.
Top nubbage Post subject: Re: Driving low impedancePosted:
Mon Nov 23, 2009 4:10 am General Joined: Fri Feb
17, 2006 12:07 pm Posts: 218 Location: London UK Hi mmf
you will probably have more success by using a high-current differential
line driver, also used for CATV, but not the single-ended unbalanced
50ohm type. These are designed for high current drive into a twisted
pair cable, so you connect the one turn coil across the two balaced
outputs. Also recall that discrete component audio amplifiers used
a complementary pair emitter-follower circuit to drive 4 ohm loudspeakers.
If you design a similar arrangement but using HF/VHF techniques for
component choice and circuit layout, it should be possible to drive
significant current through the coil and achieve the objective. I have
used this idea to drive a low resistance bridge to measure low values
of resistance accurately. It might be more helpful to think in terms
of current and mmf rather than voltage in this low impedance situation.
_________________ At bottom, life is all about Sucking in
and blowing out. Posted 11/12/2012
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