A simple generic Simsmith model of a linear push-pull Class B broadband HF power amplifier

This article outlines a Simsmith model developed to explore / confirm behavior of some linear Class B push-pull HF broadband power amplifiers.

The design is for a system power output of about 80W on a 24V supply, it is a combination that should work with practical system components with good efficiency.

Above is a first step, an estimate of an initial load line for the PA. The calculator is written in valve terms, but is quite applicable to this scenario. Continue reading A simple generic Simsmith model of a linear push-pull Class B broadband HF power amplifier

Last update: 11th June, 2021, 7:42 AM

Ferrite cored inductors at HF – flux, loss and saturation

I see online experts opine that small signal characteristics (eg complex permeability curves) of ferrite toroids are not valid for applications such as RF common mode choke in transmitting antennas.

Others opine that saturation is a practical design limit, and for example that Bs/2 is a safe / appropriate design target.

Let us consider a ferrite cored inductor at 7MHz. The inductor comprises 11t on a 11t on Fair-rite 5943003801 (FT240-43) toroid. This is a medium to high permeability ferrite material, and for that reason, has significant loss at HF. Higher and lower permeability materials are fashionable at different times, the higher permeability #31 mix is fashionable at this time.

I will work in MKS units.

Above is Fair-rite’s B-H curves for #43 material. Let’s take saturation flux density Bs to be 1500gauss or 0.15T. Continue reading Ferrite cored inductors at HF – flux, loss and saturation

Last update: 2nd June, 2021, 8:57 AM

SNTP synchronised clock v1 – boxing it up

The SNTP synchronised clock (ssc) is an ESP8266 based time of day clock with an LED display.

The code is fairly mature, and the boxed prototype will be build with a large 4 digit 7 segment LED display (1.2″ or 30mm) using the HT16K33 driver chip.

The prototype will be housed in an ABS Jiffy box, and a new lid cut from dark red transparent acrylic on the CNC router.

Above is the sketch of the layout. The screw holes in the display are M1.6, and there is no clearance for larger screws. Continue reading SNTP synchronised clock v1 – boxing it up

Last update: 5th June, 2021, 4:52 PM

EB104 alternative output circuit

One of the very important designs of HF broadband MOSFET power amplifiers was that of Helge Granberg in Motorola application note EB104.

This article offers an explanation of how the the alternative output circuit at Fig 5 of EB104 works.

Let’s look at the schematic diagram of the PA.

Above is the schematic from EB104, of interest for this article is the output circuit comprising T2 and T3 which are intended ideally to provide a drain to drain load of 50/9=5.55Ω. Continue reading EB104 alternative output circuit

Last update: 25th May, 2021, 10:05 AM

A desk review of the MiniPa100 kit – #2: transformer T2 turns ratio

This article is on in a series of a desk review, a pre-purchase study if you like, of the MiniPa100 kit widely sold on eBay and elsewhere online.

Previously…

The above pic from an eBay advertisement of the 2020 version of the PA would suggest very strongly that there are three turns on the secondary of the output transformer, and a half turn on each drain. Interestingly the 70W versions also appear to use three turns, alarm bells ring!

Here is a little table that shows the maximum power obtainable with a transformer of this type for various supply voltages and secondary turns. Continue reading A desk review of the MiniPa100 kit – #2: transformer T2 turns ratio

Last update: 11th June, 2021, 7:42 AM

Desk study of M0DGQ’s 150W HF PA

M0DGQ described a broadband HF PA in the Wythal Radio Club’s newsletter 2017-01, and rated it at 150W output. Note that this module does not include the necessary output filter which will probably lose 5-10% of the power from this module.

The PA uses a MRF9180 dual MOSFET operating on 26V supply.

Above is the prototype PA. The text states very clearly that the output transformer uses a secondary of two turns of PTFE insulated wire, and the pic above does not provide evidence to the contrary.

Hmmm, experience suggests that may be too few turns. Continue reading Desk study of M0DGQ’s 150W HF PA

Last update: 11th June, 2021, 7:41 AM

A desk review of the MiniPa100 kit – #1: characterise the output transformer

This article is one in a series of a desk review, a pre-purchase study if you like, of the MiniPa100 kit widely sold on eBay and elsewhere online.

One of the first questions to mind is whether it is likely to deliver the rated power, so let’s review the MOSFET output circuit design from that perspective.

Sellers mostly seem to need to obscure the MOSFET type in their pics, so essentially you buy this with no assurance as to what is supplied, no comeback if the supplied MOSFET is not up to the task. Online experts suggest the MOSFET is probably a MRF9120 (or 2x IRF640 in a 70W build). The amplifier claims 100W from 12-16V DC supply.

Note that this module does not include the necessary output filter which will lose 5-10% of the power from this module.

In this case Carlos, VK1EA, connected a sample output transformer (T2) core from a recently purchased MiniPa100 kit to a EU1KY antenna analyser. The fixture is critically important, it is at my specification. Continue reading A desk review of the MiniPa100 kit – #1: characterise the output transformer

Last update: 11th June, 2021, 7:41 AM

SNTP synchronised clock v1 – display options

The SNTP synchronised clock (ssc) is an ESP8266 based time of day clock with an LED display.

Code development progresses and a working prototype exists with three display options.

Directly supported 4 digit 7 segment LED displays

Several common driver boards are supported directly by the source code, most of these and variants can be purchased online for small money.

HT16K33

Above is a large 4 digit 7 segment LED display (1.2″ or 30mm) using the HT16K33 driver chip. Continue reading SNTP synchronised clock v1 – display options

Last update: 21st May, 2021, 7:02 PM

SNTP synchronised clock v1 – a preview

The SNTP synchronised clock (ssc) is an ESP8266 based time of day clock with an LED display.

Design criteria

The design criteria are:

  • small, portable, powered from a 5V USB power supply;
  • synchronised to a SNPT (simple network time protocol) server;
  • flexibility for a range of displays (74HC595 static, TM1637, HT16K33);
  • configurable time zone offset and daylight saving offset;
  • configurable from a web page;
  • switchable HH:MM and MM:SS display formats; and
  • switchable daylight saving mode.

The concept is that it is synchronised to net time, and the only adjustment needed through the year is to flick the daylight saving mode.

The hardware comprises:

  • ESP8266 dev board of some kind;
  • 5V to 3.3V power supply; and
  • 4 digit 7 segment LED display, preferably with a colon in the middle.

Above is a development prototype using a Wemos D1 mini ESP8266 board and 4 digit display using 74HC595 shift registers. Continue reading SNTP synchronised clock v1 – a preview

Last update: 20th May, 2021, 8:39 PM

Fix for a certain TM1637 LED display

There are a miriad of low cost displays for hobbyists in online shops, particularly targeting Arduinos where libraries exist to drive the most common chips.

This article looks at a 4 digit LED module particularly suited to a digital clock display. The driver chip is a TM1637, and it requires Vcc and Gnd, a data IO wire and data clock wire.

Above is an example that just didn’t work. Continue reading Fix for a certain TM1637 LED display

Last update: 14th May, 2021, 6:37 PM