Application of three basic circuits of amplifier circuit and selection of its parameters.

* Question

Application of three basic circuits of amplifier circuit and selection of its parameters.

* Answer

Three basic amplifier topologies (BJT ↔ MOSFET mapping):

Common-Emitter (CE) ↔ Common-Source (CS): voltage-gain stage

Common-Collector (CC, emitter follower) ↔ Common-Drain (CD, source follower): buffer/impedance translator

Common-Base (CB) ↔ Common-Gate (CG): wideband, low-input-impedance/current-buffer stage

1) CE / CS — Voltage-gain stage

Applications: general analog gain blocks, audio preamps, sensor interfaces, mid-band IF/RF gain (with cascode).
Key traits: high voltage gain, moderate input Z (CE) / high input Z (CS), Miller-limited bandwidth.
Parameter selection:

Bias point: set V_CE or V_DS ≈ 0.4–0.6 × V_CC for max swing; choose I_C/I_D from linearity/noise/power targets.

Transconductance: g_m≈I_C/V_T(BJT), gm≈2I_D/V_ov (MOS).

Gain target: A_v≈−g_m(R_C∥R_L) (or −g_m(R_D∥R_L)). Pick R_C/R_D to meet A_v with headroom.

Emitter/Source degeneration R_E/R_S: improves linearity & temp stability; A_v≈−g_m(R_C∥R_L)/1+g_mR′_E (use un-bypassed part R′_E).

Bias network: divider current ≈ 5–10× base current (BJT); set V_GS for target I_D (MOS).

Caps for low-cutoff f_L: choose C so X_C≤1/10 of the seen resistance at f_L: C≳10/2πf_LR.

Bandwidth: mitigate Miller via smaller gain at first stage, cascode, or CB/CG front-end.

2) CC / CD — Buffer (emitter/source follower)

Applications: impedance matching, level shift, line/headphone drivers, ADC drivers.
Key traits: A_v≈0.9−1, very high input Z, low output Z.
Parameter selection:

Bias for swing: I_Q≳V_pp/(2R_L) to avoid clipping.

Output impedance: Z_out≈(1/g_m)∥R_E (or (1/g_m)∥R_S); pick I_Q (thus g_m) and emitter/source resistor to meet drive spec.

Headroom: ensure V_BE/V_GS margins so device stays in active/saturation region across swing.

Coupling caps: size for target f_L using rule above.

3) CB / CG — Wideband / low-Zin / current buffer

Applications: RF LNA input match (e.g., 50 Ω), cascode upper device, photodiode/TIA front-ends, wideband amplifiers.
Key traits: low input Z (≈1/g_m), good reverse isolation, minimal Miller, wide bandwidth.
Parameter selection:

Match source: set g_m so R_in≈1/g_m≈R_S(e.g., for 50 Ω, I_C≈V_T/50 (≈0.5 mA at 300 K)).

Gain: A_v≈g_m(R_C∥R_L)(or g_mR_D).

Stability/linearity: add small degeneration R_E/ or LC match; ensure bias network is RF-bypassed.

Noise: higher g_m(more bias) lowers input-referred noise but raises power.

Cross-cutting parameter choices

Define specs first: required A_v, Z_in/Z_out, bandwidth, noise, load swing, supply & power.

Check dissipation: P_D≈V_CEI_C or V_DSI_D; heat-sink as needed.

Compensation: if global feedback is used, add small C across feedback or partial degeneration to guarantee phase margin.

Iterate: compute → simulate (AC/TRANS/NOISE) → prototype → trim R/C/bias.