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Re: LF: Class E PA

To: [email protected]
Subject: Re: LF: Class E PA
From: Johan Bodin <[email protected]>
Date: Tue, 9 Oct 2018 17:00:17 +0200
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Hello Tom,

most schoolbook class-E designs assume a DC feeding choke with infinite inductance i.e. a huge inductor that keeps the DC current constant without any RF ripple. This is of course totally impractical (especially at VLF !)...

I'll try to find some class-E design documents that I have saved somewhere. One of those documents has explicit design equations for class E amplifiers with finite DC feed inductance.

If memory serves me, the reactance of the DC feed inductor can be as low as the DC feed resistance (Vdc/Idc) but such a low value would require a very large decoupling capacitance on the DC supply side because of high RF ripple current flowing through the inductor.

I will send these pdf documents to you if you are interested.

Johan SM6LKM

Den 2018-10-07 kl. 19:51, skrev DK1IS:

Hello Group,

Planning a new single-ended Class E PA for my activities on the Dreamers Band (8.27 kHz) two questions came up - perhaps someone can help me:

-  Being interested only in on-off-keying I´m concerned about the capacitor in parallel to the power FET which in my case on VLF will be in the uF range. If the DC supply is on and without gate drive this capacitor is charged to the DC supply voltage. At the first positive gate pulse it discharges via the drain-source-path, the current being  limited only by parasitc impedances - a poorly defined condition which usually has to be avoided. Does this mean that I have to apply drive continously and key the DC power supply only, perhaps synchronously to the next positive drive pulse?
Is there any rule of thumb for the inductance of the rf choke in the drain-power supply path?

Thanks for any idea!

Tom, DK1IS

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