Bilateral and Unilateral Circuits: does direction matter?
Reverse the supply. If the circuit behaves identically, it's bilateral.
Take a resistor and connect it to a battery. Now flip the battery around. The current has the same magnitude, just the other way — the resistor doesn't care which way round it sits. Now do the same with a diode: one way it conducts, the other way it blocks. That asymmetry is the entire idea here, and it's what makes rectifiers possible.
The definitions
BILATERAL — the element's V–I characteristic is the SAME in
both directions. Its resistance/impedance does not depend on
the direction of current flow. The V–I curve is symmetric
about the origin.
UNILATERAL — the characteristic DIFFERS with direction.
The element conducts (or amplifies) in one direction only.
⚔️ Bilateral vs Unilateral
BilateralResistor, inductor, capacitor, transmission line. Symmetric V–I. Swapping the terminals changes nothing but the sign of the current.
Test to applyReverse the source polarity. Same current magnitude → bilateral. Different (or zero) → unilateral.
Two independent axes — don't confuse them
Students often merge this with the linear/non-linear distinction, but they classify different things and an element has one label from each pair.
Element Linear? Bilateral?
─────────────────────────────────────────────
Resistor Yes Yes
Ideal inductor Yes Yes
Filament lamp No Yes ← note!
Semiconductor diode No No
BJT / FET No No
The filament lamp is the case that proves the two ideas are independent: it is non-linear (R changes with temperature) but still bilateral (it glows the same whichever way you connect it). Every unilateral element happens to also be non-linear, but the reverse is not true.
Why the diode's asymmetry is useful
A unilateral element is not a defect — it's the whole reason we can convert AC to DC. Feed alternating current into a diode and only the half-cycles of one polarity get through. That's rectification, and it's how every phone charger produces DC from the mains supply. You'll meet this again in Section 4 as "diode as rectifier".
Worked numerical — applying the reverse-the-source test
The test above is stated in words. Here it is done with numbers, which is how an exam will ask for it.
A 10 V source drives a 500 Ω resistor. Find the current, then reverse the source and find it again.
Forward: I = V/R = 10 / 500 = 0.02 A = 20 mA
Reversed: I = V/R = 10 / 500 = 20 mA, flowing the other way
Same MAGNITUDE, opposite direction → BILATERAL
Now replace the resistor with a silicon diode in series with that 500 Ω. A conducting silicon diode drops about 0.7 V.
Forward bias:
Voltage left for the resistor = 10 − 0.7 = 9.3 V
I = 9.3 / 500 = 0.0186 A = 18.6 mA
Reverse bias:
The diode blocks. Only a tiny leakage current flows.
I ≈ 0 A
18.6 mA one way, ~0 the other → UNILATERAL
Notice what the numbers show that the words alone do not. The diode is unilateral and non-linear at the same time. Why non-linear? Because the 0.7 V drop stays fixed and does not scale with the supply — so doubling the source does not double the current. The resistor is bilateral and linear. But as the table above shows, a filament lamp is bilateral yet non-linear, which is why the two labels must be decided separately.
💡 Exam angle: short 2-mark question, often paired with linear/non-linear in the same part. Give the reverse-the-source test, two examples each side, and mention the lamp as the element that is non-linear yet bilateral — that detail shows you understand the two classifications are separate.
Syllabus points
Difference with examples
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