_lab quiz note

Module 1: Bipolar Junction Transistor (BJT) Amplifiers

1.1 Common Emitter (CE) Configuration

The CE amplifier is the most widely used configuration because it provides both voltage and current amplification.

Terminals & Biasing: The input AC signal is applied to the base (relative to ground/emitter), and the output is taken from the collector (relative to ground/emitter). For active linear amplification, the base-emitter junction must be forward-biased (approx. 0.6V–0.7V) and the collector-base junction must be reverse-biased.
Voltage Gain ( $A_{v}$ ): Typically much greater than unity ( $A_{v} > 1$ ).
Phase Shift: It produces a 180° phase inversion between the input and output signals. As the input voltage increases, the collector current increases, which causes a larger voltage drop across the collector resistor, thereby lowering the output voltage at the collector.
Impedance: The overall output impedance is roughly equal to the value of the collector resistor ( $R_{C}$ ).

1.2 Common Collector (CC) / Emitter Follower Circuit

The CC configuration is primarily used as a buffer or impedance matching device.

Terminals: The input is applied between the base and ground, and the output is taken across the emitter and ground.
Voltage Gain ( $A_{v}$ ): Always less than unity (slightly less than 1). This is because the emitter voltage strictly “follows” the base voltage minus the 0.7V diode drop.
Phase Shift: The output is exactly in-phase (0° phase shift) with the input signal.
Impedance: It features an exceptionally high input impedance and low output impedance, allowing it to safely connect a high-impedance source to a low-impedance load without signal loss.

1.3 The Darlington Pair

A Darlington pair consists of two cascaded transistors where the emitter of the first is tied directly to the base of the second, and their collectors are tied together.

Current Gain ( $β$ ): This configuration provides the highest current gain possible, equal to the mathematical product of their individual betas ( $β_{T o t a l} = β_{1} \times β_{2}$ ).
Beta Calculation: Beta ( $β$ ) is calculated as the ratio of the change in collector current to the change in base current ( $Δ I_{C} /Δ I_{B}$ ).
Impedance: It reflects an extraordinarily high input impedance and extremely low output impedance, functioning essentially as a “super” emitter follower.

Module 2: Multistage Amplifiers & Coupling Methods

2.1 RC Coupled Amplifiers

RC coupling connects the output of one amplifier stage to the input of the next using a coupling capacitor.

Overall Voltage Gain: The total voltage gain of a cascaded amplifier is the product of the individual gains of each stage ( $A_{t o t a l} = A_{1} \times A_{2}$ ).
Loading Effects: Adding an external load to the output heavily decreases the overall voltage gain. The AC load resistance for the first stage is calculated as its own collector resistor in parallel with the input resistance of the second stage.
Coupling Capacitors: Electrolytic coupling capacitors must be connected with proper polarity, have low AC reactance at operating frequencies to allow the signal to pass, and possess low DC leakage to prevent the bias of one stage from disrupting the next.
Emitter Bypass Capacitor: Adding a capacitor across the emitter resistor acts as an AC short circuit. This eliminates negative feedback, which significantly increases the frequency response, voltage gain, and emitter current.
Frequency Response: The response curve is relatively flat over the mid-band (audio) frequencies. The gain drops off at low frequencies due to the reactance of the coupling/bypass capacitors, and at high frequencies due to parasitic transistor capacitances.

2.2 Direct Coupled Amplifiers

Direct coupling wires the output of one stage directly to the input of the next without any capacitors or transformers.

Component Efficiency: It naturally requires fewer components than RC or transformer coupling and eliminates both capacitive and inductive reactance between stages.
Frequency Response: Direct coupling provides excellent low-frequency response, capable of amplifying signals all the way down to DC (0 Hz).
DC Biasing & Stability: Complementary pairs (an NPN driving a PNP) are frequently used to make DC biasing easier, allowing the voltages to stack appropriately. A major disadvantage is that it is highly temperature-sensitive; any change or drift in the DC bias of the first stage changes the collector voltage of that stage and is amplified by all subsequent stages.

Module 3: Power Amplifiers

3.1 Amplifier Classifications

Power amplifiers are classified by the portion of the AC cycle during which the transistor conducts:

Class A: Conducts for the full 360° of the cycle. Low efficiency (25%-50%) because power is continually dissipated as heat even with no signal.
Class B: Biased exactly at cutoff. Conducts for exactly 180° (half-cycle). Much higher efficiency (up to 78.5%) but creates severe distortion on its own.
Class AB: Conducts for slightly more than 180° to eliminate crossover distortion. Efficiency falls between Class A and B.
Class C / D: Class C conducts for less than 180° (used in tuned communications). Class D uses digital pulse operation with >90% efficiency.

3.2 Complementary Power Amplifier (Push-Pull)

Operation: Uses matched complementary transistors (one NPN and one PNP). The NPN handles the positive half-cycle (“push”) and the PNP handles the negative half-cycle (“pull”).
Advantages: It can produce push-pull operation and completely eliminates the need for bulky input and output transformers, keeping the output impedance naturally low.
Class AB Biasing: If unbiased (Class B), a “dead zone” occurs where signals between -0.7V and +0.7V are ignored, resulting in crossover distortion. To fix this, the amplifier is normally slightly forward-biased into Class AB operation so that both transistors constantly idle just on the edge of conduction.

Module 4: Field Effect Transistors (JFETs & MOSFETs)

4.1 JFET Voltage Amplifiers

JFETs are voltage-controlled devices characterized by extremely high input impedances.

Biasing Principles: The gate-source internal diode must always remain reverse-biased (e.g., the gate must be negative relative to the source for an N-channel JFET). If the input signal exceeds this bias and forward-biases the junction, gate current will flow, input impedance will crash, and severe distortion will result.
Self-Bias Circuit: A highly popular biasing method where the necessary negative gate bias voltage is naturally developed as a voltage drop across the source resistor ( $R_{S}$ ).
Stabilization vs. Gain: Leaving the source resistor unbypassed provides negative feedback that stabilizes the circuit against inherent variations in transistor characteristics. However, bypassing this source resistor with a capacitor maximizes the stage’s voltage gain.
Voltage Gain Factors: The overall gain of a JFET is strictly dependent on the device’s transconductance ( $g_{m}$ ), the external load resistance, and the chosen gate bias.

4.2 MOSFET Voltage Amplifiers

MOSFETs have their gates physically insulated from the channel by a layer of Silicon Dioxide ( $S i O_{2}$ ), leading to even higher input impedances than JFETs. Varying the gate bias neither increases nor decreases this input impedance; it is permanently maintained by the insulation.

Depletion-Mode MOSFETs: These are “Normally ON” devices and are biased very similarly to JFETs. However, because they inherently possess both depletion and enhancement characteristics, they can uniquely be operated directly at zero bias ( $V_{GS} = 0 V$ ). In a P-channel depletion MOSFET, the gate must be biased positive to restrict/reduce the channel current. For maximum voltage gain, the source resistor should be bypassed.
Enhancement-Mode MOSFETs: These are “Normally OFF” devices. To be used as a linear amplifier, the gate strictly requires a forward bias voltage (greater than the threshold) to induce a channel and turn the device on.

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