Biasing an IRFP460 Mosfet - Page 1
Forum section: electronics / projects

author: Tracotron
author_rank: Newbie
timestamp: September 13, 2025, 07:02:58 am
content: Hello Everyone,I'm new here, and I'm trying to build this circuit. But I don't know how to determine the values of each of these components (where the ? mark is).Can anyone help?Thanks in advanced.

author: ledtester
author_rank: Super Contributor
timestamp: September 13, 2025, 02:45:17 pm
content: This looks like it was taken from this video:Hartley Oscillator using enhancement MOSFET Animation -- ee diaryhttps://youtu.be/yJI-zSlqmTgThe ee-diary blogspot content isn't available anymore, but you can find it on the Internet Archive:https://web.archive.org/web/20231211094420/https://www.ee-diary.com/2022/01/Hartley-Oscillator-Design-with-Enhancement-MOSFET.htmlThe circuitry around the MOSFET is a class A amplifier, so search for "MOSFET class A amplifier" (or ask AI) for help with the the resistor values, e.g. like this video:Class A MOSFET amplifier using one transistor - with schematic -- JohnAudioTechhttps://youtu.be/D4tHgHfljyYFor L1, L2 and C, read up on the Hartley oscillator.

author: ledtester
author_rank: Super Contributor
timestamp: September 13, 2025, 08:11:33 pm
content: FWIW, here is an LTSpice simulation of the circuit with a 2n7000:I've also attached the asc file and model definitions for the 2n7000 and IRFP460.-https://github.com/cellularmitosis/LTSpice-parts-https://groups.io/g/LTspice/topic/mosfet_irfp460/50197307Note that the IRFP460 model required reformatting.

author: Tracotron
author_rank: Newbie
timestamp: September 14, 2025, 01:24:25 am
content: Thank you for your reply.I'll give it a shot.Thanks again.

author: ledtester
author_rank: Super Contributor
timestamp: September 14, 2025, 02:55:30 am
content: Trying the irfp460a model in Windows LTspice gave me a lot of errors. Here is an updated version which loads error-free (and hopefully is correct).

author: Alaezae
author_rank: Contributor
timestamp: September 14, 2025, 03:05:14 am
content: Sometimes, simulations can be deceiving with LC oscillators like this. The simulation model in something like LTSpice will be very poor at actually deriving those values when you go to build the circuit. Although LTSpice will take into account parasitic capacitances for a given transistor as well as beta/transconductance, that doesn’t necessarily mean that your specific transistor will fit those quantities exactly. When working with RF, differences of pF and nF will matter a whole lot!In addition to that, you must take into account trace inductances between the loop elements. This will throw off the Q of the network, which should ideally be high to get it to oscillate. Even a few cm of wire spac/trace length between the output of the transistor and the resonant network can throw off the phase of the feedback network. That’s really important if you’re trying to meet the Barkhausen criterion for oscillation.As for picking resistor values, recall that an oscillator must have a loop gain greater than one and a loop phase shift of 180 degrees. Since the resonant network is purely passive, it provides no gain and you must compensate for losses in it with the transistor amplifier. Therefore, the drain and source resistor ratio which determines the gain of the amplifier stage must be high enough to compensate for that loss. In addition, the input resistors should be chosen to not load the output of the feedback network, which is determined by the parallel combination of the gate bias resistors. You will probably also need to accommodate the internal gate capacitance and miller capacitance (related to the Cgd of the transistor) to properly determine what the feedback network and loop should be.While all that is great, it will probably still take some experimentation on the bench. Nothing in a transistor’s datasheet is all that consistent between actual devices, so you may find that even if you account for every minute detail, you may just have to fiddle with it on the bench to get it to work. One thing you might try is characterizing both the stages first by building them separately just to get an understanding of the time-domain/frequency response. When you put them together, you’ll end up with a totally different response as their interaction. At least you’ll have an understanding of the circuit and the knobs you have to play with.I’ve recently found a set of videos from the famed R. D. Middlebrook discussing methods of analytically finding these parameters. You can look him up on YouTube, they’re pretty much the only thing by him. He speaks on the General Feedback Theorem, which goes over this exact scrnario(lecture 10?. I think it’s a worthwhile effort (only about an hour and a half of your time) and will give you a lot more insight as opposed to trial and error.

author: Tracotron
author_rank: Newbie
timestamp: September 14, 2025, 03:17:56 am
content: Thank you for your advice and guidance.I'll check out R. D. Middlebrook discussion.