Bipolar junction transistors and biasing notes — Unit 2

The Bipolar Junction Transistor (BJT) is arguably the most important invention of the 20th century. It is a 3-terminal semiconductor device that can act as either a microscopic switch or an electronic amplifier.

⇒Why 'Bipolar'?

It is called Bipolar because the conduction of electric current inside the transistor is due to TWO types of charge carriers simultaneously: Electrons (negative) and Holes (positive).

⇒Construction Details

A BJT consists of three sandwiched layers of doped silicon, forming two separate P-N junctions back-to-back.

Depending on how the three layers are stacked, we get two completely different types of transistors, operating on opposite electrical polarities.

⇒The NPN Transistor

A thin P-type Base is sandwiched between two N-type blocks (Emitter and Collector). The majority charge carriers are free electrons.

• It is vastly more popular in industry than PNP because electrons move much faster than holes, making NPN transistors capable of switching at much higher speeds.

⇒The PNP Transistor

A thin N-type Base is sandwiched between two P-type blocks. The majority charge carriers are holes.

⇒The Schematic Symbols

To tell them apart on a circuit diagram, look at the arrow on the Emitter leg. The arrow always points in the direction of conventional current flow (from P to N).

Because a BJT has two P-N junctions (Emitter-Base junction and Collector-Base junction), and each junction can be either Forward Biased (FB) or Reverse Biased (RB), the transistor has four distinct mathematical modes of operation.

⇒The Transistor as a Switch

In digital computers (CPUs), transistors are only ever slammed back and forth between Cut-off (Output = 0) and Saturation (Output = 1). They never linger in the Active region.

⇒The Transistor as an Amplifier

To amplify a weak microphone signal into a loud speaker signal, the BJT is locked into the Active Region. A tiny change in Base current creates a massive, proportionally identical change in Collector current.

To understand how amplification occurs, we must trace the exact flow of electrons inside an NPN transistor biased in the Active Region (E-B Forward, C-B Reverse).

⇒Step-by-Step Mechanism

• 1. Injection: The Forward bias on the E-B junction pushes a massive tsunami of electrons from the heavily doped N-type Emitter into the P-type Base.
• 2. The Trap: The Base is P-type, meaning it is full of 'Holes'. Normally, electrons would fall into these holes and exit through the Base wire. BUT, the base is intentionally manufactured to be extremely thin and lightly doped.
• 3. The Base Current (IB): Because there are so few holes, only about 2% to 5% of the injected electrons manage to fall into a hole. This tiny trickle becomes the Base Current (IB).
• 4. The Sweep: The remaining 95% to 98% of electrons have too much momentum and overshoot the thin base. They instantly hit the Reverse-biased Collector-Base junction. The intense electric field of the reverse bias grabs them and sweeps them violently into the Collector. This massive flow becomes the Collector Current (IC).

⇒The Fundamental Current Equation

A transistor has 3 terminals, but an electrical circuit requires 4 terminals (2 for input, 2 for output). Therefore, one terminal MUST be made common to both the input and the output circuits.

⇒1. Common Base (CB) Configuration

• Input is applied between Emitter and Base. Output is taken across Collector and Base.
• Provides Voltage Gain, but NO Current Gain (Output current IC is slightly less than input current IE).
• Primarily used in high-frequency RF applications.

⇒2. Common Emitter (CE) Configuration

• Input applied to Base. Output taken from Collector. Emitter is grounded.
• Provides massive Voltage Gain AND massive Current Gain. Therefore, it provides the highest overall Power Gain.
• This is the universally standard configuration for almost all amplifier circuits.

⇒3. Common Collector (CC) Configuration

• Input applied to Base. Output taken from Emitter. Collector is connected to Vcc.
• Also known as an 'Emitter Follower'. Voltage gain is exactly 1 (Output voltage perfectly copies the input).
• Used strictly for Impedance Matching (High input impedance, extremely low output impedance).

We define mathematical ratios to measure exactly how much a transistor amplifies the current. These are Alpha (α) and Beta (β).

⇒Alpha (α) — Common Base Current Gain

The ratio of Collector current (Output) to Emitter current (Input) in a CB configuration.

⇒Beta (β) — Common Emitter Current Gain

The ratio of Collector current (Output) to Base current (Input) in a CE configuration.

⇒The Relationship between α and β

This derivation is frequently asked in exams.

To design an amplifier, engineers rely on the empirical graphs of the transistor's behavior. The Common Emitter (CE) characteristics are the most important.

⇒1. Input Characteristics

A graph of Base Current (IB) vs Base-Emitter Voltage (VBE), keeping VCE constant.

• Because the E-B junction is forward-biased, the input curve looks exactly like a standard Forward-Biased Diode curve.
• IB is practically zero until VBE reaches the cut-in voltage (~0.7V for Silicon). After 0.7V, IB shoots up exponentially.

⇒2. Output Characteristics

A graph of Collector Current (IC) vs Collector-Emitter Voltage (VCE), plotted for several fixed values of IB.

• Saturation Region: Near the Y-axis (VCE < 0.2V). IC rises sharply and depends heavily on VCE. Transistor acts as a closed switch.
• Active Region: The flat, horizontal portion of the curves. IC is almost completely independent of VCE. IC is dictated entirely by IB (IC = β*IB). This flat region is where linear amplification happens.
• Cut-off Region: The area below the IB = 0 line. No collector current flows. Transistor acts as an open switch.

When a transistor is hooked up to a power supply (Vcc) and a load resistor (Rc), its operation is mathematically bounded by the external circuit. We represent this bound using the DC Load Line.

