MAE 170 Experimental Techniques

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MAE 170Experimental Techniques

• Oct. 18, 2004

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Topics

• Operational amplifiers
• OPerational AMPlifiers (op-amps)
• Wheatstone bridge
• How to measure an unknown resistance
• Based on Ohms Law (VIR) and on Kirchoffs Law
  (current in current out)

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Objectives of this weeks lab experiments

• Construct Op-Amp amplifier circuit
• Inverting mode
• Non-inverting mode
• Construct a Wheatstone bridge circuit
• Measure an unknown resistance

Implement above using LabView VIs
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General characteristics of signal amplification

• Many transducers produce signals with low
  voltage (mV)
• Difficult to transmit low signals
• Many processing systems require voltages in the
  range 1-10 V
• Amplitude can be increased with an amplifier
• Gain Vout/Vin

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Gain

• Gain can be lt1 or gt1
• Commonly expressed in a log scale in decibels
• GdB 20 log (Vout/Vin)
• Amplifier can distort signal
• Frequency distortion
• Phase distortion
• Source loading

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Op-Amps

• An op-amp is a large integrated circuit, which
  consists of gt 50 transistors
• An op-amp has
• non-inverting input ()
• an inverting input (-)
• one output
• The output voltage is the difference between the
  and - inputs multiplied by the open-loop gain
• Vout (V - V-) Gopenloop.

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Op-Amps
Gain G Vout/Vin
Vin
Vout
Voltage gain of up to 106 (1 mV ? 1 Volt)
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Op-Ampsopen loop
Two input terminals (,-) Vs and Vs-
power supply terminals
V and V- input voltages Vo output
voltage g(V - V-)
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Review principals of feedback
Xo A Xi Input is amplified Xf b
Xo Output is fed-back to the input Now, Xi
(Xs Xf), positive feedback, -
negative feedback
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Why feedback?

• Coupling the output back to reinforce/cancel
  some of the input
• Better control
• Reduce the effect of noise
• Reduces output distortion
• Gain is independent of signal level

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Examples of positive and negative feedback

• Positive feedback
• oscillators
• Negative feedback
• op-amps

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An op-amp schematic of the 747 op-amp
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Types of op-amps
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Golden rules for op-amps

• Voltage rule
• The output attempts to do whatever is necessary
  to make the voltage difference between the inputs
  zero.
• Current rule
• The inputs draw no current.

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Ideal op-amp

• Infinite voltage gain
• Infinite input impedance
• Zero output impedance
• Infinite bandwidth
• Zero input offset voltage (i.e., exactly zero
  out if zero in).

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Real and ideal op-amps
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Inverting and non-inverting op-amps

• Inverting op-amps
• Output voltage is the same sign as input voltage
• Non-inverting op-amps
• Sign of output voltage is the same as the input
  voltage

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Inverting op-amp
For an inverting amplifier, the current rule
tries to drive the current to zero at point A.
This requires This gives the voltage
amplification
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Non-inverting op-amp
Iin
A
Vin
-
Vout
B
R2
R1
Current flow from B?op-amp negligible (high input
impedence from op-amp Current flowing through R1
IR1 Vout/(R1R2) Vn VB IR1R1
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Non-inverting op-amp