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A DC to 20 MHz Coax Driver Using
This is what we used
to call "wide
band" in the old days.
It works very well as the output amplifier for a 20 MHz function/sweep
generator on my workbench.
A differential amplifier followed by a differentially
driven class A voltage gain
and then a Class A-B complimentary pair buffer for the output stage.
Find updates and
corrections at www.projects.cappels.org
Related Project: MAX038-Based
Sweep/Function Generator With Markers
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
Upon opening the case, I found that the Ztex ZXFY202N8 wide band
opamp had been destroyed and the output termination resistor burnt and
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
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.
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
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.
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"
it is sometimes called. Just start at one end, and start adding parts.
Contents ©2010 Richard Cappels All Rights Reserved. Find updates
First posted in October,
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