Analog Circuits & EDC
Diodes and rectifiers, BJT and MOSFET biasing and small-signal models, op-amp circuits, feedback and oscillators.
About this subject
Analog Circuits covers the devices and circuits that handle continuous signals: diodes and rectifiers, bipolar and MOS transistors, single-stage and multistage amplifiers, feedback, operational amplifiers, oscillators and comparators.
Nearly every amplifier question follows the same two steps. First find the DC bias point; then use it to get the small-signal parameters such as $g_m$ and $r_\pi$, and analyse the small-signal circuit. Keeping those two steps separate is the most reliable habit in the subject.
Solved questions
Each question is a practice problem written for this portal at GATE standard. Try it first, then open the full derivation, and read the note on the idea behind it.
- NAT Hysteresis width of a non-inverting op-amp Schmitt trigger Comparators · 2 marks
- NAT Transconductance of a saturated NMOS from its bias point MOSFET small signal · 1 mark
- NAT Mid-band voltage gain of a common-emitter amplifier BJT amplifiers · 1 mark
- NAT Average output voltage of a full-wave bridge rectifier Diode circuits · 2 marks
Key formulas
The results these questions lean on most. Know what each symbol means, and when each one applies.
| What | Formula |
|---|---|
| BJT transconductance | $g_m = I_C / V_T$, with $V_T \approx 25$ mV at room temperature |
| BJT input resistance | $r_\pi = \beta / g_m$ |
| MOSFET in saturation | $I_D = \tfrac{1}{2}\mu_n C_{ox}\tfrac{W}{L}V_{OV}^2$, $\;g_m = 2I_D / V_{OV}$ |
| Common-emitter gain | $A_v = -g_m R_C$ with the emitter bypassed |
| Full-wave bridge (ideal) | $V_{dc} = 2V_m / \pi$; with real diodes subtract two drops from $V_m$ |
| Inverting op-amp | $A_v = -R_f / R_{in}$ |
| Non-inverting op-amp | $A_v = 1 + R_f / R_{in}$ |
| Non-inverting Schmitt trigger | thresholds $\pm V_{sat} R_1 / R_2$, width $2V_{sat}R_1/R_2$ |
How to prepare
- Never mix up BJT and MOSFET small-signal formulas: $g_m = I_C/V_T$ for one, $2I_D/V_{OV}$ for the other.
- Always solve DC bias before AC; the small-signal parameters depend on it.
- For op-amp circuits, apply the two golden rules (no input current, equal input voltages) with negative feedback, and saturation reasoning without it.
- Draw the conduction path for each half cycle in rectifier questions to count the diode drops.
Common traps
Each of these costs marks every year. The questions above are built to catch them.
- Using the thermal voltage in a MOSFET transconductance.
- Letting $\beta$ into the common-emitter voltage gain.
- Using the inverting Schmitt-trigger thresholds for a non-inverting one.
- Subtracting one diode drop instead of two for a bridge rectifier.