Intrinsically capacitive transducers and other high-impedance signal sources usually require ac coupling and a buffer amplifier to condition the signal for further processing. Buffers take many forms, but most of them compromise signal quality through the use of external resistors that provide a dc path for the input bias current. Recent improvements in op-amp technology allow ac-coupled inputs without the need for bias resistors. The new op amps have inputs that operate within the supply-voltage span. Some are CMOS, but many use complementary bipolar transistors. For the latter, each input connects to both a npn-difference pair and a pnp-difference pair. Combining these parallel input stages ensures that the second stage has a wide input common-mode range. Thus, some op amps exhibit an input-voltage level for which bias current to the npn pair exactly equals that from the opposite-polarity pnp pair, causing the input bias current to go to zero (Figure 1).
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| Figure 1. | A self-biasing amplifier buffers a high-impedance signal without the need for bias resistors. |
For the amplifier shown, this bias-canceling effect self-biases the inputs at a level consistently close to 2VCC/3. To avoid the distortion effects that increase with signal deviations from 2VCC/3, you should limit the input signal swings to 500 mV p-p. If necessary, downstream circuitry can remove the dc offset while providing gain and filtering. Even with small, low-cost coupling capacitors, this circuit's ultrahigh input impedance and absence of bias resistors allow operation at frequencies well below 1 Hz.
