Most RF amplifiers have what has come to be known as“standard” 50Ω output impedance. The AR RF/Microwave Instrumentation model 350AH1A RF amplifier is unique in that its output impedance is typically less than 1Ω. With such low output impedance and a broad frequency range of 10Hz to 1MHz, one might mistakenly assume that the 350AH1A is merely a high-end audio amplifier. This amplifier, however, was not designed to be used in any stereo system. It was specifically designed as a very versatile and rugged bench-top instrument for lab use in R&D applications, product quality testing, magnetic research, and Electromagnetic compatibility (EMC) testing per MIL-STD-461F: CS101, CS109, CS114, RS109, DO160F sections 18 & 19, as well as many automotive EMC standards. The extremely rugged design ensures that the 350AH1A will deliver the required voltage and/or current to any load without fear of amplifier shut-down or failure. This Application Note will explore this unique amplifier's characteristics, cover methods of interconnecting amplifiers for enhanced performance, and discuss some of the applications addressed by this robust, versatile amplifier.
350AH1A Basic Specifications
(Please refer to the data sheet for full and updated specifications).
|Frequency Response:||10Hz – 1MHz|
|Output:||14 Amps & 25 Volts, 10Hz - 300 kHz
De-rated at higher frequencies with a gradual sloping down to 5.5 Amps & 10 Volts at 1 MHz
|Input Impedance:||600 Ω typical|
|Output Impedance:||<Ω typical|
|Mismatch tolerance:||100% rated voltage or current available into any load without amplifier shutdown or failure|
|Display:||Color with both voltage & current or VA displayed|
|Interface:||GPIB, serial RS232, Ethernet, and USB|
The 350AH1A amplifier differs from all other AR amplifiers in that it guarantees a minimum value of voltage and/or current delivered to the load rather than output power. The load impedance determines whether the output will be voltage or current limited in a manner similar to that of a conventional DC power supply.
In particular, for a load impedance exceeding 1.8 Ω, the output voltage is limited. Typically one could expect a minimum of 25 volts, but no more than 37 volts. Conversely, an output impedance of less than 1.8Ω results in output current limiting. In this situation, one can expect an output current of at least 14A but no more than 16.5A. Figure 1 clearly shows the minimum guaranteed voltage and current values as well as the typical levels. Note that the “gray” area containing typical voltage and current voltage values assumes minimal transmission loss in the output lead wires.
Output power is generally considered the major criteria when specifying a power amplifier. However, some applications call for either a fixed voltage or current, and in this situation, the power delivered to the load can be determined by applying Ohm’s law. The Ohm’s law pie chart shown in Fig. 2 shows the various combinations of the four variables, I, V, Ω and W. As shown in Fig. 3, an ideal output impedance of 1.8Ω will deliver a minimum output power of 350 W. Above this impedance, the amplifier will voltage limit, and below this impedance, the amplifier will current limit. More information is available in RF Amplifier Output Voltage, Current, Power, and Impedance Relationship.
When operating at frequencies up to about 100 kHz the amplifier’s output is very predictable. Unfortunately, as frequency increases, the inductive reactance of the output cables begin to adversely affect delivered power. While insertion loss from 100 kHz to 300 kHz is noticeable, it is not excessive. However, beyond 300 kHz losses can be significant and one must pay particular attention to output cabling to ensure adequate power is delivered to the load.
To minimize reactive losses when the load is close to the amplifier, ensure that the wired output is as short as possible and physically large. Bus bars are particularly well suited for this application. By keeping the inductance low, the losses are tolerable. Often the application involves some degree of separation between the amplifier and the load. In this case, matched coaxial cables are recommended to minimize VSWR and maximize delivered power. Since the amplifier's output impedance is approximately 1.8Ω, the matched cable will need to be <2 Ω. Unfortunately, 2 Ω cable is not readily available. However, with a little ingenuity, one can fashion a serviceable low impedance cable from standard cables. For example, a 2 Ω coaxial cable can be formed by combining ten 20 Ω coaxial cables in parallel.
A less attractive method is to use a wide copper strap or thick wire braid. The more surface area the strap has the better. This approach works well at lower frequencies but may be limited at 1MHz.
Standard wire is only viable at the lower frequencies or for short distances. Much of the power will be lost as a result of the inductive reactance at the higher frequencies.
For applications that require voltage or current levels beyond the capabilities of a single 350AH1A, the outputs from two or more 350AH1A amplifiers can be combined. Depending on the physical configuration, one can double either the voltage or current, quadruple the current, or double both the voltage and the current.
To double the output current, tie both outputs together in a parallel configuration.
If the application requires more voltage than a single 350AH1A can provide, two amplifiers can be combined in a bridged configuration. The second amplifier is set to invert the output signal and the load is placed between the two outputs. The effect is a doubling of the output voltage. This technique is analogous to a residential 220VAC line (commonly used in the United States) where two 110VAC lines 180° out of phase are combined to provide 220VAC.
By combining four amplifiers, one can quadruple the output current via a parallel configuration or double both the voltage and current via a bridged & parallel configuration.
Note: In parallel configuration 2, 3, 4, 5 … amplifiers can be combined; it is not limited to just 2 and 4.
The parameters listed in the table below are useful when operating at frequencies <300 kHz. At higher frequencies, inductive losses become significant, and as noted in the spec sheet, current and voltage are gradually de-rated from 300 kHz to 1 MHz.
|Voltage||Current||Effective Output Impedance (EOI)||Power at (EOI)*|
V / A = Ω
V * A = VA
|A single 350AH1A||25||14||1.8||350|
|2X 350AH1A in Parallel||25||28||0.89||700|
|2X 350AH1A Bridged||50||14||3.57||700|
|4X 350AH1A in Parallel||25||56||0.45||1400|
|4X 350AH1A Bridged & in Parallel||50||28||1.8||1400|
Due to the versatility of the 350AH1A’s rugged design, it lends itself for use in a myriad of applications. One of the most promising applications is EMC testing. Prior to the introduction of the 350AH1A, there were no instrument amplifiers available for these demanding tests. Test engineers were forced to compromise with audio amplifiers and sacrifice some of the following desirable features: broad frequency response, durable design, full mismatch tolerance into opens and shorts, and voltage and current output display.