OZ2OE-144 MHz Pre Amplifier With BF981
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14/02/12 144 MH] preamplifier with BF981 1/4 o]2oe.dk/radio/bf981/981.html OZ2OE Technical Home Page - 144 MH] low noise amplifier. Uploaded ApUil 19'Wh 2000 DeVign of a pUeamplifieU foU 144 MH] ZiWh 1 dB NF and 23 dB gain XVing BF981. The finiVhed amplifieU. Background ThiV amplifieU iV XVing a loZ coVW Vilicon MOSFET (BF981 fUom PhilipV) Wo giYe moUe Whan 20 dB gain ZiWh aUoXnd 1 dB noiVe figXUe on 2 meWeU. YoX can bXild moUe adYanced loZ noiVe amplifieUV XVing GaAV FET'V, bXW Whe dXal gaWe MOSFET iV no bad peUfoUmeU - Whe diffeUence in noiVe figXUe ma\ noW eYen VhoZ Xp on WeUUeVWUial paWh. BeVideV iW'V a loW moUe foUgiYing in WeUmV of accidenWal deVWUXcWion - and iW'V cheapeU! Mechanical design The amplifieU iV bXild on a doXble Vided boaUd meaVXUing 25[50 mm. The XppeU Vide fXncWionV aV a gUoXnd plane and Whe WUaceV aUe on Whe loZeU Vide. The boaUd can eiWheU be bXild inWo Whe Uadio oU moXnWed in a Vmall meWal bo[ ZiWh BNC connecWoUV. The amplifieU iV bXild on a Vmall PCB. Electrical design The diagUam iV VhoZn beloZ. The inpXW ciUcXiW iV a VeUieV WUanVfoUmaWion, Whe oXWpXW ciUcXiW a "noUmal" paUallel ciUcXiW ZiWh a capaciWiYe oXWpXW Wap. AV WhiV amplifeU haV high gain, WheUe iV pUoYiVion foU a pi-aWWenXaWoU on Whe oXWpXW. BiaVing of Whe WUanViVWoU iV done ZiWh a VoXUce UeViVWoU/YolWage diYideU and ZiWh YolWage diYideUV on boWh gaWe1 and gaWe2. ThiV ciUcXiW WighWl\ conWUolV Whe DC paUameWeUV of Whe WUanViVWoU.
Transcript of OZ2OE-144 MHz Pre Amplifier With BF981
14/02/12 144 MHz preamplifier with BF981
1/4oz2oe.dk/radio/bf981/981.html
OZ2OE Technical Home Page - 144 MHz low noise amplifier.
Uploaded April 19'th 2000
Design of a preamplifier for 144 MHz with 1 dB NF and 23 dB gain using BF981.
The finished amplifier.
Background
This amplifier is using a low cost silicon MOSFET (BF981 from Philips) to give more than 20 dB gain with around 1
dB noise figure on 2 meter.
You can build more advanced low noise amplifiers using GaAs FET's, but the dual gate MOSFET is no bad
performer - the difference in noise figure may not even show up on terrestrial path. Besides it's a lot more forgiving interms of accidental destruction - and it's cheaper!
Mechanical design
The amplifier is build on a double sided board measuring 25x50 mm. The upper side functions as a ground plane and
the traces are on the lower side.
The board can either be build into the radio or mounted in a small metal box with BNC connectors.
The amplifier is build on a small PCB.
Electrical design
The diagram is shown below. The input circuit is a series transformation, the output circuit a "normal" parallel circuit
with a capacitive output tap. As this amplifer has high gain, there is provision for a pi-attenuator on the output.Biasing of the transistor is done with a source resistor/voltage divider and with voltage dividers on both gate1 and
gate2. This circuit tightly controls the DC parameters of the transistor.
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gate2. This circuit tightly controls the DC parameters of the transistor.
L2, L3: 5.25 turn ø0.25 mm cu-wire on Neosid form K3312, F100 slug.
Circuit details
The input circuit is a bit unusual as it uses a series inductor L2 to step the 50 ohm generator impedance up to 1600
ohm, which is the optimum gate impedance for noise matching. L2 resonates with C4 in parallel with the gate1 input
capacitance.This minimises the loaded Q of the input circuit - important to keep insertion loss down - and the correct impedance
(noise) match is fixed by design.
The output circuit is a parallel circuit with with L3, C6 and C8. The capacitive tap C6/C8 sets the drain impedance
to 2 kohm. This also determines the gain.
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Component layout.
Tune up
When powering up for the first time, start by checking the DC current consumption of abt. 20 mA. Then adjust L2
and L3 for maximum signal.
For best noise match use a weak unmodulated signal and a receiver in FM mode. Then tune only L2 for best noise
suppression.By using a signal at the noise threshold, the FM (or PM) detector in the receiver will "amplify" any improvement of
the C/N, so you can tune for optimum noisefigure by ear! Any resulting variation of the preamplifier gain will be
nullified by the limiter in the FM receiver.
Attenuator
The can be taylored to your specific needs - which should be "not more than enough" - by having an attenuator at the
output of the amplifier. The following table shows resistor values for different attenuation.
PI - attenuator values.
Performance
I have build and measured many of these amplifiers, and they have almost identical performance.
The typical result with a 3-dB output attenuator is shown here:
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Noise figure and gain from 130 to 160 MHz. NF is 1.3 dB and gain 20 dB on 145 MHz.
PCB's and components
When I have the PCB layout converted to a proper file format, I'm going to post it here - until then the best I canoffer, is a scan of the original artwork. Ask me.
At the moment I'm unable to help with etched PCB's, however I still have the special Neosid coil forms and cans.
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