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G8MNY > TECH 05.03.26 18:59l 49 Lines 1841 Bytes #5 (0) @ WW
BID : 53582_GB7CIP
Read: GUEST
Subj: Transistor PA Biasing
Path: JH4XSY<N3HYM<N2NOV<VE2PKT<PY2BIL<HB9ON<VK7AX<GB7CIP
Sent: 260305/0952Z @:GB7CIP.#32.GBR.EURO #:53582 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated May 17)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here is a simple circuit that can deliver the peak base current needed for
linear SSB operation, that works very well. It uses 2 TIP type NPN transistors,
3 Rs, and suitable RF filtering for the band in use. T1 must be bolted close to
the PA devices so that it's emitter base voltage can tracks the PA temperature,
the base is protected from RF with a ferrite bead. T2 needs heat sinking so it
is also convenient to mount it on close to T1.
Bias adjustment can be done by adjusting the 470R, but never change it just for
a preset without a safe minimum series R!
PTT
Switched
+12V ﾄﾄﾂﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ
ｳ ｳ 10R 10W
ｳ 470R T2 ｳ
10u ｳ+ ｳ ｳ/c
=== ﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄｴTIP
ｳ ｳ NPNｳ\e +570mV
ｳ ｳ T1 ﾃﾄﾄﾄﾄﾂﾄﾄﾄﾂﾄﾄﾄ RFC to PA
ｳ c\ｳ ｳ ｳ ｳ Bases
ｳ ﾃﾄﾄFBﾄﾄﾄﾄﾄｴ ｳ +ｳ
ｳ e/ｳ NPN 5R6 === ===
ｳ ｳ ｳ 1nｳ ｳ10u
0V ﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾄ
The MAX base current, about 1A is available, this is limited by the 10R and
should be enough current for a 50W UHF or 100W VHF or 200W HF PA.
The difference in power is because bipolar transistors have 2 gain figures, the
DC gain HFE (also called Beta) and the much smaller AC gain Hfe at the
frequency of operation. The bias current needed is Hfe mode, where the gain for
most RF transistors drops from a DC HFE of up to a 100, down to below single
figures like Hfe of 0.1 to 5 @ RF! Even at 1 there still as lot of power gain,
due to the voltage swing difference.
See my Tech Bul on "AF 2 Tone Test Osc Design"
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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