This unit is ≈4% of the A-Level Physics, across 8 lessons. Full syllabus
Lesson 4 of 8 · Options and board-specific extensions
Electronics: semiconductor devices and operational amplifiers
10 min read · about 1 h with practice3 quick checks<1% of the testStretch: Stretch: harder material that separates the top grades
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This lesson covers the analogue half of the AQA Electronics option (7408 sections 3.13.1, 3.13.3 and 3.13.4): discrete semiconductor devices, LC filters and operational amplifiers. The digital and communications half is in Digital Signals, Imaging and Communication Systems. The option is Section B of Paper 3 (35 marks, about 14% of the A-level). Its questions reward exact device behaviour, confident use of the data-booklet equations and clear circuit reasoning.
By the end you’ll be able to
Describe the characteristics and uses of MOSFETs, Zener diodes (as voltage references), photodiodes and Hall sensors
Explain resonance in a parallel LC circuit as a tuned filter and calculate its resonant frequency
Describe ideal op-amp properties and use gain equations for inverting, non-inverting, summing and difference amplifiers
Use an op-amp as a comparator and explain frequency response and gain–bandwidth product
What the exam asks
MOSFET (n-channel, enhancement mode): drain, source and gate; VDS, VGS, and the threshold voltage ; use as a switch and as a device with a very high input resistance.
vii.Check your understanding
3 questions on electronics: semiconductor devices and operational amplifiers. Every option is explained once you answer.
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PromptCard 1 of 3
List the properties of an ideal op-amp.
IDS
Vth
Zener diode: characteristic curve, breakdown voltage and minimum operating current; use with a series resistor as a constant voltage (reference) source.
Photodiode: characteristic and spectral response curves, photoconductive mode, and use with a scintillator.
Hall-effect sensor: uses in attitude monitoring and tachometers (the principle of operation is not required).
LC filters:f0=2πLC1, the mass–spring analogy, the energy response curve and Q=f0/fB.
Op-amps: ideal properties, open-loop Vout=AOL(V+−V−), the comparator, and inverting, non-inverting, summing and difference amplifiers. Real op-amps: saturation, frequency response and gain × bandwidth = constant.
Core ideas
MOSFET (n-channel enhancement)
The gate is separated from the channel by a thin insulating oxide layer, so the gate draws essentially no current: its input resistance is typically above 1012Ω. A sensor, logic gate or op-amp can therefore drive it without being loaded.
Condition
Behaviour
VGS<Vth
off: IDS≈0 (only a tiny leakage current)
VGS>Vth
a channel forms and I rises steeply as increases
VDS above a volt or two
IDS is almost independent of (saturation) and set by
As a switch, the gate is driven well above Vth (for example 5 V against Vth≈2 V). The MOSFET then turns fully on with a very small drain–source resistance, so a lamp, motor or relay in the drain circuit gets almost the full supply pd. Driving the gate to 0 V turns it off.
Zener diode
In forward bias a Zener diode behaves like an ordinary diode (about 0.7 V). In reverse bias almost no current flows until the breakdown voltageVZ. After that, the current rises steeply while the pd stays almost constant at VZ. To stay on this steep part (beyond the “knee”), the current must be at least a minimum operating current, typically a few mA.
Reference voltage circuit: a series resistor R from the supply, with the Zener reverse-biased (cathode to +). A load connected across the Zener gets a steady VZ:
IR=RVs−VZ=IZ+Iload
Choose R so that IZ is still at least the minimum at the largest load current. The Zener dissipates the most power when there is no load: P=VZIR.
Photodiode
In photoconductive mode the photodiode is reverse-biased. Absorbed photons create electron–hole pairs, so the reverse current is proportional to the light intensity, on top of a tiny dark current. Its characteristic curves in the reverse-bias region are almost horizontal lines, evenly spaced for equal steps in intensity. It responds in nanoseconds (an LDR takes milliseconds), so it suits optical-fibre receivers and other optical detectors.
The spectral response curve plots relative response against wavelength. Silicon peaks at around 800–900 nm and cuts off near 1100 nm, where the photon energy falls below the band gap of about 1.1 eV. With a scintillator, each particle or gamma photon produces a flash of light in a crystal. The photodiode turns each flash into a current pulse, so the particles can be counted, and the pulse size indicates the particle’s energy.