⇒The DC Load Line Equation

Applying Kirchhoff's Voltage Law to the output loop of a CE amplifier:

⇒The Quiescent Point (Q-Point)

The DC Load Line is physically drawn over the Output Characteristics graph. The exact point on this line where the transistor rests when NO AC signal is applied is called the Q-Point.

• For a perfect, undistorted amplifier, the Q-point MUST be located exactly in the geometric center of the DC load line.
• If the Q-point is too close to Saturation, the top of the AC output wave gets chopped off (clipped).
• If the Q-point is too close to Cut-off, the bottom of the AC output wave gets clipped.

Transistor Biasing is the process of setting up external DC resistors to guarantee the transistor establishes a perfectly stable Q-point in the center of the active region, BEFORE any AC audio signal is applied.

⇒The Thermal Runaway Nightmare

Why can't we just set the Q-point once and forget it? Because BJT parameters are highly unstable with temperature.

• As the transistor operates, it naturally heats up.
• In silicon, heat generates extra minority carriers, which drastically increases the leakage current (ICo). ICo doubles for every 10°C rise.
• Because IC = βIB + (1+β)ICo, an increase in ICo causes a massive increase in total Collector Current (IC).
• Higher IC means the transistor dissipates even MORE heat. More heat means even MORE ICo. This vicious, exponential cycle destroys the transistor in seconds. This is called Thermal Runaway.

⇒The Goal of a Biasing Circuit

A good biasing circuit must automatically detect when IC tries to increase due to temperature, and actively fight back by decreasing IB, thus holding IC perfectly constant. We measure this defensive capability using the Stability Factor (S).

⇒1. Fixed Bias (Base Resistor Method)

The simplest possible circuit. A single resistor (RB) connects the Base directly to the Vcc supply.

• Advantage: Extremely simple, uses minimum components.
• Disadvantage: Absolutely zero thermal stability. Because IB is rigidly fixed, if IC rises due to heat, the circuit does nothing to stop it. The Q-point shifts wildly. Thermal runaway is almost guaranteed.

⇒2. Collector-to-Base Bias

A slight improvement. The resistor RB is connected from the Collector terminal (instead of Vcc) to the Base.

• Mechanism: If heat causes IC to rise, the voltage drop across Rc increases. This means the voltage at the Collector drops.
• Because RB is fed from the Collector, a lower Collector voltage forces IB to decrease.
• A decreased IB immediately forces IC back down. This is negative feedback!
• Disadvantage: While it stabilizes temperature, this negative feedback also accidentally kills the AC amplification gain.

Also known as Universal Bias or Self-Bias. This is the absolute industry standard. 99% of all commercial BJT amplifiers use this exact circuit because it provides near-perfect thermal stability.

⇒The Circuit Design

• Two resistors (R1 and R2) form a voltage divider across Vcc to provide a rock-solid, constant voltage to the Base.
• A crucial resistor (RE) is placed on the Emitter leg.

⇒How it defeats Thermal Runaway

The Q-point is locked in place. The transistor is rendered completely immune to temperature changes and variations in Beta (β).

Once the transistor is properly DC biased (Q-point is stable), we inject the AC audio signal. To mathematically calculate exactly how much voltage gain the amplifier produces, engineers use the Hybrid (h-parameter) or re model.

⇒The AC Equivalent Circuit Rules

To analyze the AC behavior, we must strip away the DC components using the Superposition Theorem:

• 1. Kill all DC Sources: Treat the Vcc supply as a dead short to Ground (0V).
• 2. Short all Capacitors: Coupling and Bypass capacitors are chosen to have extremely low reactance at AC frequencies. We draw them as solid wires (short circuits).
• 3. Replace BJT with Model: The physical transistor is replaced with an equivalent circuit containing a dynamic input resistance (re) and a controlled current source (β * ib).

⇒The Dynamic Emitter Resistance (re)

The internal AC resistance of the forward-biased base-emitter junction.

Using the AC equivalent model of a standard Common Emitter amplifier (with the Emitter bypassed to ground via a capacitor), we can derive the formula for Voltage Gain (Av).

⇒The Derivation

⇒Significance of the Formula

• Massive Gain: Because Rc is usually in Kilo-ohms and re is in tiny ohms, the gain is huge (often >100).
• The Negative Sign: It proves mathematically that a Common Emitter amplifier inherently introduces a 180° phase shift. When the input audio wave goes UP, the output voltage wave goes DOWN.

⇒End-of-Unit Verification

• Draw the physical structure and schematic symbols for NPN and PNP transistors.
• List the 4 operating regions based on junction biasing and state the application of each (Switch vs Amplifier).
• Explain why the Base is made extremely thin and lightly doped, and the Collector is made physically largest.
• Write the fundamental current equation (IE = IB + IC) and define the current amplification factors Alpha (α) and Beta (β).
• Derive the mathematical relationship linking α and β.
• Explain why the Common Emitter (CE) configuration is universally preferred over CB and CC for amplification.
• Draw the Input and Output V-I characteristic curves for a CE configuration. Identify the Cut-off, Active, and Saturation regions.
• Define the DC Load Line and the Q-Point. Explain why the Q-point must be perfectly centered.
• Explain the destructive chain-reaction of Thermal Runaway.
• Draw the circuit diagram for Voltage Divider Bias and explain the step-by-step mechanism by which it actively stabilizes the Q-point against temperature changes.
• Draw the AC equivalent model of a CE amplifier and derive the formula `Av = - Rc / re`.