Op Amps (E7G12)
The 2020-2024 Extra License question pool wants to ensure you know a thing or two about operational amplifiers, an analog integrated circuit introduced in the General License Course:
E7G12: What is an operational amplifier?
A. A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance
B. A digital audio amplifier whose characteristics are determined by components external to the amplifier
C. An amplifier used to increase the average output of frequency modulated amateur signals to the legal limit
D. A RF amplifier used in the UHF and microwave regions
Many questions about various types of amplifiers are in the Extra Class question pool, and this one refers specifically to the ubiquitous operational amplifier, or simply “op amp.” Op amps are widely used in electronic devices, and they have broad application as low-distortion signal amplifiers, as fundamental components in oscillators, as amplifiers for precision measurement instruments, and much more. Here, we will provide just a surface-scratch overview of op amp characteristics to address question E7G12.
The name “operational amplifier” comes from the fact that devices of this design were originally used in analog computers to help perform mathematical operations such as addition, multiplication, square root extraction, and more. The schematic symbol is a triangle with two signal inputs on one side (sometimes shown as a curved side), usually labelled “+” and “–,” and one output at the opposing corner to the input side. Sometimes the +V and –V terminals where powering voltage is provided are depicted on the other two sides of the triangle. Op amps are typically packaged as small integrated circuit chips for surface mounting or through-the-hole mounting techniques. A single op amp chip may contain several individual op amps of a particular design.
Internally, the op amp is basically a three-stage amplifier circuit using multiple transistors. The three stages are: 1) a differential stage that makes a comparison of the two input voltage signals and outputs the amplified difference value, 2) a gain stage that receives the differential stage output and amplifies it further, and 3) an (optional) output buffer stage that may serve as a “power amplifier” stage for driving large capacitive or resistive loads and for providing a low output impedance.
At its heart, the op amp is a differential amplifier (stage 1), boosting the voltage difference that occurs between its two input terminals. The ‘+’ terminal is a non-inverting input, while the ‘–‘ terminal is a signal inverting input. The summation of the two yields a difference value to be amplified. The input impedance of an op amp is made to be very high so that very little current flows in the signal inputs, and thus any preceding circuit providing the input signal has very little load imposed upon it and very little power is consumed.
Unlike other signal boosting amplifier circuits, including the popular common emitter arrangement, the op amp circuit uses no coupling capacitors at the input or output terminals of its transistors. The direct coupling method allows the op amp to work with DC voltages as well as AC.
The voltage gain of an operational amplifier in an “open loop” configuration (no output feedback provided) is typically quite high, in the range of 10,000 to 100,000, and some may exceed 1,000,000 (120 dB). For most circuit application a closed loop configuration is used in which feedback to an input terminal is provided from the output. Closed loop gain values are much lower than open loop and determined by the values of resistors inserted in the feedback circuit.
Here is one example of a closed loop amplifier circuit using negative feedback – that is, feedback from the output is routed to the inverting (–) input via resistor R2. The input signal is also routed to the inverting input via resistor R1. The non-inverting (+) input is tied to ground voltage. The gain of this op amp circuit may be calculated as Av= -R2/R1. The negative sign in this formula indicates that the signal is inverted as compared to the input signal.
Given the resistance values:
R1 = 10 Ω R2 = 470 Ω
The magnitude of the gain (ignoring the inversion) is
|AV|= 470 Ω / 10 Ω = 47 [see question E7G07]
Some questions in the Extra Class pool require you to calculate the gain of the inverting amplifier circuit of Exam Figure E7-3 with different assigned values of RF and R1, as shown in the figure.
The answer to 2016-2020 Extra Class question E7G12, “What is an integrated circuit operational amplifier?” is “A. A high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance.”
-- Stu WØSTU