1.2 Watt Bullet Proof Amp For The FM-10 or FM-25

By Mycal

The follwing is a very easy to build amplifier that was designed to follow a Ramsey FM-10 and FM-25 transmitter. It is built on top of a simple PCB board surface style (all parts tacked on top, no holes in PCB.) The performance is excellent with power levels of up to 1.5 watts achevable and harmonic suppression greater than 50db. Using this amp in conjunction with an Ramsey FM-10/25 can provide you with the ideal micro power radio station with usable range of up to 2 miles or more. If you use this unit to amplify a Ramsey FM-25, build the FM-25 in the low power output configuration.

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.. < 50mw.) R2 (20K) provides the bias voltage making the amp class-A. Current is fed to the stage through R3 (300ohms) with a ferrite bead on the power side to help keep RF out of the power grid. An inductor of 1uh or better can be substituted if a ferrite bead cannot be found (use only small resistor type inductors, not giant monsters from Radio Shack). The first stage is isolated from the interstage maching circuit by C12 (.001).

Intra-stage matching is done by C1, C2, and L1. This network trys to match the output impeadance of the first stage with the input impeadance of the second stage. It also provides a little bit of filtering. It should be noted here that the power output of the first stage affects it's output impeadance which is effected by the input power level. Any change in power outputs of the transmitter (most likly from frequency or power supply change) will require re-tuning of this amplifier.

The second amplifier is running Class-C pushing it's output into the output matching circuit made up of C3,C4 and L2. The output is fairly clean and can drive an antenna or another stage.

Original Schematic

PCB board:

3 3/8" x 1 3/8"

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.

Updated Schematic

New Updated Schematic

Parts List


R1                              1.5K ohms
R2                              20K ohms
R3                              300 ohms


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


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


Q1                              mpf901 -or- mrf901
Q2                              2sc1970

Last Updated Febuary 25, 1996.
©1996 Mycal Labs, All Rights Reserved