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G8MNY > TECH 10.08.04 00:28l 71 Lines 3358 Bytes #999 (0) @ WW
BID : 4891_GB7CIP
Read: GUEST OE7FMI
Subj: 12V RF immune AF Booster Amp
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0BEL<7M3TJZ<HA3PG<IK1ZNW<GB7CRV<GB7CIP
Sent: 040809/2058Z @:GB7CIP.#32.GBR.EU #:4891 [Caterham] $:4891_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
47u 1K ³ ³+ 3A
Rig LS>ÄÂÄÄ´ÃÄÄÂÄÄÄ¿ 220u + ³ TR2³/ C6 ===470uF
³ C1 )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 === 222u ³ TR1³\ ÀÂÙ\LS ³
³ ³ C2³SOT 10v 1K Ge ³ ³ 7W 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 clos ull 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.
For stability C6 is included across the power rails.
Why don't U send an interesting bul?
/QSL
73 De John, G8MNY @ GB7CIP
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