A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 1 Advanced Instrumentation By A.L. Wicks Department of Mechanical Engineering Virginia Tech A course in current techniques of measurement, acquisition and processing test data

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 2 Amplifiers Ideal amplifier Amplifier has gain A In this case, the gain relates the output of the source to the voltage delivered to the load This is in contrast to the input voltage and output voltage of the amplifier

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 3 Amplifiers Av in R out What are the desirable characteristics of an amplifier?

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 4 Amplifiers Operational amplifier

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 5 Amplifiers Development of summing point constraint Consider the following amplifier with feedback

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 6 Definition of Gain Voltage gain is equal to the ratio of voltage output to voltage in. Power gain is equal to the ratio of output power to power input.

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 7 Amplifiers Non-inverting amplifier Gain for the non-inverting amp

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 8 Amplifiers Voltage follower Often referred to as a buffer A=1

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 9 Amplifiers Summing amplifier

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 10 Amplifiers Differential Amplifier Assuming the op amp is ideal

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 11 Amplifiers Differential Amplifier Differential mode and Common Mode components Assume some error in R 2

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 12 Amplifiers Differential Amplifier Differential mode and Common Mode components For 1 % resistors the CMRR= 48.5 dB Note The larger the differential gain R 2 /R 1 the higher the CMRR

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 13 Amplifiers Differential Amplifier Use of differential amplifiers to eliminate ground loops Note differential amp removes the ground loop

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 14 Amplifiers Charge amplifiers Some transducers such as piezoelectric accels operate by converting force to charge Piezo model Thevinin equiv. Norton equiv. model Note: charge amps require high input impedance common to have FET circuits

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 15 Amplifiers Charge amplifiers Practical circuit for a charge amp. R 2 provides a dc shunt for the input bias current. Without this resistor, the bias currents would charge C f to saturation R 1 stabilizes the feedback Resistor criteria R 2 1M-1G ohms R ohms

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 16 Comparator

A. L. Wicks Dept. of Mechanical Engineering Virginia Tech 17