Login: GUEST @ JH4XSY.14.JNET1.JPN.AS [Tsuchiura]
home | newest check | boards | help index | log | ps | userlogin | send sysop | slog | status forward | bcm news | users | version | remove cookieG8MNY > TECH 23.06.25 16:04l 97 Lines 4410 Bytes #2 (0) @ WW BID : 35298_GB7CIP Read: GUEST Subj: 12V RF immune AF Booster Amp Path: JH4XSY<IW0QNL<IZ3LSV<ED1ZAC<GB7CIP Sent: 250623/0653Z @:GB7CIP.#32.GBR.EURO #:35298 [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 100R up ù)º(___³ C4 + ³ ³/e C5 ³ ³ ³ T1 ³ ³ ÀÄÄÄ´ÃÄÄÄÄÂÄÄÁÄÄÄ´ PNP ÚÁ¿/³4R 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 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. _ +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, in this design it 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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