Message 29247 in TECHNI.WW
From: G8MNY
Date: Wed, 02 Sep 26 08:58:00 Z
Newsgroups: TECHNI.WW
Subject: 12V RF immune AF Booster Amp
Message-ID: <3965_GB7CIP>
Path: N2MH4|GB7BED|K1AJD|VE3CGR|OK2PEN|GB7CIP
R:260902/0859Z 36533@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260902/0859Z 16916@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260902/0858Z 8978@K1AJD.#AUG.GA.USA.NOAM LinBPQ6.0.25
R:260902/0858Z 93140@VE3CGR.#SCON.ON.CAN.NOAM LinBPQ6.0.25
R:260902/0858Z 31452@OK2PEN.SP.BRA.SOAM [Sao Jose dos Campos] $:3965_GB7CIP
R:260902/0858Z @:GB7CIP.#32.GBR.EURO #:3965 [Caterham Surrey GBR] $:3965_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is a simple AF 5W booster amp for handhelds etc. I built mine into a large
RF PA box and included a 5" LS so it is really loud. It has the advantage of
being fairly RF immune unlike IC Amplifiers. Handhelds and small rigs don't
produce a full power rail LS swing (often on low voltage) so a small 2:1 step
up transformer is used to make sure the double emitter follower current gain
circuit can go into full clipping with low drive swing. Using a low 4R or 3R LS
plenty of AF power is available.
INPUT CIRCUIT
The input 100ohm is only there to provide a DC path if the rig needs a DC load
for bootstrap current etc. It can be omitted to save handheld battery life if
this is not required.
The input load on the handheld will be the 1/4x (LS x HFE // 500ohm) which
should be no lower than 15ohm.
T1 is typically a small transistor radio output transformer of 50mW or so
rating, that has 2 identical windings. This ensures the Transistors will be
driven with voltage swings above & below the power rails for max power.
_
Input Base ~ Drive Full Swing
> 0 & 12V Output 5W
The value of C1 limits the LF response, and with the inductance of T1 plus the
input load can give a good LF cut off below 300Hz of a rate of 12dB/Octave.
C1 only needs to be low voltage type.
6dB/O LF cut 6dB/O HF cut
Gain
0 80 150 300 700 1k5 3k 6k 12k
The value of C2 is best found on test (SOT), it's effect together with the non
mutual inductance leakage L of T1 provide a sharp HF cut off point above 3kHz
and also gives a slope of 12dB/Octave. Typical C2 Value is 1nF.
BIASING
A single Silicon diode D1 is used with 2x 1k to give the required 0.7V bias @
half power rail point. To maintain the bias over the input current drive cycle
2 large capacitors C3 & 4 are used. These only need to be 10V working.
TRANSISTORS
Use a metal box for the construction & bolt down TR1, a PNP Germanium
transistor like a TO3 OC35, with no insulation kit, & TR2 a Silicon NPN like a
2N3055 with an insulation kit. As these are a double emitter follower output,
then the bias voltage of 0.7V is just about right, so no emitter Rs are needed.
As there is 100% NFB the distortion is low even with non gain matched devices,
but there will be some low level crossover distortion!
OUTPUT
This is fed through a large C5 1000uF to maintain the low output Z and low
than the power rails, in this design it will be very close to full power rails.
into 4R giving about 5.8W RMS 11.6W "USA music rating" (e.g. full square wave
power.)
For stability C6 is included across the power rails.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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