Driving low impedance - RF Cafe Forums
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Post subject: Driving low impedance Posted: Mon Nov 23, 2009 2:21 am
Joined: Mon Nov 23, 2009 1:21 am
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.
Post subject: Re: Driving low impedancePosted: Mon Nov 23, 2009 4:10 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
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.
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