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F6ITV - PA 144 MHz  3W-100W  14.5V
Creation: 01/10/2002 by   F6ITV 
Subject:
2m Linear Amplifier - SSB/FM-100W, issue 5



The amplifier I proposed is not of the latest generation simply because I built it in 1985, at this time the objective were:

q Non-intensive SSB operation and eventually FM operation at reduced power.
q Output power: >= 80W with a MRF245 Motorola transistor.
q Input power: 2.5W  with a FT290R YAESU transceiver.

DESIGN

q According to Motorola's data sheets a MRF245 is 80W capable at 145 MHz with 10W drive at 12.5VDC and in my opinion 12W drive is a maximum regarding linearity. So, an intermediate stage with 6dB power gain is required.
q As a first approximation, in this kind of application, the collector current of a BJT almost follows the power supply voltage. I mean that if the voltage changes from 12.5 to 13.8VDC, that is to say a 1.1 ratio, when also applied to the current it will give a power ratio of about 1.1^2 = 1.22 and therefore 80*1.22 = ~100W output power can be expected at 145MHz.
q Data sheets also show that when the power supply is increased the linearity is a slightly improved.
q Transistors are class AB biased by means of two resistors and a diode in charge of the temperature compensation.
q T/R switching circuit will use the DC voltage superimposed to the VHF signal when the FT290R is in TX mode, so a VOX unit is not necessary.
q Test of printed air striplines and make them operating in narrow band. They are greatly inspired from a Motorola Engineering Bulletin.
q Manual switching to ground of the  transistor bases for FM operation.



                      
Text (pdf)                 Schematic                 Printed Circuit Board                   Photos
TESTING

Unfortunately, the notes I kept on this job are briefs.

Implementation

Process

q Heat scattering is an essential problem to solve. As a first approximation, in this kind of circuit, BJT collector efficiency is about 60%. As the wished output power is 100W with 10W drive then the heat peak power to scatter is ((100+10)/0.6)*0.4= ~75W and let say that the  average power is about 50W in SSB. I didn't perform any calculation regarding heat scattering and I let me guided by experience and intuition ! My investigation leads me to buy a 190x120x40mm heat sink that seemed suitable to me.
q Test of a  MRF5590 or equivalent as first stage, it provides 6dB gain at 145MHz
q The first tests were carried out with the transistor bases grounded through a VK200, at 12VDC and with drive reduced to 300mW.
q Measuring : SWR between TRX and PA and output power on a suitable 50R dummy load.
q Determination of the adjustable capacitors values and also the values of the essential UNELCO or SEMCO chip capacitors
q Implementation of two biasing circuits from a regulated +5VDC.

Biasing

q For SSB linearity purpose at weak signals transistors have to be biased in AB class.
q Each biasing circuit is a classic bridge of two resistors supplied by a +5VDC regulated voltage provided by VR1.
q A negative temperature compensation of the transistor base current is obtained from a 1N4007 diode directly mounted on the transistor top package (photo). The reason of this device is that the transistor base current naturally increases according to the temperature, so it would induce a collector current increase that in turn would produce a temperature increasing from which there is a potential risk of thermal run away as well as a linearity degradation. The 1N4007 diode sees its current increasing according to the temperature and consequently it's going to shunt a part of the base current. A perfect temperature compensation is not easy to get, it would require several trials because it involves the bridge current, the base current and the diode current. Other methods could be used as well.
q Each bridge resistors are calculated to have a going through current much higher than the related transistor base and diode ones and to obtain a collector quiescent current of 12 to 15mA max for T2 and 120 to 150mA max for T3. There is every chance to have to adjust the resistance connected to +5VDC, this is due to resistor tolerance and transistor characteristic dispersion.
q An accurate biasing adjustment requires.

