7/27/2019 FM Radio Peramplifier

1/2

CIRCUIT IDEAS

ELECTRONICS FOR YOUNOVEMBER 2003

Fig. 1: Circuit of MOSFET-based preamplifier for FM DXing

Fig. 2: Different antennae used for FM DXing

MOSFET-BASEDPREAMPLIFIERFOR FMRADIODXing

SANITH

EO

N.S. HARISANKAR,VU3NSH

FM transmissions can be receivedwithin a range of 40 km. If you arein fringe areas, you may get a very

weak signal. FM DXing refers to hearingdistant stations (1500 km or more) on theFM band (88-108 MHz). The term DX is

borrowed from amateur radio operators.It means distance unknown; D standsfor distance and X stands for unknown.For an FM receiver lacking gain, or hav-ing a poor signal-to-noise ratio, using anexternal preamplifier improves the signallevel.

The dual-gate MOSFET preamplifier cir-

cuit shown inFig. 1 givesan excellentgain of about18 dB. It

costs less andis simple todesign.

Field-ef-fect transis-tors (FETs)are superiorto bipolartransistors inmany appli-cations asthese have a

much higher gainapproaching that of avacuum tube. These are classified intojunction FETs and MOSFETs. On compar-ing the FETs with a vacuum tube, the gateimplies the grid, the source implies thecathode, and the drain implies the plate.In a transistor, the base implies the grid,the emitter implies the source, and thecollector implies the drain.

In dual-gate FETs, gate 1 is the signalgate and gate 2 is the control gate. Thegates are effectively in series, making iteasy to control the dynamic range of the

device by varying the bias on gate 2.The MOSFET is more flexible becauseit can be controlled by a positive or nega-tive voltage at gate 2. The resistance be-tween the gate and rest of the device isextremely high because these are separatedby a thin dielectric layer. Thus the MOSFEThas an extremely high input impedance.

Dual-gate MOSFETs (DG MOSFETs) arevery popular among radio amateurs. Theseare being used in IF amplifiers, mixers,and preamplifiers in HF-VHF transceivers.The isolation between the gates (G1 andG2) is relatively high in mixer applications.

This reduces oscillator pulling and radia-tion. The oscillator pulling is troublesomeparticularly in shortwave communications.It is a characteristic in many unsophisti-cated frequency-changer stages, where theincoming signal, if large, pulls the oscilla-tor frequency slightly off the frequency setby the tuning knob and towards a fre-quency favourable to the (large) incomingsignal. A DG MOSFET can also be usedfor automatic gain control in RF amplifi-ers.

DG MOSFET BF966S is an n-channeldepletion-type MOSFET that is used for

general-purpose FM and VHF applications.In this configuration, it is used for FMradio band. The quadratic input character-istic of the FET input stage gives betterresults than the exponential characteristicof a bipolar transistor.

Gate 1 is meant for input and gate 2 isfor gain control. The input from the an-tenna is fed to gate G1 via C1 and L1.Trimmer VC1 is used to tune and selectthe input frequencies. Capacitor C4 (100kpF) at the gain control electrode (gate 2)

7/27/2019 FM Radio Peramplifier

2/2

CIRCUIT IDEAS

ELECTRONICS FOR YOU NOVEMBER 2003

decouples any variation in G2 volt-age at radio frequencies to main-tain constant gain. Set preset VR(47k) to adjust the gain or connecta fixed resistor for fixed gain. Theoutput of the circuit is obtained viacapacitor C5 and fed to the FM re-ceiver amplifier.

For indoor use, connect a -wavelength whip antenna, -wave-length 1.5m wire antenna, or anyother indoor antenna set-up withthis circuit. You may use a 9Vbattery without the transformer anddiode 1N4007, or any 6V-12V powersupply to power the circuit (referFig. 1). The RF output can be takendirectly through capacitor C5. For an im-proved input and output impedance,changeC1 from 1 kpF to 22 pF and C5from 1 kpF to 100 kpF.

For outdoor use at top mast, like a TVbooster, connect the C5 output to thepower supply unit (PSU) line. Use RG58U/RG11 or RG174 cable for feeding the powersupply to the receiver amplifier. The PSUfor the circuit is the same as that of a TVbooster. For TV boosters, two types ofmountings are employed: The fixed tunedbooster is mounted on the mast of theantenna. The tunable booster consistingof the PSU is placed near the TV set forgain control of various TV channels. (Fordetails, refer High-Gain 4-Stage TV Booster

Coil & Capacitor Details for DXing of Various Frequency Bands

10m band 6m band 3m band 2m band

(28 MHz) (50 MHz) (98 MHz) (144 MHz)

amateur radio amateur radio FM radio FM radio

L1 core 17T, 28 SWG 12T, 26 SWG 7T, 5mm dia., 5T, 20SWG,

Amindoncore OnT-37-10,Tap 20SWG -inch ID, -inch L

(T-50-6), Tap at 5T from GND Aircore, 1cm Length, Tap 2 from GNDat 6T from GND Tap 3T from GND

L2 core 17T, 28 SWG 12T, 26 SWG 8T, close winding, 4T, 20 SWG,

Amindoncore On T-37-10, 5mm dia. Air-core, No tap

T-50-6, without tap No tap

Without tap

VC1 & VC2 60 pF 22 pF 22 pF 15 pF

on page 72ofElectroni cs Projects Vol . 8.)Mount the DG MOSFET BF966S at the

solder side of the PCB to keep parasiticcapacitance as small as possible. Use anepoxy PCB. After soldering, clean the PCBwith isopropyl alcohol. Use a suitableenclosure for the circuit. All componentleads must be small. Avoid shambled wir-ing to prevent poor gain or self oscilla-tions. Connecting a single-element cubicalquad antenna to the circuit results inOpen Sesam for DXing.

You can use a folded dipole or anyother antenna. However, an excellent per-formance is obtained witha cubical quadantenna (refer Fig. 2) and Sangean ATS-803 world-band receiver.

In an amplifier, FET is immune to

strong signal overloading. It produces lesscross-modulation than a conventional tran-sistor having negative temperature coeffi-cient, doesnt succumb to thermal run-away at high frequencies, and decreasesnoise. In VHF and UHF, the MOSFETproduces less noise and is comparablewith JFETs. DG FETs reduce the feedbackcapacitance as well as the noise powercoupled to the gate from the channel,giving stable unneutralised power gain forwide-band applications.

This circuit can be used for otherfrequency bands by changing the inputand the output LC networks. The tablehere gives details of the network compo-nents for DXing of stations at variousfrequency bands.