Signal-powered signal squarer

I need a square-wave generator to test out the step response of the amplifier in my HP 400C AC VTVM

Signal-powered signal squarer

I've been fighting some oscillation problems in the circuit. But none of my square-wave generators are in service – they are all in the restoration queue.

I remembered an old article from the October 1967 issue of Popular Electronics. (Many thanks to AmericanRadioHistory.com for making scans of Popular Electronics available.) Leslie Solomon published an add-on Schmidt trigger squaring circuit for an audio sine wave generator. The squarer was powered by a voltage doubler running off the input sine wave - a clever idea.

So, I built one to run off my HP 200CD. The original design used 2N404 PNP germanium transistors. I used 2N2222A NPN silicon transistors in my version, so all the power polarities are reversed from the published circuit. I built it on a piece of proto board and started testing it. 

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At low input voltages, the circuit worked fine.

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But as I turned the output higher on the HP 200CD, I started getting some odd bumps on the low side of the waveform.

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At full voltage, you can see that the negative portion of the input sine wave is breaking through. 

I puzzled over this for a while, until I realized that the circuit puts the full negative portion of the input sine wave on the base of the first transistor. Those old 2N404's must have had a very high Vbeo reverse breakdown voltage to withstand that rough treatment. The base-emitter junction of modern silicon transistors usually breaks down at around 5 or 6 volts, so when the input reverse-biased the base-emitter junction on the first transistor, the junction broke down and applied a negative voltage to the emitter of the second transistor. Now on the low portion of the output square wave, the second transistor is saturated, so the negative half sine wave on the emitter of the first transistor is coupled directly to the output.

fixed schematic

The fix is to add a 10K series resistor to the base of the first transistor, and add a diode clamp to prevent the base-emitter junction from breaking down.

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With this modification, I get nice square waves up to the maximum output voltage. I also added a zener on the voltage doubler power supply to limit the voltage to less than 40 volts, as the 2N2222A is only rated for 40 volts maximum Vceo.

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I packaged the circuit in a small die-cast aluminum box from Hammond. The banana plugs are spaced at 3/4 inch to plug directly into the output of the HP 200CD. They are insulated by some fiber shoulder washers. The output uses a BNC connector.

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 It was a bit of a squeeze getting the banana plugs connected up.

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So, I now have another nice addition to the collection of test fixtures in my lab.

© Steve Byan 2011-2016