Micropower and low-voltage op amps allow you to build high-performance analog-signal processors that require no batteries or wall transformers. Instead, you can use a capacitor as a power source.
Using a capacitor as a voltage supply has many potential advantages vs regular disposable or rechargeable batteries. The Supercap has a typical charge time of 10 to 20 sec. To charge the capacitor, you can use any regular wall transformer unit with a filtered or unfiltered, regulated or unregulated, 5 to 10V-dc output. There is no risk of overcharging the capacitor and therefore no need for accurate charge-current and time monitoring. Capacitor charge/discharge cycles are practically infinite. These voltage supplies have no disposable parts and no leaks. When using a capacitor, there is no chemical contamination or oxidation of electrodes. The capacitor can withstand short circuits for an unlimited time and may be discharged to any level without damage. You can easily connect capacitors in parallel without side effects, such as bias current between paralleled batteries.
The circuit is straightforward. Supercap C1 with C2, C3, R1, and R2 form the split supply with a virtual ground. Three stages of the LMC6574 with accompanying discrete components comprise the microphone amplifier. The three-op-amp topology provides good dynamic performance and low distortion. (Any stage has a maximum closed-loop gain of 10.)
The first stage, IC1A, performs bandpass filtering to remove high- and low-frequency noise. The frequency bandwidth for the microphone input is approximately 100 Hz to 10 kHz. To extend the upper cutoff frequency, you need to reduce C4 to 100 pF. You can adjust the gain of the second stage, IC1B. The third stage, IC1C, is an inverting amplifier with a gain of 10. IC1D works as an inverting signal mixer with unity gain. The circuit targets a standard load of 100 kilohms with a guaranteed 1V p-p output signal in a supply-voltage range from 2.7 to 10V. Actually, the load capability of the LMC6574 is much higher, which makes the circuit and its variations fairly universal and well suited for many signal-processing applications.
You can use the circuit without a power switch because charging the capacitor immediately before use is simple. To operate the device, you have to connect the Supercap to a dc source for 10 to 30 sec. For example, with 9V of initial voltage (standard), you have at least 2 hours of continuous operating time until the voltage drops to 2.7V. You can assume that the discharging rate of the power capacitor under load is approximately 3V/hour.
It is important that any change in the supply voltage within the op amp’s operating range not affect the output signal level and overall performance. The performance of this mixer design is dependent on the op amp you choose. You can substitute the LPC660 family of parts, but the circuit will start operating at 5V instead of at 2.7V. If an application fits a more limited frequency range, biomedical instrumentation below 1 kHz for instance, you can use the LMC6044 and LMC6064 families, which have typical supply currents of 10 and 16 µA/amp, respectively. These currents dramatically lengthen the continuous supply time of the Supercap. (DI #2007)
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