Note: The low-pass filter values for 21-28 Mhz are incorrected. They will be uploaded after testing the filter
RF386 is the Minima's companion RF power amplifier for 5 watts with just one coil! It keeps the complexity down for the builders while providing a very clean signal.
- The RF386 is named after the venerable LM386
- Just a single coil on a cheap FT37-43
- It works with almost any power FET. From IRF510 to RD16HHF10.
- The bandswitching uses just a single digital line
The Minima has an output of 50 mV peak into a 50 ohm load (approximately -10dbm). The first three transistors Q1-Q3 boosts this power leve to 3 volts peak. As all the loads are resistive, the amplifier chain provides 3V peak (6v peak to peak) at the collector of Q3.
Driving MOSFET power devices has traditionally been a challenge for amateurs. The MOSFETS like the IRF510 exhibit a very high gate capacitance that needs a low impedance drive that can handle a reactive load. This is usually accomplished by loading input with a low value parallel resistor (either in parallel or series), thereby dissipating most of the driving power into the dummy resistor.
A more efficient approach is to provide a low impedance drive through emitter followers. We borrow a configuration called the 'Complementary Emitter Follower Pair' (quite popular among the audio amplifier designers). The Q4 is an emitter follower that provides a current gain into the 100 ohms emitter load. The two diodes in the emitter of the Q4 provide a constant difference of 1.2V between the bases of Q5 and Q6. The Q5 and Q6 provide a further current gain to the Q4, thereby lowering the output impedance even further. W7EL described a similar circuit in Experimental Methods in RF Design (ARRL pub.)
The 10 ohms resistor between the emitters of the Q5 and Q6 limits the standing current. The capacitor across the resistor allows the RF current to rise.
The gate of the power MOSFET is driven without any loading resistor. 3 volts peak RF at the gate provides 5 watts of power on 14 MHz. Slightly higher power is available on lower bands and lower power is available on the higher bands.
There are some small but significant circuit details that entirely eliminate any instability :
- The drain of the power MOSFET is bypassed at RF as well as audio frequencies with a large 500 uF cap. This capacitor is significant in both the builds of the amplifier.
- The Q5 and Q6, though directly connected to the +T line had individual bypass close to their leads. These are needed to provide low impedance returns to ground to prevent feedback.
- Use a coax between RL-IN and RL-OUT. Note that the RL-IN shorts the coaxial cable to ground on transmit to prevent any RF output from leaking back into the amp's input.
- Construct over a ground plane and follow a 'straight line' layout for the amp stages.
The low-pass filters
Four of 3-section low-pass filters with sharp cut-off are used. This helps in using just four filters to cover from 3 to 30 MHz. The bandswitching arrangement is capable of adding upto seven more filters if the need be.
Bandswitching the low-pass filters
The band switching is accomplished by a commonly available CMOS IC - the CD4017. The way this IC works is like this :
- Only one of the ten outputs is high at any time
- Each time the clock output goes high, the current output goes low and the next output goes high.
- A high on the reset pin sets back the first output to high
The rest is coupled through a resistor and capacitor. Short pulses of less than 0.5 msec don't provide enough charge for the capacitor to reach the high voltage required to reset the IC. Thus, a long pulse resets the band switch and short pulses select the correct low-pass filter. A single digital line is sufficient to switch to the correct LPF. These pulses are generated by the Arduino of the Minima.
Two versions of this amplifier have been built. Both followed the same circuit. A few bypass caps on the power supply line and the 500uF was left out from the second build. This resulted in low frequency oscillations. The 500 uF cured it.
- Pay attention : the CD4017 and the MOSFET are permanently powered-up by 12Vs.
- The T line goes to +12v on transmit.
- Capacitors on the relays are needed to prevent stray RF pick-up by the relay coils.
Setting up the RM386
Only the bias needs adjustment.
- Before powering-up, keep the 10K bias pot completely at the ground end.
- Disconnect the input from any signal.
- Monitor the power supply with an amp meter. Increase the bias until the current increases by 150mA.
- Now, inject a clean 50mV signal (key-down on Minima) and the MOSFET current should climb to about 1A(for an IRF510, your milage with others will vary).
- You're done!
Laziness, Hubris and Impatience
Larry Wall, the famous programmer said that Laziness, Hubris and Impatience are three virtues of a programmer. I think that the same applies to Radio Amateurs as well.
This project started, ironically, with a need to replace the LM386 with something better in the audio stages. Articles by Rick Campbell and the amazing treasure-house at www.qrp.pops.net provided enough material to develop a very compact and sweet audio amplifier for the Minima.
In the meantime, the search continued for a suitable linear amplifier as the companion for the Minima.
Impatience prompted reuse of existing circuits for the purpose rather than experimenting with new ones. The three stages of drivers are directly lifted from the BITX. The Q4, Q5 and Q6 circuit is not very different from the audio amplifier of the Minima.
Laziness prompted looking for design that saved on winding more transformers and coils.
Hubris is what drives us to produce circuits that are easy to replicate and work well.