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G8MNY > TECH 28.02.05 16:06l 72 Lines 3403 Bytes #999 (0) @ WW
BID : 34567_GB7CIP
Read: GUEST
Subj: 12V RF immune AF Booster Amp
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<7M3TJZ<HG8LXL<ON4HU<GB7YFS<GB7CIP
Sent: 050228/1304Z @:GB7CIP.#32.GBR.EU #:34567 [Caterham] $:34567_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Apr 04)
This is a simple AF 5W booster for handhelds etc. I built mine into a large 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.
ÚÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄoÄÄoÄ<+12V
C1 47u 1K ³ ³+ 3A
Rig LS>ÄÂÄÄ´ÃÄÄÂÄÄÄ¿ 220u + ³ TR2³/ C6 ===470uF
³ )º( ÚÄÄÄ´ÃÄÄÄÁÄÄÂÄÄÄ´ NPN ³ 18v
³ 2x )º( ³ C3 _³_ ³\e + 1000uF ³
³ Step )º( ³ D1 \_/ ÃÄÄÄÄ´ÃÄÄ¿ 10v ³
100ê up )º(___³ C4 + ³ ³/e C5 ³ ³
³ T1³ ³ ÀÄÄÄ´ÃÄÄÄÂÄÄÁÄÄÄ´ PNP ÚÁ¿/4ê ³
³ ³ === 220u ³ TR1³\ ÀÂÙ\LS ³
³ ³ C2³SOT 10v 1K Ge ³ ³ 8W ³
ÀÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ-< - 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.
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.
T1 is typically a small transistor radio O/P transformer of 50mW or so rating
that has 2 identical windings. This ensures the Transistors with be driven with
voltage swings above & below the power rails for max power.
The value of C2 is best found on test (SOT), it's effect together with the non
mutual inductance of T1 provide a sharp HF cut off point above 3KHz & also
gives a slope of 12dB/Octave. Typical C2 Value 1nF.
BIASING
A single Silicon diode D1 is used with 2x 1Ks 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 constructuion & bolt down TR1, a PNP Germainium
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 no gain matched devices,
but there will be some low level crossover distortion!
OUTPUT
This is fed though a large C5 1000uF to maintain the low O/P 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. eg on 14v should give ñ6.8V swing into 4ê giving about
5.8W RMS 11.6W USA AMP rating (eg full square wave).
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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