Saturday, February 27, 2010

a low-cost MOSTFET CW RF power amplifier, using an IRF710 power mosfet

Someone gave me a sleeve of IRF710's, so, this week i decided to try
to build an RF power amplifier out of one of them. These transistors
were never intended for RF applications, they were built for motor
control, solenoid control, and other high power switching applications.

Making this amplifier was remarkably easy. Without impedance
matching or feedback neutralization i was able to make a 7 watt out
(200 mW in) RF power amplifier at 7 MHz. The amplifier was likely
operating in class C mode. Perfect for QRP operations!

I should be able to achieve much higher output power and greater
efficiency when i properly match the input and output. Many articles
have been written on this topic in QST, this is nothing new. Some
authors have written that they have achieved power added efficiencies
upwards of 75% at high frequencies (in the Mhz range) operating in
class D mode.

From this result i am very encouraged, and i am considering building
a CW transmitter using one of these and pushing it as hard as i

Saturday, February 20, 2010

getting back to basics, build a shortwave radio using only 2N4124's (NPN BJT's) and one 2N3906 (PNP BJT)

During my undergraduate days at MSU a math professor friend of mine told me about designing a receiver out of only one type of transistor back in the 60's when transistors were more expensive. Again, a colleague of mine shared stories with me about a receiver that he developed using only one type of transistor and a surplus WW2 radio chassis with a very nice gear reduction tuning mechanism and capacitor. This was done in the early 60's.

Considering the intuition that this generation has from their experiences in developing RF systems using discrete devices i decided to challenge myself to build my own one-type-of transistor receiver.

Looking around my laboratory i found that i have an abundance of 2N4124's, which are general purpose NPN transistors that are not specifically designed for RF necessarily. I challenged myself to develop a receiver, a transistor radio from scratch, using only 2N4124's.

Using the 1970 ARRL handbook, Art of Electronics, and some web articles on the Norton RF amplifier circuit i successfully developed the receiver shown here. It is very sensitive, better than -110 dBm (as low as my HP606A will go) and is capable of tuning in shortwave AM, +- 500 Kc's tuning range centered at 5 Mc. The IF was at 455 Kc's because i had a bunch of old 455 Kc tuned transformers lying around. With this receiver I can tune in numerous shortwave stations with the use of only a 10' long wire inside of my basement.

A few things i have learned:

It is easier to make an oscillator than an amplifier.

Be cognizant of your envelope detector output impedance.

Norton amplifier design topology (or for that matter any common base configuration) is a great way to use a NPN junction transistor at higher frequencies without oscillation problems.

Do not expect more than 30 dB of gain for any one IF amplifier stage. Actually, it should probably be kept below 30 dB to 25 or so for stability. I preferred to use degenerative emitter feedback to stabilize my IF amplifiers.

I really like Hartley oscillators. The one i developed produces alot of power and is very stable. (but i did have to cheat with this and use a PNP transistor, the 2N3906, because the only variable caps available as a tuning capacitor were grounded to the chassis on one leg).

This was lots of fun! You should try building your own transistor radio from scratch.

Friday, February 12, 2010

Portable Tube Preamp

Audio Express Magazine has published an article that i wrote on a portable tube preamplifier. This preamp was developed so that we could record live concerts with high fidelity. The hard part was to use tubes for this (or any) portable application. If you find this interesting, please read on by checking out Audio Express Magazine online.

If you find audio projects to be interesting then support this community by buying a subscription to Audio Express, it is a fascinating magazine with numerous makers building audio projects including; amplifiers, preamps, solid state amplifiers, tube amplifiers, speakers, and etc. I plan to contribute with more articles in the future, if you have anything interesting that you have built i'm sure they would want to hear about it :)

Sunday, February 7, 2010

a synthetic aperture radar image of my Cannondale M300 bicycle

I acquired this using my rail SAR a few years back. Just sifting
through some old data tonight re-processing images.

Saturday, February 6, 2010

Working on SAR imaging

SAR imaging is fun. Here is an image of a 1:32 scale model F14
acquired off of my rail SAR that i built in my garage: