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|18 dB LT1253 DDS Amplifier
This 18 dB gain amplifier was built Manhattan-style to evaluate the Linear Technology LT1253 dual video amplifier as a candidate output amplifier for DDS systems employing the Analog Devices AD9850 and similar devices. This dual amplifier was chosen to take advantage of its high gain-bandwidth product. By utilizing both amplifier sections, a very broadband (100 KHz to 30+ MHz), high gain system could be built. The amplifier schematic is shown above. It is configured to have 50 Ohm input and output impedances, and a bit over 18 dB of power gain. The LT1253 contains two current feedback amplifier sections. The voltage gain is given by the equation: A = 1 + Rf/Rg where Rf is a 620 Ohm resistor and Rg a 200 Ohm resistor. Each amplifier section is running at a voltage gain of 4.1.
Here is the amplifier that was constructed and tested. The BNC connector on the left is the input, and the one on the right is the output. The amplifier can be powered using a supply of 8 to 14 volts with little change in performance. An optional low pass filter is shown in the schematic, but that circuitry was neither constructed nor tested.
A Bode or frequency response plot of the LT1253 DDS amplifier. Note there is very little change in amplitude (about 1/2 dB) from 1 MHz to 30 MHz. In fact, the amplifier is quite useable out to 40 MHz as configured. With small changes in overall gain (lower), it could probably be pushed to 50 MHz, allowing it to be used at 6 meters.
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