Engineering with Mr KellySQA / QS National 5 & Higher revision
National 5Higher
Electronics & control · Higher

Operational amplifiers

7 cards · 4 quiz · 0 exam

Engineering Science · Higher · mrkellydt.co.uk

What you're learning in this topic

Learning intention WALT

We are learning about operational amplifiers, specifically to:

know

  • inverting, non-inverting, summing, difference and comparator op-amp circuits

understand

  • how a comparator switches an output

be able to

  • calculate gain and output voltage, including saturation at about 85% of supply
  • design an op-amp circuit with suitable resistor values

Success criteria WILF

  • I can identify inverting, non-inverting, summing, difference and comparator op-amp circuits
  • I can calculate gain and output voltage, including saturation at about 85% of supply
  • I can design an op-amp circuit with suitable resistor values
  • I can describe how a comparator switches an output

Op-amp basics HIGHER

An op-amp amplifies the difference between its two inputs: inverting (−) and non-inverting (+). It is powered from a supply, and its output can't exceed the supply: in SQA questions the output saturates at about 85% of the supply (e.g. 0.85 × 5 = 4.25 V).

−+VoutRinVinRf0 VInverting amplifier: Vout = −(Rf ÷ Rin) × Vin
The feedback resistor Rf from the output to the inverting input sets the gain. The output is inverted and saturates at about 85% of the supply.

Configurations and formulae HIGHER

ConfigurationOutputUse
InvertingVout = −(Rf ÷ Rin) × VinAmplify and invert a signal
Non-invertingVout = (1 + Rf ÷ R1) × VinAmplify without inverting
ComparatorOutput high (saturated) when V+ > V−, otherwise lowSwitch at a set level: sensor divider vs reference divider
Difference amplifierVout = (Rf ÷ R1) × (V2 − V1)Amplify the difference between two signals, e.g. a strain-gauge bridge
Summing amplifierVout = −Rf × (V1/R1 + V2/R2 + …)Add several signals, e.g. a mixer or digital-to-analogue converter
Voltage followerVout = VinBuffer: isolates a sensor from the load

Gain = Vout ÷ Vin. Watch for saturation: if the formula gives more than 85% of the supply, the output saturates.

Windows and two comparators HIGHER

Two comparators with reference voltages from a divider chain (e.g. 1.2 kΩ, 3.3 kΩ, 1.2 kΩ on 5 V give 1.1 V and 3.9 V) can switch a motor on only when the input is between the two references. Describe what each op-amp, transistor or MOSFET and the motor do below, between and above the window.

Key terms

Inverting amplifierVout = −(Rf/Rin) × Vin
Non-inverting amplifierVout = (1 + Rf/R1) × Vin
Summing amplifierVout = −Rf × (V1/R1 + V2/R2 + …)
Difference amplifierVout = (Rf/R1) × (V2 − V1)
Voltage followerVout = Vin; used as a buffer.
ComparatorOutput high when V+ > V−, otherwise low.
SaturationOutput limited to about 85% of the supply.

Quick check

1. Rf = 100 kΩ, Rin = 10 kΩ, Vin = 0.2 V (inverting). Vout is…

  1. −2 V
  2. 2 V
  3. 0.02 V
  4. −0.2 V
Show answer
A: −2 V. −(10) × 0.2.

2. Which configuration is used as a buffer?

  1. Summing amplifier
  2. Voltage follower
  3. Comparator
  4. Inverting amplifier
Show answer
B: Voltage follower. Gain of 1.

3. A comparator output goes high when…

  1. V− > V+
  2. Both are equal
  3. V+ > V−
  4. Supply is off
Show answer
C: V+ > V−. Non-inverting input higher.

4. On a 12 V supply, an op-amp saturates at about…

  1. 12 V
  2. 0.7 V
  3. 6 V
  4. 10.2 V
Show answer
D: 10.2 V. 85% of 12.

Printable worksheet

An I do, we do, you do worksheet for this topic, with a separate answer sheet.

Worksheet (PDF)Answers (PDF)

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