It is sometimes necessary to have a microphone preamplifier mounted in the mike head. Obviously, the preamplifier should be as small as possible, battery operated and consume a small amount of power. In the past this was accomplished with bipolar and FET op amps. The primary disadvantage of these circuits is the comparatively large physical size of both the amplifier and power source. Another major factor is relatively large power consumption, which requires frequent battery replacement. The most obvious next choice would be a CMOS op amp. While this approach seems logical at first, it has some disadvantages. A metal-gate CMOS op amp can be operated from a low-voltage supply and has low power consumption; but, it suffers from input offset voltage instability. This input-offset drift is due primarily to the differential input signals at the op amp input terminals. The LinCMOS op amp overcomes these problems. In addition, it has the advantage of low power comsumption and low voltage operation (down to 1.0 V).
A microphone preamplifier using a LinCMOS op amp is illustrated in Figure 3-37. This unit comes complete with its own battery and is small enough to be put in a small mike case. The amplifier illustrated was designed to be operated from a 1.5-V mercury cell battery at low supply currents.
This preamplifier will operate at very low power levels and maintain a reasonable frequency response as well. The TLC251 operated in the low bias mode (operating at 1.5 V) draws a supply current of only 10µA and has a -3-dB frequency response of 27 Hz to 4.8 kHz. With pin 8 grounded, which is designated as the high bias condition, the upper limit increases to 25 kHz. Supply current is only 30µA under those conditions.
If improved higher frequency performance is desired, the VDD may be increased. For example, when using a 5-V supply the frequency response is from 27 Hz to 11 kHz for the low bias, and from 27 Hz to 220 kHz for the high bias modes respectively. Operating in the high bias mode at 5-V VDD the amplifier requires a supply current of less than 500µA. Frequency response for the amplifier is shown in Figure 3-38.