At the 0:27 mark of the video there is a close up of some of the transformers. Is that surface mount parts on the two big transformers on the left hand side?
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What's on the transformers...HopperBuilt staged 2x8?
- Thread starter snippits75
- Start date
Yes, more and more builders are using surface mount in that position with the logic being that they will handle the torturing RF current that cap has to endure better than silver mica or metal clad. It my opinion, time will tell on that assumption. lol
OK, I thought it was surface mount.
I thought metal clad was the top end, and silver mica was the standard.
Here in the last couple weeks, and I am not mentioning any names. A builder told me that he took off the cheap sh..it(SMT), and put on the metal clad.
Anyway, I have a friend that wants a staged amp built like the one in the video, but after seeing the SMT parts on the transformers I am having second thoughts. Need opinions!
I thought metal clad was the top end, and silver mica was the standard.
Here in the last couple weeks, and I am not mentioning any names. A builder told me that he took off the cheap sh..it(SMT), and put on the metal clad.
Anyway, I have a friend that wants a staged amp built like the one in the video, but after seeing the SMT parts on the transformers I am having second thoughts. Need opinions!
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The chip capacitors work fine if the right one is used for the application. That’s what manufacturers use on FET’s, LDMOS and the like for RF generators and on broadcast modules.
The SMT chip caps are CHEAP. A lot of idiot builders have been using 3-12 cheap poorly rated blue or yellow dipped Chinesium ceramics all over the the input and output circuits for years. Those are about as heat tolerant as Albion's with no sweat glands to equatorial locations! They keep trying to get the cheapest Chinese parts money can buy that will get the job done. Metal Clad Mica's are really expensive as are Arco's!
Due to their small physical size, in RF applications where current is high or impedance is very low, it is not uncommon to see more than one chip cap in parallel, to share the current flow and reduce RF heating.
APC has developed a line of high current RF chip capacitors that are exceptionally good. The chip mount part eliminates all lead inductance too. Now I just wish they would recognize these parts are only suitable for older low voltage applications and they will need to design a new line of chip caps to work with the next generation of high voltage RF MOSFET's.
The new transistors have arrived but the supporting components able to handle the voltage and current, not so much.
The new transistors have arrived but the supporting components able to handle the voltage and current, not so much.
For example, the newest MOSFET's can operate with a VDD well into the hundreds of volts. The largest ATC high current RF cap is rated at 300 working volts and many values are not made above 250 volts. In most applications, that can handle the DC but once we combine the DC in a tuned RF circuit, as I'm sure you know, we can quickly exceed the working voltage of these caps if the load impedance is ever above 50 ohms.
Certain phase angles at the load can make the peak RF voltage rise well over double the DC and you still have the RF on top of the DC. With that in mind, there are applications where I need to see a breakdown voltage of at least 3 times the DC and strive to achieve 4 times, when possible. That eliminates the chance of a cap failure due to any spike in RF voltage that might occur under the worst mismatch conditions.
Certain phase angles at the load can make the peak RF voltage rise well over double the DC and you still have the RF on top of the DC. With that in mind, there are applications where I need to see a breakdown voltage of at least 3 times the DC and strive to achieve 4 times, when possible. That eliminates the chance of a cap failure due to any spike in RF voltage that might occur under the worst mismatch conditions.
Roger that. Not a big deal with the metal clads and a 12-Volt circuit. Big deal with surface mount caps and a 28 or 50-Volt transistor.
73
OK fine. I was just asking because when normally dealing with mobile amps etc the norm is 12 volts nominal and even 50 volt devices may be considered "high" or at least "higher"voltage devices by some. Yeah commonly devices are now being rated at a couple hundred volts. I see some switch mode MOSFETs are even rated at 4500 volts. Now THAT is a high voltage device for sure.
Combining high voltage, high frequency RF and operation in the linear mode is the challenge. High voltage transistors for low frequency or class C have been around since the horizontal output tube went solid state in televisions. While MOSFET's have high operating voltages, check out the Insulated Gate Bipolar Transistor (IGBT). They have about the highest operating voltage of any transistors but only run switch mode too.
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