WHAT TO KNOW BEFORE YOU BUY YOUR NEXT AMPLIFIER
Product Announcement from AR Modular RF
People come to AR Modular RF to buy an RF amplifier system or RF module of some kind because we make amplifiers from 10kHz up to 6GHz in powers from a few watts all the way up to 5kW, but the truth is that they rarely just want to buy an amplifier. The amplifier is always just a means to an end. It's only part of the "big picture" which is why we ask a lot of questions about what our customer is trying to achieve not what he thinks he needs to buy. Sometimes that turns out to be the same thing, often we find that it isn't. AR Modular RF has provided amplifiers for just about every conceivable end user form, for instance, for heating things such as plastics, cookies, all kinds of industrial processes; for radiating things, to sterilize them, to soften, to improve blood or fluids flow, to change the state of a compound or fluid; for communications near and far of every conceivable type; for surveillance of all kinds, the detection of weather related events, radars and so on. Sometimes because of the powers involved (up to 5kW) the systems are large rack mounted systems, with heavy power supplies that create large amounts of energy and hot air from the cooling systems. All the way down to a small 2 inch square module that barely gets warm. The physical size and configuration of the system for many customers is as important or sometimes more important as the actual technical specifications, which is where science and art collide! The key questions and specifications for RF amplifiers are as follows:
1. How much output power do I need?
Output power this can be listed in a number of ways:
• Watts or kilo watts (1000's), 10W or 5kW
• dBm's, 40dBm, or 67dBm
The power number by itself doesn't really tell you the whole story because there are several ways to convey that number and it makes a big difference sometimes when comparing amplifier if you get the wrong reference.
• P1dB, this is the power at the point where the output has just begun to stop following linear changes to the input
• P3dB, this is a similar point a little further up the scale
• PSat, this is the point where the amplifier will not increase its output regardless of the increase to the input. It has reached it fully "Saturated" level
It's important to know which of the above your amplifier is specifying. Often these terms are left off the spec sheet in the hope that the customer will make an assumption that the amplifier performance is higher than it really is. You'd be very unhappy to buy a 100W amplifier for an application that needed a linear amplifier only to find out that the 100W quoted was really P3dB and that the most you could really get out of your amplifier was closer to 50W rather than the 100W that you thought you purchased.
2. What about gain of the amplifier?
The gain of the amplifier is important too, you have a signal of a certain level that needs to be amplified to a higher level. The gain is the ratio of the input to the output of the amplifier, usually related in dB's.
• A gain of 10 is 10dB
• A gain of 20 is 13dB
• A gain of 100 is 20dB
• A gain of 1000 is 30dB
Your input of 1 Watt needs to become an output of 10 Watts so you'll need a gain of 10 or 10dB.
3. Does the gain have to be linear or not?
This can depend on a lot of factors, but one of the key factors is the bias condition of the amplifier. The 3 most common are:
• Class A……….Most linear of all, but it consumes a lot of power all the time and gets the amplifier very hot
• Class AB…… Nearly as good as Class A for most applications, is a close second in linearity to the Class A amplifier and only consumes full power when the input is applied to the amplifier. So doesn't run so hot or consume so much power when idle
• Class C………Not at all linear, can only be used for applications that don't need a linear output signal. Applications such as FM radio or Phase shift data transmission, or perhaps simple heating within a process.
4. Harmonics are related to the linearity of the amplifier; the more linear it is, the lower the level of the unwanted harmonics. Harmonics are related to the level of the carrier in dB's again such as -10dBc which says that the harmonic is one tenth the level of the carrier. The bigger the negative number, the smaller the harmonic; and in most cases harmonics are a bad thing. We have to take steps to reduce them by:
• Carefully selecting and controlling the bias point of the amplifier
• Not over driving the input of the amplifier and running it in its non- linear mode
• Limiting the bandwidth of the amplifier or adding filters to the output so that the harmonics are reduced to a level that they won't affect the intended application.
5. What kind of power is needed to run the amplifier?
• DC, what voltage?
• 12, 24, 28, 40 or higher
• Does it have to be filtered against spikes or protected from EMI interference?
• Does it have to be regulated so that the amplifier has a constant DC voltage?
• AC, what voltage?
• 110, 208, 220, 240, 440
• Will it be single or 3 phase and what configuration of 3 phase: Star or Delta?
• How much current can your power supply provide? Is it large enough and stable enough to power the amplifier?
6. What kind of packaging and heat sinking is available or required for this project?
The possibilities are almost endless. When it comes to heat sinks, size is important. Too small a heatsink and the amplifier will overheat and shut down or worse permanently, damage it. Keeping it cool, by having the correctly sized heat sink and enough air flow, or water flow in some cases improves the reliability of the amplifier dramatically so this area needs some careful consideration and is often overlooked with disastrous results.
7. Can the vendor actually do what they say they are going to do?
Can they give you an example of performance data for the same or similar amplifier? We here at AR not only provide a lot of sample data from the lab or from previous shipments to our customers, but we also "fast sample" amplifiers to get you something in your hands for a "real live test", which is by far the best way to be assured of success. AR Modular RF has been building RF amplifiers for over 30 years. You should be looking for a vendor like AR who can draw on years of experience to be able help you in your design. Plus you'll have the assurance that should you have a problem during the warranty period (3 years for AR) that the company will actually be there to take care of you when you need it.