Linear (analogue) integrated circuits
Logic ICs deal in 1s and 0s. Linear ICs handle signals that vary continuously — a sensor voltage, an audio signal, a servo error. The family includes operational amplifiers, comparators, voltage regulators (78xx fixed positive, 79xx fixed negative, LM317 adjustable), audio and power amplifiers, instrumentation amplifiers for strain gauges and bridges, and mixed-signal parts such as the 555 timer and analogue-to-digital converters.
On an aircraft they appear everywhere signals are measured or conditioned: amplifying thermocouple and pressure-transducer outputs, servo amplifiers in autopilots, interphone and audio amplifiers, active filters, threshold detectors for overheat and overspeed warnings, and the regulators that power every circuit card in an LRU.
The operational amplifier
An operational amplifier (op-amp) is a very high-gain, direct-coupled amplifier with:
- two inputs — the inverting input (−) and the non-inverting input (+);
- one output;
- power-supply connections, usually a dual supply such as ±15 V, which are often left off circuit diagrams.
It amplifies the difference between its inputs: Vout = AOL × (V+ − V−), where AOL is the open-loop gain. A signal on the − input appears inverted at the output; a signal on the + input appears in phase. The output cannot go beyond the supply rails. When the gain demands more, the output saturates about 1–2 V inside them.
Treat the op-amp as ideal: infinite gain, infinite input impedance (no current flows into the inputs), zero output impedance, infinite bandwidth and no offset. A real 741 has a gain of about 200 000, an input resistance of about 2 MΩ and an output resistance of about 75 Ω — close enough. With negative feedback, two rules then solve almost every circuit: (1) no current flows into either input; (2) the output does whatever is needed to make V− = V+.
The classic 741 in an 8-pin DIP has the inverting input on pin 2, non-inverting pin 3, output pin 6, supplies on pins 7 (V+) and 4 (V−), and offset-null adjustment on pins 1 and 5. Most later single op-amps keep the same pinout.
Open loop: the comparator
Used with no feedback, the op-amp's enormous gain means that even a microvolt of difference between the inputs drives the output into saturation: +Vsat when V+ > V−, and −Vsat when V+ < V−. The circuit therefore compares two voltages and gives a yes/no answer. That makes level detectors, zero-crossing detectors, and overheat or overspeed thresholds that switch on a warning. Dedicated comparator ICs are optimised for this job. Adding a little positive feedback gives hysteresis (a Schmitt trigger), which stops the output chattering when the input hovers near the threshold.
Closed loop: amplifiers with negative feedback
The open-loop gain is huge and varies from chip to chip and with temperature. Negative feedback — feeding part of the output back to the inverting input — trades that unruly gain for a smaller, precise one, set only by resistors.
- Inverting amplifier: the input goes through Rin to the − input, with Rf from the output back to the − input and the + input grounded. Gain = −Rf ÷ Rin; the minus sign means the output is inverted (180°). Because the + input is at 0 V, rule 2 holds the − input at 0 V too — a virtual earth. Example: Rf = 100 kΩ and Rin = 10 kΩ give a gain of −10, so 0.2 V in gives −2 V out.
- Non-inverting amplifier: the input goes to the + input, and the − input is fed from a divider (Rf and R1) across the output. Gain = 1 + Rf ÷ R1, in phase, with a very high input impedance. Example: Rf = 90 kΩ and R1 = 10 kΩ give a gain of 10.
- Voltage follower: the output is tied straight to the − input and the signal goes to the + input. Gain = 1, with very high input impedance and very low output impedance. It is a buffer that lets a weak, high-impedance source (a sensor, a potentiometer wiper) drive a load without being dragged down.
- Summing amplifier: several input resistors feed the − input of an inverting amplifier. Vout = −Rf(V1/R1 + V2/R2 + …), which adds and scales signals.
- Differential amplifier: amplifies the difference between two inputs and rejects anything common to both (common-mode rejection). It is ideal for bridge sensors and for long sensor wiring that picks up noise.
B2 extension: integrators, differentiators and feedback
B1 stops at the description and operation of linear circuits. The following belongs to the B2 syllabus but completes the picture.
- Integrator — a capacitor replaces Rf. The output is proportional to the (inverted) integral of the input over time, so a square wave becomes a triangle. It is used in ramp generators, analogue computing and the "I" term of PID controllers (Chapter 8).
- Differentiator — a capacitor replaces Rin. The output is proportional to the rate of change of the input, so a triangle becomes a square wave. It is very sensitive to noise.
- Negative feedback — advantages: stable, predictable gain, wider bandwidth, less distortion and noise, and controlled input and output impedances. Disadvantages: less gain, and if phase shift at high frequency turns it positive, the amplifier can oscillate.
- Positive feedback — part of the output fed back in phase. Advantages: it produces oscillators and snap-action switching with hysteresis (Schmitt triggers). Disadvantages: instability and distortion in an amplifier.
- Coupling amplifier stages — RC (resistive-capacitive) coupling blocks DC between stages; transformer (inductive) coupling also matches impedances; direct coupling passes DC and very low frequencies, and is how op-amps are built inside.