A_DC_to_20_MHz_Coax_Driver_Using_Discreet

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Related Project: MAX038-Based Sweep/Function Generator With Markers

Introduction

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.

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 transistors. The collector currents are differentially amplified by a 2N3906, which has its 1k collector load connected to - 5 volts though a bias network for the complementary pair buffers the high impedance collector of the second amplifier stage to drive the output termination resistor.

The relatively low values of the resistors are needed to allow sufficient output current to provide the large output range of over 5 volts (- 2.6 volts to + 2.6 volts) while driving terminations, to keep sufficient current through the transistors for them to have adequate bandwidth and so that capacitances don't cause the amplifier stages to roll off at frequencies that are too low.

Once the bandwidth and load driving signal swing were confirmed, the next problem was how to stabilize the amplifier. Having three stages means that at some frequency, there is 180 degrees phase shift between the inverting input and the output. Dominant pole compensation would not work because there was not enough excess gain in the circuit. That left me with the trick of shunting the amplifier's input with a broad band network to reduce the loop gain. That is the purpose of the 0.1 uf capacitor in series with the 100 ohm pot.

The 0.1 uf capacitor and 100 ohm pot is connected between the an input stage's two inputs. The pot is set to about 50 ohms and the impedance of the two 1k resistors that set the closed loop ;gain is 500 ohms, which results in the loop gain being lowered by about 40 db at frequencies above the corner frequency of the 0.1uf capacitor and the 50 ohm resistor (about 30 kHz), which is enough to keep the gain at 80 Mhz, where the amplifier tended to oscillate, less than 0 db. I tried using a fixed resistor, but I noticed that to get excellent square wave response, the loop gain had to be critically adjusted. Using a pot was much easier than selecting a set of resistors.

Construction

Functionality and frugality take precedence over appearance.

This is going to be hidden inside the sweep/function generator's enclosure, so there was little worry about keeping it neat or even rugged, for that matter. I built it dead bug, or "ugly bug" style as it is sometimes called. Just start at one end, and start adding parts.

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