BJT as an amplifier (common emitter) — Hinglish notes

2.4.5 · Hardware › Transistors: BJT & FET

Common-emitter amplifier KYA hota hai?

Active region KYU chahiye? Sirf wahan $I_{C} = β I_{B}$ cleanly hold karta hai. Cutoff mein modulate karne ke liye koi current nahi hoti; saturation mein collector "room se bahar" ho jaata hai aur gain collapse ho jaata hai. Hum Q-point (quiescent point) ke beech mein baithna chahte hain taaki AC signal bina clipping ke upar aur neeche swing kar sake.

Ye KAISE kaam karta hai — gain scratch se derive karna

Step 1 — DC bias (operating point)

Output loop ke around Kirchhoff's Voltage Law (KVL) apply karo: $V_{C C} = I_{C} R_{C} + V_{C E}$

Ye step KYU? Supply voltage ko $R_{C}$ ke across drop aur transistor ke across bache hue voltage ke beech share hona chahiye. Yahi load line hai.

$I_{C}$ choose karne ke liye, fixed base voltage $V_{B}$ ke saath base loop pe KVL: $V_{B E} = 0.7 V, I_{B} = \frac{V _{B} - 0.7}{R _{B}}, I_{C} = β I_{B}$

Step 2 — Transconductance (dil ki baat)

Collector current, $V_{B E}$ pe exponentially depend karta hai (Ebers–Moll): $I_{C} = I_{S} e^{V_{B E} / V_{T}}, V_{T} = \frac{k T}{q} \approx 26 mV at room temp$

Differentiate karo ye jaanne ke liye ki $V_{B E}$ mein choti si change se $I_{C}$ kitna change hota hai: $g_{m} \equiv \frac{d I _{C}}{d V _{B E}} = \frac{I _{S}}{V _{T}} e^{V_{B E} / V_{T}} = \frac{I _{C}}{V _{T}}$

Step 3 — Small-signal voltage gain

Output voltage (AC part). KVL: $v_{o u t} = V_{C C} - I_{C} R_{C}$ . Signal ke liye sirf changing part matter karta hai, aur $V_{C C}$ constant hai, isliye: $v_{o u t} = - R_{C} i_{c} = - R_{C} (g_{m} v_{b e}) = - g_{m} R_{C} v_{in}$

Minus sign KYU hai (gehra reason): Zyada base voltage → zyada $I_{C}$ → $R_{C}$ ke across bada drop → collector node ground ki taraf neeche khicha jaata hai. Rising input ⇒ falling output.

Figure — BJT as an amplifier (common emitter)

Emitter degeneration (practical CE)

Agar emitter mein $R_{E}$ add karo, toh gain stabilized ho jaati hai aur wobbly $g_{m}$ pe kam dependent hoti hai: $A_{v} \approx - \frac{R _{C}}{R _{E}} (when g_{m} R_{E} ≫ 1)$ KYU: emitter resistor negative feedback deta hai — $I_{C}$ mein rise se $V_{E}$ badhta hai, jo $V_{B E}$ ko reduce karta hai, rise ko oppose karta hai. Tum gain ko stability aur linearity ke liye trade karte ho.

Worked examples

Common mistakes

Active recall

Recall Feynman: 12-saal ke bacche ko samjhao

Socho ek garden tap jahan tumhari ek choti si ungli ki twist se paani ka bada flow khulta hai. Ungli ki twist choti base signal hai; bada paani ka flow collector current hai. Us flow ko "voltage picture" mein badalne ke liye, hum paani ko ek narrow pipe ( $R_{C}$ ) se push karte hain aur pressure measure karte hain. Zyada paani push karo → zyada pressure drop → doosra sira neeche sarakta hai. Toh jab tum input ko upar push karte ho, output neeche jaata hai — isliye amplifier signal ko flip karta hai. Aur kyunki ek choti twist se bada flow control hota hai, output pe picture tumhari tiny ungli-wiggle ki ek giant copy hoti hai.

Flashcards

Input aur output ke beech emitter share karne wala configuration kaun sa hai?

Common-emitter

Transconductance

g_{m}

ka formula?

g_{m} = I_{C} / V_{T} \approx I_{C} /26 mV

Small-signal CE voltage gain?

A_{v} = - g_{m} R_{C} = - I_{C} R_{C} / V_{T}

CE gain mein minus sign KYU hota hai?

Zyada

I_{C}

→

R_{C}

ke across bada drop → collector node neeche khicha jaata hai → output inverted (180°)

Room temperature pe

V_{T}

kya hota hai?

\approx 26 mV

(

= k T / q

)

Emitter degeneration ke saath CE gain (strong feedback)?

A_{v} \approx - R_{C} / R_{E}

R_{E}

KYU add karte hain?

Negative feedback →

β

/temperature ke against gain stabilize karta hai, linearity improve karta hai (gain ko stability ke liye trade karta hai)

Amplify karne ke liye BJT ko kis region mein operate karna chahiye?

Active region (

V_{B E} \approx 0.7

V forward, base–collector reverse)

CE output loop ke liye load-line KVL?

V_{C C} = I_{C} R_{C} + V_{C E}

Maximum symmetric swing ke liye Q-point kahan hona chahiye?

Mid-supply ke paas (

V_{C E} \approx V_{C C} /2

)

β

aur

A_{v}

mein kya fark hai?

β

= device ka DC current gain;

A_{v}

= small-signal voltage gain

= - g_{m} R_{C}

Connections

BJT operating regions (cutoff, active, saturation)
Ebers–Moll model
Small-signal hybrid-pi model
Load line analysis
Negative feedback in amplifiers
FET as an amplifier (common source) — field-effect analogue
Thermal voltage $V_T=kT/q$