Op-amp
So far we have learned about a couple of physical sensors, including a thermistor, diode temperature sensor, transistor temperature sensor, thermistor, strain gauge, and more. For practical applications, the voltage signals generated from these sensors mu
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Op-amp
So far we have learned about a couple of physical sensors, including a thermistor, diode temperature sensor, transistor temperature sensor, thermistor, strain gauge, and more. For practical applications, the voltage signals generated from these sensors must be conditioned and appropriately amplified. This task is usually achieved by an operational amplifier, more commonly referred to as an op-amp, which we briefly used in our Chap. 3 laboratory (Sect. 3.5).
6.1
Op-amp
The op-amp is an integrated circuit (IC); more specifically, an analog IC or linear IC. Most ICs deal with digital signals, called digital IC or logic IC, and are used primarily in microprocessors and memory applications. Op-amps, however, deal mostly with analog signals (voltage signals). Other examples of analog or linear IC include timers and oscillators. A typical op-amp, shown in Fig. 6.1, is an integrated device with a non-inverting input, an inverting input, two DC power supply leads (positive and negative), an output terminal, and a few other specialized leads used for fine-tuning (Figs. 6.2 and 6.3).
J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, DOI 10.1007/978-1-4419-6022-1_6, # Springer Science+Business Media New York 2013
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88
6 Op-amp
Fig. 6.1 Op-amps (LM741, LM324, and TL082)
positive supply voltage (+Vs) inverting input (V–)
–
non-inverting input (V+)
+
output voltage (Vout)
negative supply voltage (–Vs)
offset null
1
V–
2
V+
3
–Vs
4
8
NC
–
7
+Vs
+
6
Vout
5
offset null
Fig. 6.2 A schematic symbol of an op-amp (left) and its pin configuration (right; for LM741)
Fig. 6.3 A component level diagram of LM741, consisting of transistors and resistors
6.2 Basics of Op-amp
6.2
89
Basics of Op-amp
The following is an expression for an op-amp’s output voltage as a function of its input voltages V+ (non-inverting) and V (inverting) and of its open-loop voltage gain Ao: Vout ¼ Ao ðVþ V Þ
(6.1)
Figure 6.4 compares an ideal op-amp (left) with a real op-amp (right). We must take into account the real features of an op-amp, such as its input resistance Rin and output resistance Rout. To understand and compare the ideal and real equivalent circuits, know that the values of Ao, Rin, and Rout are defined by the following rules: • Rule 1: For an ideal op-amp, the open-loop voltage gain is infinite (Ao ¼ 1). For a real op-amp, the gain is between 104 and 106. • Rule 2: For an ideal op-amp, the input resistance is infinite (Rin ¼ 1). For a real op-amp, the input resistance is finite, typically between 106 and 1012. The output resistance for an ideal op-amp is 0 (Rout ¼ 0). For a real op-amp, Rout is typically between 10 and 1,000 Ω. • Rule 3: The input terminals of an ideal op-amp draw no current. Practically speaking, this is also true for real op-amps since input current is usually in the pA or nA range. As an example, we can solve for the gain of the circuit shown in Fig. 6.5. Since V is grounded (0 V) and V+ is simply Vin, the open-loop voltage gain expression beco
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