PUSH-PULL AMPLIFIER FOR 14 MHz
By: S. Rama Mohan Rao, VU2RM
After working many years on my RM-96 with IRF830...etc. in the final PA, I have tried the same tried the same circuit for 14 MHz transceiver. The same broad band is used with IRF830. It has given an output of 4watts or so. Then, I have gone deep into the subject of VMOS & TMOS power FET's. I have decided to use these devices with Low CISS & COSS (input & output capacitance) values. I have finally chosen to use IRF510 devices. This has a maximum Voltage (VDS) of 110 V.
I have built a push-pull stage because of the following reasons:
When a tuned circuit is used in the final tank circuit with a single stage amplifier, the quah/ of the signal sounds good.
Nevertheless, with a single stage broad band/t is not the same quality.
I felt that this difference is due to the flywheel effect of the tuned circuit.
During the technical discussions with some of the home brewers, it is learnt that the)/also found the same when the wave shape was seen on the oscilloscope.
The pos/five and the negative half cycles are not the same
Therefore, I have decided to make a push-pull amplifier.
Also for the reason that as we are using broad band amplifier, there are bound to be some harmonic generations, due core saturation of the final toroid and various reasons.
In push-pull these can be avoided largely and therefore less spurious outputs and better quality.
Keeping in view al/these points the following circuit is evolved and it is working satisfactorily.
Experimentation made on this circuit:
After building this circuit, I found, one IRF is getting hotter than the other. I made all checks I know, but it is observed that one gate is getting more voltage than the other. Then I reversed the secondary windings. Now, the other gets more heated. So evidently, IRF has nothing to do with this unequal drive voltages to the gate. I tried T with
an imported toroid. No luck got fed up and thought of introducing negative feedback to equalizes the gain of each FET. So, tried combination of RC network, which is a general practice. About 100 Kilo ohms in series with
150pf capacitance from drain to gate. Now both the positive hail cycles and negative half cycles are equaled and the defect of one FET getting hotter than the other vanished. The quality also improved. Even I operated for one or two hours the heat sink is not getting heated up. i am using 22 Volts supply on the drain and it draws about 2 Amps.
(i.e. 44 watt DC input). It is giving a power output of 25 watts into a 50 ohms antenna as measured on a Daiwa power/SWR meter. Further, there is no reflected power from the PA stage. So, RF filters are not used.
I have not built a 21 MHz RM-96, but looking at CISS value of IRF510, I have a strong belief that it can work on 21 MHz also. So this will be a good choice for 7/14/21 MHz power amplifier. When one Ham from North India told me that he is using IRF840 in 14 MHz and is getting about 20 watts output, I immediately got hold of IRF840 and replaced the IRF510 with IRF840. The power output is just 4 watts. I have increased the voltage from 22 V to 40 V, the output power went upto 10 watts. However, the heat sink is getting hot indicating that the efficiency is Low. Therefore, I left the PA like that, settling with IRF510 only. Happy, go lucky, now that is all my experimentation on the push-pull power amplifier.