Electronics & Circuits · Study deck

Op-Amp Sensor Front-End Contracts

Picture a greenhouse probe whose voltage changes only a little as the soil dries.

Voltage Vera is your guide for this deck.

electricitysensorfront

Voltage Vera, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Use the negative-feedback op-amp golden rules without applying them outside their valid range.
Choose a buffer, non-inverting amplifier, inverting amplifier, or comparator for a sensor front-end job.
Explain how a unity-gain follower prevents voltage-divider loading errors.
Check output swing, input common-mode range, and supply rails before trusting an amplified signal.

Major section

Start Simple

An amplified number should not become an unchecked act on a heater or pump.

The reading unit cannot use the full change well, so the designer adds an amplifier.
Too much gain can clip the wet end, and the wrong input load can change the probe itself.
This opening does not choose a part or prove the whole instrument.

Major section

Phoebe's Field Notes: Why Gain and Bandwidth Trade, Not Just Correlate

The mathematical gist: Negative feedback sets non-inverting gain $A_v=1+R_f/R_g=1/\beta$ , while a single-pole op-amp supplies about $GBW/f$ open-loop gain.

The loop reaches its bandwidth boundary when $\beta GBW/f=1$ , so $f_{-3dB}=GBW/A_v$ .
A 1 MHz part at gain 100 therefore reaches about 10.0 kHz.

Major section

Two Rules for Op-Amps

The photograph shows a plastic chip body with two rows of metal legs and a printed part mark.

The two rows of legs must be matched to the part's pinout before wiring.
An operational amplifier (op-amp) is a high-gain differential amplifier — it amplifies the difference between its + and − inputs.
An amplifier is the active opposite of an attenuator: it takes energy from a supply and uses it to produce a larger output signal while preserving the input shape as much as the circuit allows.
A package diagram is part of that contract.

Major section

Two Rules for Op-Amps (continued)

Those labels show a UA741 packages the op-amp's differential inputs, high-gain stages, compensation, and output driver into one eight-pin IC; the feedback components outside the package decide how that gain is used.

This connects the visual to two rules for op-amps by making the relevant component behaviour part of the design check.
Specification matching starts with the two voltages you actually have: the board-side signal level and the project-side requirement.
Output slams to a rail depending on which input is higher — an analog threshold detector.
The same triangle symbol can hide very different assumptions.

A photographed uA741C eight-pin op-amp packages differential inputs, gain stages, compensation and output driver; external feedback sets the used gain. Photo: Teravolt, CC BY 3.0

Major section

Sensor Op-Amp Five Checks

Buffer a voltage divider so it stops sagging: Earlier in this chapter a resistor divider's output dropped when a real load drew current.

A unity-gain buffer fixes exactly that: it presents near-infinite impedance to the divider (so it draws no divider current) and near-zero impedance to the load (so it can supply current).
If the source impedance is high, the capacitor steals charge and the conversion reads low or depends on the previous channel.
A buffer, smaller divider resistors, a longer acquisition time, or a local charge reservoir fixes that contract; the right choice depends on current budget and sample rate.

The buffer is the simplest op-amp contract: preserve the sensor voltage while changing the interface strength, so a weak high-impedance node can drive an ADC input, cable, or next stage without being pulled away from its intended value.

Major section

Virtual Short Feedback Limits

Dynamic limits are separate from small-signal bandwidth.

The virtual short is not magic — it is a consequence of enormous open-loop gain plus feedback.
Two real limits bound these circuits.
The math is still Ohm's Law and feedback, but the reference node defines what "zero" means to the circuit.

Major section

Release Checklist

Input common-mode range includes the expected sensor voltage across temperature and tolerance.

Output swing leaves enough headroom below the rails for the chosen op-amp and load.
Closed-loop bandwidth from GBW / gain exceeds the signal bandwidth with margin.
The op-amp can drive the ADC sampling input, load capacitance, cable, or next-stage impedance without instability.

Deck summary

Key takeaways

An amplified number should not become an unchecked act on a heater or pump.

The mathematical gist: Negative feedback sets non-inverting gain $A_v=1+R_f/R_g=1/\beta$ , while a single-pole op-amp supplies about $GBW/f$ open-loop gain.
The photograph shows a plastic chip body with two rows of metal legs and a printed part mark.
Those labels show a UA741 packages the op-amp's differential inputs, high-gain stages, compensation, and output driver into one eight-pin IC; the feedback components outside the package decide how that gain is used.
Dynamic limits are separate from small-signal bandwidth.

Retrieval practice

Recall check 1 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q1With negative feedback and an unsaturated output, what do the two op-amp golden rules say?

AThe inputs draw large current, and the output equals the supply voltage.

BThe gain is exactly 1 because the resistor values cancel for this amplifier configuration.

CThe virtual short holds even with no feedback, as in a comparator.

DThe inputs draw essentially no current, and the output drives until V+ equals V-

Show answer

Answer: D Near-infinite input impedance means no input current, and negative feedback forces V+ = V-.

Retrieval practice

Recall check 2 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q2An op-amp circuit is powered from a single rail. Which check must precede a claim that its full sensor range is readable?

ACheck input common-mode range and loaded output swing against sensor and ADC.

BCheck gain alone as if amplifier input limits could not matter for the sensor.

CAdd software averaging as if it could recover signal peaks clipped in analog.

DAssume amplifier output reaches both supply rails without a measured swing check.

Show answer

Answer: A Rail limits can clip either the input stage or the output stage.

Retrieval practice

Recall check 3 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q3A resistor divider gives the right voltage in theory, but the reading sags whenever it is connected to the load. Which op-amp circuit fixes this without changing the voltage?

AAn inverting amplifier changing voltage rather than buffering the divider output

BUnity-gain buffer isolating the divider from load without changing its voltage

CComparator converting the analog divider output to a two-state digital level

DMake no circuit fix because buffering cannot prevent divider sag under load.

Show answer

Answer: B The follower isolates the divider from the load, so the divider no longer sags.

Retrieval practice

Recall check 4 of 4

Voltage Vera says: answer from memory, then check your reasoning.

Q4You build a non-inverting amplifier with a gain of 100 using an op-amp whose gain-bandwidth product is 1 MHz. Approximately what -3 dB bandwidth do you get?

AAbout 1 MHz because closed-loop gain supposedly leaves bandwidth fixed

BAbout 100 MHz because gain supposedly multiplies the available bandwidth

CUnlimited bandwidth if output voltage remains between the op-amp supply rails

DAbout 10 kHz from the 1 MHz gain-bandwidth product divided by gain 100

Show answer

Answer: D Gain and bandwidth trade through the gain-bandwidth product: at a gain of 100, a 1 MHz-GBW part gives roughly 10 kHz of bandwidth.

Print reference

Answers

Answer key.

D · Near-infinite input impedance means no input current, and negative feedback forces V+ = V-.
A · Rail limits can clip either the input stage or the output stage.
B · The follower isolates the divider from the load, so the divider no longer sags.
D · Gain and bandwidth trade through the gain-bandwidth product: at a gain of 100, a 1 MHz-GBW part gives roughly 10 kHz of bandwidth.