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Introduction to the Operational Amplifier: Inverting and Non-Inverting Op Amp

Introduction to the Operational Amplifier | Inverting Op Amp | Non-Inverting Op Amp | Op Amp Characteristics

Introduction to the Operational Amplifier

The operational amplifier (also called op amp) is an integrated circuit commonly referred to as an IC. An integrated circuit is a component consisting of transistors that have been doped onto a substrate (typically Silicon wafer) with interconnecting wires to external pins and encased in a ceramic or plastic material. The transistors have been configured and interconnected to perform unique functions. In the case of the operational amplifier, several stages of discrete transistor amplifier circuits have been interconnected. Both bipolar junction transistor technology and MOSFET technology have been utilized in the construction of integrated circuits.

The desired characteristics of the input stage will be high input impedance with a low input current. The output stage characteristic will be low source impedance with a reasonable ability to deliver the desired voltages for a wide range of load impedances. The middle stage or stages will require high current gain, amplifying the input signal.

Although it can be extremely fascinating and important to study the solid state characteristics of the integrated circuit, our focus in this course is on the analysis and design of circuits using the operation amplifier.

A simple 2-port circuit model is shown in Figure 1 and will be used to characterize and analyze the operational amplifier integrated circuit. The two ports consist of the input port and the output port. Each port has two terminals.

The input port of the operational amplifier is comprised of one terminal identified as the non-inverting input (+ terminal) and another terminal identified as the inverting input (- terminal). The complex internal circuitry is simplified and represented as the op amp internal input impedance ZIN measured between the non-inverting and inverting input terminals. As previously stated, the input impedance optimal value should be extremely high, ideally an open circuit or infinite impedance. The high input impedance results in low input positive and negative terminal bias currents.

The op amp output port consists of one terminal identified as the signal output (output) and the other terminal identified as the signal reference (ground). The complex internal circuitry is simplified and represented as the op amp internal dependent Thevenin voltage source AVOL x ( VP - VN ) in series with the op amp internal Thevenin source resistance ZOUT. AVOL is the op amp open loop voltage gain. VP is the voltage applied to the non-inverting op amp input terminal. VN is the voltage applied to the inverting op amp input terminal.

Figure 1: Op amp circuit model

For the integrated circuit to function, external power must be applied to the op amp circuit. Power supplies were not included in our circuit model but are extremely critical for proper circuit operation. Some op amp circuits need a single voltage power source, whereas other op amp circuits require a positive and a negative power supply. Do you know how to configure your tri-power supply to produce both a positive DC voltage and a negative DC voltage simultaneously?

Inverting Op Amp

Now that we have evaluated the op amp circuit model, we will consider the effects of connecting a resistor between the output and the inverting input terminal. The first circuit that we will consider is shown in Figure 2. The circuit is known as the inverting amplifier.

Figure 3 provides the op amp model inserted in place of the op amp schematic symbol, allowing us to evaluate the circuit operation.

The op amp positive terminal is connected to ground. Therefore, the voltage VP is 0 Volts. Using the ideal amplifier characteristics, the op amp bias currents IBN and IBP are 0 Amps. Using Ohm's law, the voltage appearing across the op amp internal impedance is equal to the bias current IBN times the input impedance ZIN. The resulting value is 0 Volts. With 0 Volts between the two op amp input terminals, the negative terminal voltage VN will be 0 V as well. Since the negative terminal voltage is 0 Volts, the negative terminal of the inverting op amp circuit is called a virtual ground.

Having established the negative terminal voltage, the effect of the input signal voltage can be considered. Using Kirchhoff's Voltage Law, the input signal voltage equation is

All the signal input voltage appears across the resistor R1, resulting in the current I1.

Applying Kirchhoff's Current Law to the negative op amp terminal, the equation is

Remembering that the bias current is 0 Amps, the feedback current I2 is the same as the input current I1. The output voltage will be calculated by writing the KVL equation around the feedback loop.

Recalling that the definition for the circuit voltage gain AV is the output voltage divided by the input voltage, the voltage gain of the inverting amplifier becomes

For a positive input signal voltage, the output signal will be a negative voltage. A negative input voltage will result in a positive output signal voltage. When a sinusoidal waveform signal is applied, the output signal will be inverted from the input signal represented as 180 phase angle shift between the signals.

Non-Inverting Op Amp

We will now analyze the non-inverting amplifier shown in Figure 4.

Figure 4: Non-inverting op amp circuit

Figure 5: Non-inverting op amp circuit model

Figure 5 provides the op amp model inserted in place of the op amp schematic symbol, allowing us to evaluate the circuit operation.

The op amp positive terminal is connected to the signal input. Therefore, the voltage VP is equal to VIN. Using the ideal amplifier characteristics, the op amp bias currents IBN and IBP are 0 Amps. Using Ohm's law, the voltage appearing across the op amp internal impedance is equal to the bias current IBN times the input impedance ZIN. The resulting value is 0 Volts. With 0 Volts between the two op amp input terminals, the negative terminal voltage VN will be the same as VIN, identical to the positive terminal voltage.

Having established the negative terminal voltage, the effect of the input signal voltage on R1 can be considered.

Figure 6: Non-inverting op amp circuit

The entire input signal voltage appears across the resistor R1, resulting in the current I1.

Using Kirchhoff's Voltage Law, the input signal voltage equation is

Applying Kirchhoff's Current Law to the negative op amp terminal, the equation is

Remembering that the bias current is 0 Amps, the feedback current I2 is the same as the input current I1. The output voltage will be calculated by writing the KVL equation around the feedback loop.

Recalling that the definition for the circuit voltage gain AV is the output voltage divided by the input voltage, the voltage gain of the inverting amplifier becomes

For a positive input signal voltage, the output signal will be a positive voltage. A negative input voltage will result in a negative output signal voltage. When a sinusoidal waveform signal is applied, the output signal will be in phase with the input signal, represented as 0 phase angle shift between the signals.

A special case of the non-inverting amplifier consists of no resistors as shown in Figure 7.

Figure 7: Non-inverting op amp with no resistors

The output voltage is equal to the input voltage.

Op Amp Characteristics

We have just considered the ideal op amp characteristics being used to evaluate the circuit voltage gain and output equations for the inverting and non-inverting amplifier. As we begin to look at actual op amps, we discover that the open loop voltage gain for the op amp is not infinite. Additionally, the open loop voltage gain decreases as the input signal frequency increases.

Using the inverting amplifier circuit, the frequency response of the LM324 operational amplifier will be observed as you select the different frequencies, looking at the input and output signals and the ideal and actual voltage gains.

	Inverting Op Amp Bandwidth Click on the link above and select the different frequencies. Study the changes in the input and output signals, and the ideal and actual voltage gains.

The actual voltage gain can be calculated accounting for the effect of the decreasing op amp open loop voltage gain.

Using the non-inverting amplifier circuit, the frequency response of the LM324 operational amplifier will be observed. This characteristic will be observed as you select the different frequencies, looking at the input and output signals and the ideal and actual voltage gains.

	Non-Inverting Op Amp Bandwidth Click on the link above and select the different frequencies. Study the changes in the input and output signals, and the ideal and actual voltage gains.

The actual voltage gain can be calculated accounting for the effect of the decreasing op amp open loop voltage gain.

Other op amp parameters that should be considered are input offset voltage, input bias current, slew rate, output short circuit current, output voltage swing, supply voltage requirements (single versus dual as well as voltage range), supply current, input impedance, output impedance, signal phase shift, and power dissipation.