RF Circuits

I feel this is a much better alternative than the Ramsey LPA-1 because it provides much lower harmonic output and it is relitivly bullet proof to antenna mismatches which has been known to destroy LPA-1s without the slightest warning.

The PCB board is a single sided copper board etched or grinded out to the shown layout. The board size is 3 3/8" x 1 3/8" but anything close that can accomidate the parts without any lengthing of the lead lengths is fine.

I recommend that if you don't have a good way to make the PCB that you buy the Radio Shack PCB Etching Kit, this kit works very well for this type of application. We've used laser printer iron ons for this board, but we've found that electrical tape or the resist pens work fine.

Design and Schematic:


The amplifier is a 2 stage design. The first stage uses a high gain microwave transistor amplifier running class-A to boost a 10mw signal to about 150mw.

In the first stage the resistor R1 (1.5K) gives Q1 (mpf-901 or mrf-901) and the Ramsey transmitter a nice stable input/output load to look at that should smooth out missmatches between the transmitter and the amp (note that this type of matching is only workable at flea power levels.

Printed Circuit Board (PCB):


Construction Tips:
Solder all the small low lying parts first; resistors, L3, L4, L5, L6. Then mount all the small capacitors; C5-C10, and C12. Next Q1, C1,C2,C3,C4, followed by L1, L2 and Q2. Finally add C11 and attache the input and output with coax to the connectors and/or transmitter.

Part's Layout:


For tune up you should simply tune C1, C2, C3, C4 and L1 for maximum output. This amplifier doesn't like to ocsillate, but this is always a possiblility. You can check for oscillation by tuning a FM radio up and down the FM radio band, if you hear multiple images of your broadcast your amplifier is in oscillation (not good).

Updates and Modifications
I would change the design slightly if I were to build more of these amplifiers. The input is not DC isolated. I would add a .001uh cap between the input and Q1. This is a must do mod if the amp is to be used as a stand alone device (ie not hardwared ont a FM-10).

I would get rid of C12, it is not necessary since C2 blocks the DC between stages. I would move C1 to the other side of C3, this allows C2 to be adjusted without effecting the C1/L1 low pass filter. I would add another 5-50pf cap from the input side of L2 to ground, thus adding an extra element and more flexibility to the output/matching filter (I would and have done this addition on every amp that I have built with an output power of under ~3 watts, cannot get -50db down on harmonics without it.)

The modifications listed above can easly be made to the existing circuit board if done during the assembly stage. Basically you would shift Q1 and its associated parts one pad to the right on the circuit board (since C12 is no longer necessary), and add a .001uf cap between the first pad and the base of Q1 pad. C1 can easly be moved to the pad on the output side of C2. And another 5-50pf variable cap fits nicely on the input side of L2.

L6 mostlikly can be optimized, but is most likely not that critical to the overall performace of the amp.

Parts List

Resistors:
R1 1.5K ohms
R2 20K ohms
R3 300 ohms


Capacitors:
C1, C2, C3, C4, (C14) - ~5-50pf
C12,(C13) - .001uh
C5, C7, C9 - .01uf
C6, C8, C10 - .1uf
C11 - 10uf


Inductors:
L1- .2uh adjustable digikey...
L2 - .2uh fixed coil...
L3,L4,L6 - 10uh Moulded Inductor
L5 - 3 turns of #22 enamel wire through Ferrite Bead.

Transistors:
Q1 - mpf901 -or- mrf901
Q2 - 2sc1970