Negative Voltage Generator Schematic

Posted on Dec 22, 2012

Most operational amplifier circuits require a dual-polarity power supply - one having +V and -V. However, there are times when a DP supply simply isn't conveniently available. Single-polarity modifications to DP designs often acheive their effect by referencing the input to a `synthetic ground`, derived using a resistive voltage divider. This renders the resulting circuit highly sensitive to power supply noise, since any variation in the power supply will be divided by the divider, then used as an input reference, and amplified by the circuit in question. While this is of little concern with battery sources, AC `wall-warts` and many DIY power supplies are chock full of horrendous 60Hz buzz that is sure to get into the circuit.

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By powering an op amp with a dual-polarity supply, we are able to reference all signals to a definite solid 0V ground. This circuit starts with a single-polarity (GND and +V) input and produces an approximate opposite voltage (-V), enabling dual-polarity op amp circuits to be used with single-polarity supplies. Theory of Operation This circuit was built in a pinch because I needed to power a microphone preamp from an unregulated, noisy, single-polarity 12V supply that also powers a couple of power-hungry sirens and a video camera. While said preamp is now working relatively successfully, this circuit is a quick design that has not been put through extensive testing. I do not know its current limitations. Some components on the board can be heard to be "whining", much like a switchmode power supply. This does not indicate a problem, as none of the components run particularly hot. Also like a SMPS, this noise is slightly detectable on the output. Q1, Q2, C1, C2, R1, R2, R3, and R4 form a multivibrator -- an oscillator. We are using both of its outputs which are opposites of one another: when one is on, the other is off. These outputs go through R5 and R6 to turn on Q3 and Q4 - one at a time, of course. This applies a wave oscillating from +V to GND to C3. This is where things get interesting. Q3 and Q4 form a current amplifier. When Q3 is turned on, +V is applied to C3. We can envision the current going through C3...

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