Linearity

As said previously linearity requires transistor biasing but also to control input power. Here also a two-tone test generator and an oscilloscope can show that input power has to be limited to avoid transistor saturation and therefore splatters production. To me, according to data sheets, 3W input is a recommended limit

Results & Comments

q Following some instabilities, FT290R PA (2SC1947) was destroy and replaced by a MRF237. A 220R resistor placed in parallel with the input has been providing the PA with a reliable stability in the duration.
q Printed air striplines proved to be operational because the adjustable capacitors I have used gave distinct tuning regarding minimum input SWR and maximum output power.
q Adjustable capacitors and chip capacitor value optimisation really lead to get the expected figures. Actually, when PA and TX are 14.5VDC supplied, then drive being 3W, the amplifier deliver 115W (CN630 Dawa) and overall consumption is about 15A. In  this configuration, as the driver stage current is about 1.4A and other circuits one about 0.4A, therefore the MRF245 current is about 13.2A this give about 60%  for collector efficiency.
q Caution, Motorola informs that the MRF245 capability to withstand a 20:1 SWR is decreased when voltage supply is increase above 12.5VDC and when output power is more than 100W. Consequently make sure that the PA is correctly loaded before transmitting. The antenna must be connected !!!
q I haven't tested an intensive SSB operation but at 14.5V the peak power to be scattered is about P=U*I*40%=14.5*14.6*0.4=~85W and therefore an average of 50 to 60W, so I think the heat sink I used should be suitable.
q In FM operation, continuous 85W have to be scattered, and actually after a few minutes, a heat sink cooling seems advised. A 12VDC fan controlled by a NO thermal probe (about 60) both mounted on the heat sink top and the whole enclosed in metal sheets acting as an air sheath should do the job.

CONSTRUCTION

Schematic / Components

q T1 and T2 transistors together with RLY1 form the T/R switching device.
q RLY1 is not particularly dedicated to VHF and for sure there is better
q Two internal RF links are made by means of 3mm - 50R coaxial.
q Suitable adjustable capacitors are Philips Teflon and air TRONSER at output.
q Others capacitors are ceramic 63V, chemical or Tantalum 25V or 35V.
q Switches are miniature type, the On/Off one is a triple model because of the 15A constraint.
q Monitoring lights are 12V model but as they are 14.5V supplied a serial resistor is used to protect their life duration.

Case, PCB and Transistor

q As I kept in stock a heat sink bar, 100x10mm, I used two pieces for front and rear faces, they take a small part in heat scattering but they do give a sympathetic look to the amplifier case.
q PCB is 1.5mm epoxy, double-sided, 35m copper coated and its size is 150x100mm. As recommended in the rules of thumb concerning this kind of unit both board sides are connected together by means of several rivets and there is no copper beneath printed striplines. The PCB is connected to the case at four points (photo).
q Motorola's transistor SOE (Stripline Opposed Emitter) packages like the MRF5590 stud type and the MRF245 flange type are straight in contact with the heat sink and tightened in order to ensure a good heat scattering. Motorola's AN555 note give advises concerning SOE transistor mount, in short:

- heat sink surface has to be completely clean and flat
- tightening hole edges have to be properly deburred
- a thin coat (0.01in) of thermal compound has to be applied
- recommended tightening torque is 5 to 6 lb-in max
- It must not exist any mechanical stress on transistor leads.

q To avoid any mechanical stress on transistor leads, the best way for positioning them as regard PCB is firstly to tighten them moderately on the heat sink before soldering them and with the same objective jump at the opportunity to determine the size and the type of metallic spacers that have to be used between the PCB and the heat sink.
q Finally, two soldering lugs (photo) are grounded at MRF245 package tightening level.

NOTICES

q It goes without saying that this PA construction applies to experienced  and prudent OM builders.
q I wouldn't certify that this PA is perfectly reproducible as today it's a unique example, however another one is under construction this would allow to better circle the subject.
q In my opinion, the output power optimisation is thoroughly linked to the values of UNELCO chip capacitors, which tolerance could be 20%, so I would recommend to purchase several additional judicious values.
q Finally, during adjustment, the chip capacitors will not be definitely soldered otherwise unsoldering them will be a hard job that will require a 100W iron with the risk of local overheating causing damages.
q Operation beyond 100W implies a thoroughly respect of all the above recommendations.