ESECE common mistakes
ESECE · Understand the traps and how to avoid them.
When analyzing a circuit with dependent sources, the candidate applies superposition to the dependent source itself, treating it as an independent source and setting it to zero when another source is deactivated.
Superposition is taught as a universal method for linear circuits, and candidates memorize the procedure 'turn off all sources except one' without internalizing the condition that only independent sources may be deactivated; dependent sources are controlled by circuit variables and must remain active in every subproblem.
What to do instead
Before applying superposition, explicitly classify every source as independent or dependent. For a dependent source, never set it to zero; instead, keep it in the circuit and express its value in terms of the subproblem's variables. Practice by solving a circuit with one independent and one dependent source both by superposition and by nodal analysis, and verify that the results match. Drill a checklist: (1) list independent sources, (2) list dependent sources, (3) solve one subproblem per independent source with all dependent sources left in place, (4) sum the responses.
When computing the small-signal voltage gain of a BJT or MOSFET amplifier, the candidate uses the hybrid-pi model but ignores the Early effect (r_o) or the body effect, and then is surprised when the calculated gain does not match the simulated or expected value.
Textbook introductory examples often set r_o to infinity for simplicity, and candidates carry that simplification into every problem; they also forget that the body terminal of a MOSFET is not always at the source potential, especially in stacked or source-degenerated stages.
What to do instead
Adopt a habit: after drawing the small-signal model, ask whether the problem statement gives V_A (Early voltage) or λ (channel-length modulation). If it does, include r_o in parallel with the dependent source and recompute. For MOSFETs, check whether the body is tied to the source; if not, add g_mb·v_bs. Work through a common-source stage with and without r_o and compare the gains symbolically-seeing the ratio 1 + g_m·(R_D∥r_o) makes the omission memorable.
When simplifying a Boolean expression or designing a combinational circuit, the candidate treats XOR and XNOR as if they obeyed the same distributive and associative laws as AND and OR, for example writing A ⊕ (B ⊕ C) = (A ⊕ B) ⊕ C incorrectly or distributing XOR over AND.
The algebra of XOR is less familiar than ordinary Boolean algebra, and candidates extrapolate rules from AND/OR without verifying them; they also forget that XOR is not distributive over AND in the same way OR is, and that XNOR is not associative in the same manner as XOR.
What to do instead
Memorize the valid identities: XOR is associative and commutative, A ⊕ 0 = A, A ⊕ 1 = A′, A ⊕ A = 0, and A ⊕ A′ = 1. Use truth tables to verify any identity you are unsure of before applying it. For circuit design, convert XOR-heavy expressions to sum-of-products or use Karnaugh maps with XOR-aware grouping, and always check the final expression against a truth table for all input combinations.
When determining the Fourier series or Fourier transform of a signal, the candidate forgets to check whether the signal is even or odd, and consequently includes both sine and cosine terms (or both real and imaginary parts) when symmetry would eliminate half of them.
Candidates focus on the integration formula and mechanical evaluation, overlooking the symmetry properties that simplify the result; they also confuse the symmetry of the signal with the symmetry of the transform and apply the wrong shortcut.
What to do instead
Before integrating, sketch the signal and test f(−t) = f(t) (even), f(−t) = −f(t) (odd), or half-wave symmetry f(t ± T/2) = −f(t). For an even signal, all sine coefficients are zero; for an odd signal, all cosine coefficients are zero; for half-wave symmetry, only odd harmonics are present. Practice by computing the series for a square wave and a triangular wave both with and without using symmetry, and confirm that the symmetry-based result is a subset of the general result.
When solving a transmission-line problem, the candidate computes the reflection coefficient at the load and then uses it directly at a different point on the line without accounting for the phase shift 2βl, or mixes up the sign convention for the direction of propagation.
The reflection coefficient transforms as Γ(d) = Γ_L·e^{−j2βd} (or e^{+j2βd} depending on the distance convention), and candidates often treat Γ as constant along a lossless line; they also confuse the electrical length in degrees with the physical length and forget that β = 2π/λ.
What to do instead
Write the transformation explicitly every time: choose a coordinate system, state whether d is measured from the load or from the generator, and apply Γ(d) = Γ_L·e^{−j2βd} with the correct sign. Convert physical length to electrical length using βl = 2πl/λ, and keep λ in the same units as l. Verify with a Smith chart: moving toward the generator rotates clockwise by 2βl. Do a sanity check on a quarter-wave line, where Γ should invert sign.
