Simple DC to 20 MHz Driver

Posted on Jan 15, 2013

The output cable from my 20 MHz function/sweep generator dangled over the side of the workbench, the alligator clip hovering over the floor. Deeply engrossed in a project, I moved the power strip on the floor a little closer so I could plug in the circuit upon which I was working. That's when the alligator clip contacted the 240 VAC. The function/sweep generator had been gravely injured. Poking around with a scope probe, the signal output was down in the noise, but it was there, and the other outputs worked. The expensive, rare, and discontinued MAX038 function generator chip was still working. Upon opening the case, I found that the Ztex ZXFY202N8 wide band opamp had been destroyed and the output termination resistor burnt and broken.

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I replaced the ZXFY202N8 with the spare I had stored inside the funciton/sweep generator but something was not connected correctly. The opamp is wired into the circuit point-to-point on the back of the circuit board. After a several cycles of removing the circuit board, touching up suspect solder joints, and reinstalling the board, I gave up and decided to build another amplifier on the benchtop where I could more easily troubleshoot it. I am out of the ZXFY202N8 opamps. There is noting like that available in the electronics and surplus stores here, and my mail order supplier's web site did not have anything that is even close. But I have a lot of very fast transistors. It quickly became apparent that the old reliable 2N3904 and 2N3906 transistors would be plenty fast, had sufficient gain, and were not exotic at all. It took a couple of hours to put the amplifier together and debug it. That activity was much more enjoyable than going through the uninstall-troubleshoot-install cycles. Circuit Description The circuit had some apparently tough constraints: High input impedance so as to not load down the output of the MAX038, a low impedance output to drive the coax and any loads, wide bandwidth -as flat as I could get to at least 20 Mhz, and a full bandwidth output voltage swing to within 2.5 volts of the power supply rails. For the high input impedance the circuit uses a differential pair of 2N3904...

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