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G8MNY > TECH 20.03.15 16:22l 97 Lines 4397 Bytes #999 (0) @ WW
BID : 25615_GB7CIP
Read: GUEST
Subj: 12V RF immune AF Booster Amp
Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP
Sent: 150320/1041Z @:GB7CIP.#32.GBR.EURO #:25615 [Caterham Surrey GBR]
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 & included a 5" LS so it is really loud. It has the advantage of
being fairly RF immune unlike IC Amplifiers. Handhelds & 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 4ê or 3ê LS
plenty of AF power is available.
ÚÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄoÄÄoÄ<+12V
C1 47uF 1k ³ ³+ 3A
Rig LS>ÄÂÄÄ´ÃÄÄÄÂÄÄÄ¿ 220uF ³ TR2³/ C6 === 470uF
³ + )º(ù ÚÄÄÄ´ÃÄÄÄÄÁÄÄÂÄÄÄ´ NPN ³ 18V
³ 2x )º( ³ C3 + _³_ ³\e + 1000uF ³
³ Step )º( ³ D1 \_/ ÃÄÄÄÄ´ÃÄÄ¿ 10V ³
100ê up ù)º(___³ C4 + ³ ³/e C5 ³ ³
³ T1 ³ ³ ÀÄÄÄ´ÃÄÄÄÄÂÄÄÁÄÄÄ´ PNP ÚÁ¿/³4ê ³
³ ³ === 220uF ³ TR1³\ ÀÂÙ\³LS ³
³ ³ C2³SOT 10V 1k Ge ³ ³ 8W ³
ÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ-< -ve
INPUT CIRCUIT
The input 100ê 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 // 500ê) which should
be no lower than 15ê.
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.
_
+4V_ _ +12V~³ / \ /~\
³/ \ ³³ ³ ³ ³
_³ \_/ ³ ³ ³ ³ ³
-4V 50mW 0V_³ \ / \_/
Input Base ~ Drive Full Swing
> 0 & 12V Output 5W
The value of C1 limits the LF response, & 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
Ä´ÃÄÄÂÄÄÄ>to LOAD ÄÄ((()ÄÄÂÄÄ>to Load
C1 )º ==== ³
L )º Another L === Another
)º 6dB/O C2 ³ 6dB/O
ÄÄÄÄÄÁÄÄÄ ÄÄÄÄÄÄÄÄÁÄÄÄ
Gain
0¿ .ÄÄÄÄÄÄÄÄÄÄÄÄÄÄ.
-3dB´ / \
³ / \
-12dB´ / \
ÃÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄ¿ Hz
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 &
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 & low
losses. Unlike IC amplifiers were the peak LS voltage is typically ñ1V less
than the power rails this design will be very close to full power rails. So
quite a bit more power is available. e.g. on 14V it should give ñ6.8V swing
into 4ê 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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