When analyzing a PN-junction or MOS capacitor, the candidate applies the depletion approximation outside its valid range-for example, using the square-root depletion-width formula under forward bias or high injection-and obtains a physically impossible result such as a depletion width that grows without bound.
The depletion approximation is derived under the assumption that mobile carrier concentrations are negligible in the depletion region, which fails under forward bias and high-level injection; candidates memorize the formula without its assumptions and apply it mechanically.
What to do instead
State the assumptions before using the depletion approximation: reverse bias or small forward bias, low-level injection, and abrupt or linearly graded junction as specified. For forward bias beyond a few kT/q, switch to the diffusion-capacitance model and the Shockley diode equation. For MOS capacitors, remember that the depletion approximation fails in accumulation and inversion; use the appropriate regime-specific equations. Practice by plotting depletion width versus bias and identifying where the curve departs from the square-root law.
When computing the bit error rate or symbol error rate for a digital modulation scheme, the candidate uses the wrong Q-function argument-for example, omitting the factor of 2 in Q(√(2E_b/N_0)) for BPSK or using E_b/N_0 where E_s/N_0 is required.
The arguments differ by scheme and by whether the noise is expressed per bit or per symbol; candidates memorize one formula and apply it to all schemes, and they confuse the energy per bit E_b with the energy per symbol E_s, especially for M-ary schemes where E_s = (log2 M)·E_b.
What to do instead
Build a table of BER/SER expressions for BPSK, QPSK, M-PAM, M-QAM, and M-FSK, and for each entry note the argument and the prefactor. Always convert given quantities to the required energy measure first: E_s = (log2 M)·E_b, and N_0 = kT·F for noise. For M-QAM, use the exact expression or the standard approximation with the correct 4(1−1/√M) prefactor. Verify by checking the limiting case M = 2, where the expression should reduce to the BPSK result.
When applying the Routh–Hurwitz criterion, the candidate encounters a row of all zeros or a zero in the first column and either stops or divides by zero instead of applying the auxiliary-polynomial or epsilon-substitution procedure.
The standard Routh array construction is taught for the generic case, and special cases are often skipped or memorized as isolated tricks; candidates do not recognize that a zero first-column entry signals either a pair of imaginary-axis roots or a sign change, and they mishandle the all-zero row that indicates symmetry.
What to do instead
Memorize the two special-case procedures: (1) if the first column has a zero but the row is not all zeros, replace the zero with a small positive ε and continue, then take the limit ε→0; (2) if an entire row is zero, form the auxiliary polynomial from the row above, differentiate it, and use the derivative coefficients to replace the zero row. After constructing the array, count sign changes in the first column to determine right-half-plane poles, and check the auxiliary polynomial's roots to identify imaginary-axis poles. Practice with polynomials such as s^4 + s^3 + 2s^2 + 2s + 3 and s^3 + 2s^2 + s + 2.
When writing an assembly-language routine for a microprocessor, the candidate forgets that the stack pointer must be initialized before any call or push, or assumes that a subroutine preserves registers that the calling convention does not guarantee.
In coursework, simulators sometimes pre-initialize the stack pointer, so candidates never see the failure; in real hardware or stricter simulators, an uninitialized stack pointer causes a crash or memory corruption. Similarly, candidates assume all registers are preserved across calls, which is not true for caller-saved registers.
What to do instead
At the start of every program, explicitly initialize the stack pointer to a valid RAM location, and document the calling convention in comments: list which registers are caller-saved and which are callee-saved. In every subroutine, push any callee-saved registers you modify and pop them before returning. Practice by writing a small program with a nested subroutine call and single-stepping through a simulator while watching the stack pointer and the stack contents.
When solving a power-system load-flow or fault problem, the candidate uses the per-unit system but forgets to convert all impedances to a common base, or mixes the voltage base on the two sides of a transformer without applying the turns ratio.
Per-unit conversion is mechanical and error-prone; candidates compute the per-unit impedance on the equipment's own base and then use it directly on the system base, ignoring the (V_old/V_new)^2 · (S_new/S_old) conversion factor. Transformer base voltages on the two sides are related by the turns ratio, and candidates often use the same voltage base on both sides.
What to do instead
Adopt a strict procedure: (1) choose S_base and V_base for one zone; (2) propagate V_base across each transformer using its turns ratio; (3) convert every impedance to the system base using Z_pu,new = Z_pu,old · (V_old/V_new)^2 · (S_new/S_old); (4) verify that all per-unit voltages are near 1.0 in normal operation. For fault analysis, draw the sequence networks and confirm that the base quantities are consistent in each sequence network. Practice with a two-transformer system and check the per-unit voltage profile before and after conversion.