70009 Parallax Inc, 70009 Datasheet - Page 111

• Manufacturer Part Number: 70009
• Description: BOOK UNDERSTANDING SIGNALS
• Manufacturer: Parallax Inc
• Type: Signal Analysisr

Specifications of 70009

• Style: Book
• Title: Understanding Signals
• Contents: Guide to Generate, View and Measure Wave Forms
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Other names: 70009PAR

AN OP-AMP USED AS A VOLTAGE AMPLIFIER
An op-amp can be used to amplify voltage. To do this, you have two types of circuits to
choose from, inverting and non-inverting. As you can imagine, the inverting circuit will
produce an output signal that is inverted, or the negative value of the input signal,
creating a mirror image. Both inverting and non-inverting circuits use two resistors as a
feedback loop to set the gain of the amplifier. However, their formulas and circuit
characteristics are a little different, so let’s look at each in turn.
The inverting voltage amplifier shown in Figure 8-5 has the input signal connected to Ri.
Ri is then connected to Rf and the inverting input of the op-amp. In this circuit the
connection at Ri, Rf and the inverting input is called virtual ground. This means that each
Ri equals the approximate input impedance. Therefore, the inverting circuit doesn’t allow
high input impedance. However, it does allow a gain of less then one, allowing you to
take a large signal and scale it down to a smaller signal. This is called attenuation.
The formula to calculate gain is as follows:
For example, if Ri is a 10 kΩ resistor and you know you want a gain of 2, your
calculation to determine Rf would look like this:
Gain =
Rf
=
Ri
Rf/Ri
*
Remember: Ri refers to the input resistor and Rf refers to the feedback resistor.
Gain,
Signal
Rf
Vss
=
10
Ri
k
*
Vss
2,
Rf
Rf
=
20
k
Vss
Vout
Figure 8-5:
Inverting voltage
amplifier