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LF: Re: Field effect versus bipolar transistors

To: [email protected]
Subject: LF: Re: Field effect versus bipolar transistors
From: "Vernall" <[email protected]>
Date: Tue, 29 May 2001 22:49:01 +1200
References: <[email protected]>
Reply-to: [email protected]
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Dick and others,
 
I have made several PAs for 180 kHz, mostly with bipolar power transistors.  Down this way we also have SSB nets, so at least historically, most PAs were designed for similar type of service as on HF, namely to be able to be biased to cater for linear operation, or thrashing along unbiased in Class C.
 
For linear service, there is very little difference between bipolars and power MOSFETs, both dump about 50% of input as heat!
 
In the early days we thought that "line output transistors" like BU508A (1500 volt rating) were suitable for PAs, but we subsequently found out that they have rather high saturation voltage, so efficiency is worse than bipolars with 450 or 500 volt ratings.
 
When it comes to a "digital" PA, for on-off keying, dual frequency keying or BPSK, then it has a different set of design objectives.  The final can be very efficient (squarewave at full drive) and the key clicks can be controlled to acceptable levels by envelope control of the driver (or pre-driver).
 
Bipolar switching transistors intended to work from rectified 230 volt mains in switch mode power supplies have been found to work very nicely in N+N pushpull finals on LF with a 100 volt rail.  I can produce 1 kW output from 2+2 TO3 bipolars, at 180 kHz, with 110 volt rail.
 
Having said that, my next final is going to be a power MOSFET job, with TC2247 driver chips.  I have seen a pushpull final made by Ric VK7RO that has efficiency of over 90%.  That is better than I can get from my bipolar final.
 
I rate efficiency as a more desirable parameter than drive.  I have an SSB modulator driver (a modified TS-850 and 0.5 ppm TCXO master oscillator) so I can use sound card drive for dual frequency keying, with envelope shaping.  I'm seeking to provide 1 kW out from a power MOSFET final that runs cool enough that I can confidently leave it for unattended running, for long sessions of QRSS, with natural convection cooling.  I can't do that with my bipolar final.
 
73, Bob
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