trouble getting class c amplifier circuit working - Page 1
Forum section: electronics / rf-microwave

author: drspastic
author_rank: Contributor
timestamp: October 27, 2024, 10:25:38 pm
content: so im building a low powered feldhell beacon using digispark, active crystal oscillator, and small solar panel. it needs a power amp stage to give a peak of maybe 1 or 2 watts at very low duty cycle.on the bench i tried the standard textbook class c but was unable to get any gain despite the tuned circuit being spot on. i could get a little gain with a positive bias but that just makes it a heater. i tried again with a double tuned 4:1 transformer wound binocular style on a couple of tiny ferrite beads and still only managed gain in class a mode. class c just either became unstable or flipped mode to a doubler, still with no gain. at this point i should mention i am working with a maximum of 4v supply. at best settings i could get 8v pp out with a bd135 and the 4:1 transformer but that dropped to 5vpp into a 50 ohm load.im not sure where to go from here as i think its the supply voltage being a constraint. should i use a mosfet? i dont have many rf parts on the shelf but i have some smd mosfets for fixing drones and they have a very low gate voltage: 9926a but im not familiar with design constraints using mosfets for rf. they seem to have a large capacitance on the gate which may be hard to drive.im just fishing for thoughts here, i dont want to raise supply voltage even by dc/dc converters. the unit must be very light and small to be lifted by a small party balloon etc on its dipole wire. frequency will be between 10-29mhz in the allowed places. it may have a lipo cell or tiny ultracapacitor to give oomph on the duty cycle but mainly very thin solar.what do lads?

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: October 28, 2024, 07:03:20 am
content: DL2AVH designed several 1.5V powered transceivers - maybe helpful for your project:https://dl2avh.eu/trx1v/trx1v.htmlGreetings,Rainer DG1SMD

author: ftg
author_rank: Frequent Contributor
timestamp: October 28, 2024, 02:20:46 pm
content: Class-C amplifiers have minimum drive power requirements. So you might need a small class-a driver stage in between the class-c pa and crystal oscillator.If you don't want to add some transistor stage, a modern single gate inverter from TI/Nexperia/onsemi could potentially also be usable.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 28, 2024, 02:41:11 pm
content: You might also want to consider a Class-E amplifier. A good place to start would be searching for Class-E WSPR transmitters.

author: stretchyman
author_rank: Regular Contributor
timestamp: November 05, 2024, 06:55:51 pm
content: I 2nd the class E design, so simple, why use anything else?Do a search for 'LULU'.J.

author: RFDx
author_rank: Regular Contributor
timestamp: November 07, 2024, 08:37:27 am
content: Class-E is fine but needs an amplifier for every band. For broadband 10...29MHz operation something like a push-pull current or voltage mode class-D amp would be more appropriate. Considering the 4V supply, small super/ultra logic level MOSFETS driven by digital LVCMOS gates could do the job and put out 1...2W RF while keeping efficiency high.

author: CaptDon
author_rank: Super Contributor
timestamp: November 07, 2024, 01:37:28 pm
content: Class C relies on a resonant tank circuit so that would be a big factor if you operate on multiple frequencies with large spacing between them. Class C relies on a heavy drive voltage / current but for a short duration (perhaps 100 degrees) of the operating frequency and the tank circuit forms and cleans up what would otherwise be a horrible looking carrier. In the class C vacuum tube amplifiers the control grid had to be designed to dissipate power since it was driven heavily positive with respect to the cathode and drew 'grid current'. In most cases the grid drive power also made up about 3 to 5% of the amplifier's total output power. As others have mentioned, class C may be a bad choice at low voltages. Class C tank circuits are also designed with a high Q to help reduce harmonic energy.

author: mtwieg
author_rank: Frequent Contributor
timestamp: November 07, 2024, 02:20:26 pm
content: Classes C, D, and E are all going to be fairly narrowband. A 3:1 frequency range is not practical without some sort of built-in tuning feature. It's not clear if the OP actually wants to operate across such a large range, or just a portion of that range.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 07, 2024, 02:27:56 pm
content: Indeed.Given this is supposed to be a light-weight balloon lift Hellschreiber beacon, I assumed that the transmitter would be operating on a single fixed frequency within the range 10-29MHz.I never considered it would need to have adjustable tuning for continuously variable frequency operation.

author: drspastic
author_rank: Contributor
timestamp: February 24, 2025, 11:55:56 am
content: quite correct, its only going to be fixed frequency, and the range i mentioned in op just indicates the area it might operate in depending on how a final circuit performs. antenna wire (even thin) is heavy so higher frequency has advantages there. lower frequency at the amp would be more efficient but require more copper at the antenna