Hall-effect sensor
The sensor’s output pd is proportional to the component of magnetic flux density perpendicular to it. Attitude monitoring: sensors on three perpendicular axes measure the components of the Earth’s field and so give the orientation of a satellite, drone or phone. Tachometer: a small magnet on a rotating shaft passes the sensor once per revolution and gives one pulse each time. Then rotation rate = pulse rate ÷ number of magnets.
LC resonance filters
A charged capacitor discharging through an inductor makes energy swap back and forth between the capacitor’s electric field and the inductor’s magnetic field, at
f0=2πLC1
Analogy: inductance ↔ mass, because both oppose a change (in current or velocity). Capacitance ↔ the spring, which stores energy (1/C plays the role of k). Compare f=2π1k/m.
As a filter: a signal is fed through a resistor to a parallel LC circuit. At f0 the LC circuit has its maximum impedance, so the largest pd appears across it. Well below f0 the inductor short-circuits the signal, and well above it the capacitor does. The energy (voltage) response curve has a sharp peak at f0. The bandwidthfB is the width between the 50% energy points, where the voltage is 1/2≈0.71 of its peak. Then
Q=fBf0
A high Q gives a narrow, selective filter. Resistance in the coil dissipates energy and lowers Q. A variable capacitor tunes f0.
The ideal op-amp and the comparator
An op-amp has two inputs, inverting (V−) and non-inverting (V+), one output, and supply connections (usually +Vs, 0 V and −Vs). The ideal op-amp has infinite open-loop gain and infinite input resistance, so no current flows into either input. Open loop:
Vout=AOL(V+−V−)
Because AOL is huge (about 105), even a millivolt difference saturates the output at about 1–2 V inside the supply rails. As a comparator, the output is +Vsat when V+>V− and −Vsat when V+<V−. A typical design compares a sensor’s potential divider with a Zener reference.
Amplifier configurations (negative feedback)
Circuit
Equation (data booklet)
Notes
Inverting
VinVout=−RinRf
output in antiphase; input resistance =Rin
Non-inverting
VinVout
Summing
Vout=−Rf
Difference
Vout=(V+−
Virtual-earth derivation (inverting):V+ is at 0 V. With negative feedback and a huge gain, V+−V−=Vout/AOL≈0, so V−≈0. This is the virtual earth. No current enters the op-amp, so the current through Rin equals the current through Rf:
RinVin−0=Rf0−Vout⇒VinVout=−RinRf
Real op-amps
The output cannot exceed about ±(Vs−1.5V), so large inputs are clipped.
The input resistance is finite (MΩ for a bipolar op-amp, higher for FET inputs), the output resistance is not zero, and the output current is limited to tens of mA.
Frequency response:AOL is about 105 only up to a few hertz and then falls as 1/f. As a result, gain × bandwidth = constant for a given device (for example 1 MHz). Lowering the closed-loop gain widens the bandwidth.
Worked examples
Exam technique
Circuit reasoning chains. For a comparator or MOSFET circuit, write the chain: the sensor resistance changes → the divider pd changes → V+ is compared with V− → the output saturates high or low → VGS is above or below Vth → the device switches.
Check the rails. After any gain calculation, compare the output with the supply. An answer of −17.6 V from a ±12 V op-amp means the output is saturated at about −11 V.
Signs matter: the inverting and summing amplifiers include the minus sign. For a difference amplifier, work out V+−V− in that order.
Read the response curve correctly:fB is measured at 50% of the peak energy, which is 71% of the peak voltage. Check which quantity the graph’s axis shows before choosing the points.
Hall sensors: describe the use only. Explaining the Hall mechanism earns no credit.
Common mistakes
Quick recap
MOSFET: insulated gate, so negligible gate current; off below Vth and fully on (a switch) well above it.
Zener: reverse-biased at VZ above the minimum current; R=(Vs−VZ)/(Iload+.
Photodiode: reverse-biased, current ∝ intensity, fast; spectral cut-off where hf equals the band gap; used with a scintillator for particles.
Hall sensor: pd ∝ B; used for attitude and tachometers; rpm = 60 × pulse rate ÷ magnets.
LC: f0=1/(2πLC; L ↔ mass, C ↔ spring; at the 50% energy points.
Ideal op-amp: infinite AOL and input resistance; comparator open loop; virtual earth in the inverting amplifier.
Real op-amp: saturation near the rails; gain × bandwidth = constant.
1
+
−
V−)
in
−
0
=
Rf0−Vout
DS
VGS
VDS
VGS
=
1+
R1Rf
in phase; very high input resistance; a voltage follower when Rf=0