Lesson · Problem 14
Design a Sensor Amplifier
Gain, filtering, and a short feedback loop
You should be able to
- Set non-inverting gain from the feedback divider and a low-pass from an RC.
- Keep the filter capacitor and the supply capacitor as different parts.
Analog · Schematic Reasoning
What it is
A non-inverting amplifier sets gain from the feedback divider. For the ideal model used here, gain is 1 + Rf / Rg when the divider is arranged that way. A series resistor and a capacitor to ground form a low-pass; the cutoff of that educational model is 1 / (2πRC). The feedback trace should be short so it does not pick up other nets.
The signal enters the positive pin. The divider from the output to ground sets the negative pin. Swap those and the stage is no longer the non-inverting circuit this model grades. The filter capacitor is in the signal path. The supply capacitor is across the op-amp rails. Using one part for both jobs changes the gain path and the supply at the same time. The numbers in the challenge are this model’s numbers, not a claim about every op-amp.
Why this matters
A sensor often needs both gain and a limit on how fast the signal is allowed to move before it reaches the next stage.
What you are building
A non-inverting stage with a gain divider and an RC low-pass, plus a short feedback route.
Prerequisites
This lesson uses resistor, supply capacitor, decoupling capacitor, routed length, net, device card. It introduces non-inverting gain, feedback divider, low-pass, RC cutoff.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Op-amp
- What it does
- The training amplifier. The signal enters the positive pin. The divider sets the negative pin.
- Why it is here
- Gain and the filter are the numbers. The feedback route is the layout limit beside them.
- Symbol
- A triangle. + is the signal input in this non-inverting stage.
- Pins
- IN+, IN−, output, and the two supply pins.
- Values
- Gain is 1 + Rf / Rg for the ideal model used here.
- Beginner mistake
- Using the filter capacitor as the supply decoupler.
- What an engineer checks
- Which pin is positive, and the resistor ratio the card asks for.
Resistor
- What it does
- Limits current. The voltage across it divided by its resistance is the current through it.
- Why it is here
- In a series LED it sets the current. Elsewhere it pulls a pin, filters with a capacitor, or sets gain.
- Symbol
- A rectangle, or a zigzag, with two pins.
- Footprint
- Two equal pads. Either end can be pin 1 unless the circuit cares about the label.
- Values
- The tray lists the values this challenge allows. Other values are not graded.
- Beginner mistake
- Picking a nearby value because it looks similar. 100 Ω and 330 Ω are different currents.
- What an engineer checks
- Resistance, tolerance, and power rating. This course grades the resistance it states.
Capacitor
- What it does
- Stores charge and supplies a short pulse of current. With a resistor it also sets a time.
- Why it is here
- A supply capacitor sits at a pin. A filter capacitor sits in the signal path. They are different jobs.
- Symbol
- Two parallel plates. A polarized part adds a plus on one plate.
- Footprint
- Two pads. Polarized parts mark the positive pad.
- Polarity
- A ceramic bypass in this course is not polarized. An electrolytic is. The card says which.
- Values
- 100 nF is the usual local bypass here. 1 µF and 10 µF show up where the card asks for bulk.
- Beginner mistake
- Using the filter capacitor as the supply capacitor, or one capacitor for three supply pins.
- What an engineer checks
- Capacitance, voltage rating, and whether it is polarized. The card states the value that is graded.
Interview lens
Write 1 + Rf / Rg and say which pin is the signal. Then say the filter capacitor is not the decoupler.
What PCBGrade measures
The gain and cutoff of this ideal model, the feedback length, and the decoupler distance. Not every real op-amp.
Where this shows up
A sensor front end is often exactly this: gain, a bandwidth limit, and a supply capacitor that is not the filter.
Terms
- Non-inverting
- The input signal goes to the positive op-amp pin. Gain is set by the divider on the negative pin.
- Low-pass
- A filter that reduces higher frequencies. Here it is an RC, not a complicated active filter unless the card says so.
What is happening electrically
The sensor’s voltage is often too small, or moving faster than the next stage should follow. A non-inverting amplifier takes the signal on the positive pin. The divider from the output to ground sets the negative pin, and for the ideal model here the gain is 1 + Rf / Rg. A series resistor and a capacitor to ground form a low-pass. Higher frequencies see the capacitor as a short and get smaller. The cutoff of this educational model is 1 / (2πRC).
Why the geometry matters
The feedback trace is part of the gain. A long loop picks up other nets and adds delay the ideal equation ignores. The filter capacitor belongs in the signal path. The supply capacitor belongs across the op-amp rails. They are different pieces of copper. The supply still needs its own short loop, the same idea as the decoupling lesson.
How an engineer reasons
Set the gain from the divider, then set the cutoff from R and C, then look at the copper. If you only do the arithmetic, a long feedback route can undo a correct schematic. If you only tidy the layout, the wrong resistor still sets the wrong gain.
Worked example
Gain near 3 and a low-pass near 995 Hz come from the resistor and capacitor values the rules list. Feedback length and the decoupler distance are layout limits beside the schematic values.
Good and bad
Rf and Rg that make a gain near 3, an RC near the stated cutoff, a short feedback path, and a separate supply capacitor: both the numbers and the loop are right. Moving the filter capacitor onto the supply pins empties the filter and puts the wrong part on the rails. Swapping the input onto the negative pin is no longer the non-inverting stage this model grades.
Common mistake
Using the filter capacitor as if it were the supply decoupler, or closing feedback with a long route past a noisy net.
Where this rule stops
The gain and cutoff formulas are for this ideal model. A real op-amp has bandwidth, offset, bias current, and output swing. The challenge does not claim those are zero on every amplifier. It does claim that the filter and the decoupler are different jobs.
Before the challenge
The challenge asks for a gain and a cutoff from the values you place, plus a short feedback route. Keep the filter capacitor and the supply capacitor as different parts.
Ready for the challenge
You can place Rf and Rg, put the filter in the signal path, and keep feedback short.
Question 1 of 2