meter amplifier

A meter amplifier is intended for use between a sensor or other measuring device, such as a probe, and the indi- cator. It is characterized by a high input impedance, typically 1 MQ, and a differential input. A differential input ensures that the output signal cannot be affected by hum or noise on the meter leads. The input signals are buffered by dif- ferential amplifiers Al and A2. The 22 pF capacitors in the C1and C2 positions obviate any tendency to oscillate. The output of opamp A3 is a function of the difference between the two input signals. Opamp A4 serves to compensate for any offset and also to set the amplification at exactly 1. The bandwidth of the circuit as shown is not less than 100 kHz, and the phase shift is 0°. As already mentioned, the amplifier may be used with any sensor, for instance, in computer control of the central heating, or to monitor the ambient temperature in rooms. It can also be used with a multimeter or oscilloscope. R2 meter amplifier (+) 5 . . ...