[Fwd: Re: [TenTec] RE:Using CB amps with Qrp rigs.]

[Dr. Gerald N. Johnson, P.E.]

Message-ID: <3548A62C.143A@ames.net>
Date: Thu, 30 Apr 1998 11:26:20 -0500
From: "Dr. Gerald N. Johnson, P.E." <geraldj@ames.net>
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Organization: Gerald N. Johnson, P.E.
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Subject: Re: [TenTec] RE:Using CB amps with Qrp rigs.
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It will take more than a low pass filter to make a decent amp out of a
CB amplifier. They were designed for AM and so depend on the carrier to
provide forward bias to the transistors to make them sort of linear. For
CW this may not be as great a consideration, but a pure class C
amplifier can add key clicks to a marginally hard keyed signal, and will
splatter all over when driven with SSB.

The simplest forward bias circuit uses a resistor and a diode to create
a sort of regulated bias voltage. This is unfortunately commonly used in
VHF "linear" amplifiers and leads to lots of splatter because it can't
control the bias when there is drive. The circuit in Motorola
applications notes using a couple transistors (one in thermal contact
with the RF power transistors, which can be a diode) and a 723 regulator
chip has proven to significantly improve the intermod distortion of such
a pseudo linear and make it as good as can be accomplished with
transistors.

Adding forward bias to a poorly constructed amplifier that neglects RF
feedback, shielding, and adequate bypassing can create a power
oscillator.

73, Jerry, K0CQ



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