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RE: LF: Field effect versus bipolar tran sistors

To: [email protected]
Subject: RE: LF: Field effect versus bipolar tran sistors
From: "Talbot Andrew" <[email protected]>
Date: Thu, 31 May 2001 08:28:54 +0100
Reply-to: [email protected]
Sender: <[email protected]>
That centre tapped coil in the Decca transmitters is there to 'soak up'
high current pulses generated in the short switching interval when both FETs
may be on together - a few ns duration.  This comes about since the turn-off
time of power FETs is of longer duration than the switch on time.  The 15
ohm resistor across the coil is the damping to prevent high frequency
spurious oscillations.
In a SMPSU the there is a deliberate dead band generated at the cross over,
when both devices in a push pull pair are switched off, but this is
undesirable in a very high efficiency class E transmitter.
Andy  G4JNT



protective means against this. The centre tapped coil for instance, loaded by 15 ohms, located between both power FETs in the Decca 5501 PA output, seems to me to be such a protective device.
In a certain way, my complementary bipolar PA is even showing
similarities to
this basic Decca power amplifier block mentioned, such as two
transistors in a
totem pole configuration, the output employing a series
resonant circuit . . .

73 Ha-Jo, DJ1ZB>

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