Unit - 3 Switching Characteristics of Diodes and Transistors.pdf
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The document discusses the switching characteristics of diodes and transistors, including their turn on/off times essential in electronic devices and applications. It also explains various types of multivibrators (bistable, monostable, and astable), clippers and clampers, and differential amplifiers, elaborating on their operations, applications, and importance in electronic circuits. Additionally, it covers the calculation of differential gain, common-mode gain, and common-mode rejection ratio (CMRR) which are critical for ensuring accurate measurements in signal processing.
Unit - 3 Switching Characteristics of Diodes and Transistors.pdf
- 2. R G P V द े B u n k e r s Unit—3: Switching Characteristics of Diodes and Transistors 1. Switching Characteristics of Diode and Transistor 1.1 Switching Characteristics of Diode and Transistor Turn ON/OFF Time Switching characteristics are vital for understanding the behavior of electronic devices like diodes and transistors when transitioning between different states. These characteristics are particularly relevant in digital circuits and switching applications. Diode Switching Characteristics: ● Turn ON Time: When a diode is initially in the OFF state (reverse-biased) and a forward voltage is applied across it, it takes a certain time to overcome the depletion region’s barrier and allow current flow. This time interval is known as the turn ON time of the diode. ● Turn OFF Time: Conversely, when a diode is in the ON state (forward-biased) and the voltage polarity across it is reversed, it takes a certain time for the diode to cease conducting current. This duration is called the turn OFF time of the diode. The switching times of diodes are essential in high-frequency rectifiers, switching power supplies, and signal processing applications. Transistor Switching Characteristics: ● Turn ON Time: In the context of transistors, the turn ON time refers to the time it takes for the transistor to switch from its OFF state to the ON state. This process occurs when the base-emitter junction is forward-biased sufficiently to allow current to flow between the collector and the emitter. ● Turn OFF Time: The turn OFF time of a transistor is the time taken for the transistor to switch from the ON state to the OFF state. This happens when the base-emitter junction is reversed biased, halting the current flow through the collector-emitter path.
- 3. R G P V द े B u n k e r s Transistors are commonly used as switches in digital logic circuits, amplifiers, and power control circuits. 1.2 Reverse Recovery Time of Diode When a diode is conducting current in the forward-biased state, and the applied voltage across the diode is suddenly reversed to make it reverse-biased, the diode does not instantaneously stop conducting. Instead, it takes a short period to transition from the conducting state to the non-conducting state. This time interval is known as the “Reverse Recovery Time” of the diode. During this time, the excess charge carriers present in the diode’s semiconductor regions need to recombine before the diode can completely block the current in the reverse direction. The reverse recovery time is a critical parameter in power diodes used in high-frequency rectifiers and switching circuits. It affects the diode’s switching efficiency and power loss during high-speed switching. 1.3 Transistor as a Switch Transistors, especially bipolar junction transistors (BJTs) and field-effect transistors (FETs), can be utilized as electronic switches in various applications. BJT as a Switch: ● When the BJT is in the ON state (active region), it acts as a closed switch, allowing current to flow freely between the collector and emitter terminals. ● When the BJT is in the OFF state (cutoff region), it acts as an open switch, effectively blocking any current flow between the collector and emitter terminals. BJTs are commonly used as switches in digital circuits, amplifiers, and signal processing applications. FET as a Switch: ● In the ON state, the FET operates in its saturation region, allowing a significant current flow between the source and drain terminals. ● In the OFF state, the FET operates in its cutoff region, effectively blocking any current flow between the source and drain terminals. FETs find applications in power control, low-power digital circuits, and switching applications.
- 4. R G P V द े B u n k e r s 2. Multi-vibrators 2.1 Bistable Multivibrator A bistable multivibrator, also known as a flip-flop, is a digital circuit capable of maintaining two stable states indefinitely until an external trigger is applied to change its state. The bistable multivibrator has two outputs, commonly denoted as Q and Q ̅ (complement of Q). Operation: The bistable multivibrator can be constructed using various configurations, such as cross-coupled transistors, NAND gates, or NOR gates. When one output (e.g., Q) is HIGH (logic 1), the other output (e.g., Q ̅ ) will be LOW (logic 0), and vice versa. Once set in one of the stable states, it remains in that state until an external trigger is received, causing it to switch to the other stable state. Applications: Bistable multivibrators are widely used in digital circuits for memory elements, such as D flip-flops, T flip-flops, JK flip-flops, and SR flip-flops. They also form the fundamental building blocks of counters, registers, and state machines in digital systems. 2.2 Monostable Multivibrator A monostable multivibrator, also known as a one-shot multivibrator, has only one stable state and requires an external trigger to switch to the unstable state momentarily. After a predefined time duration, it returns to its stable state automatically. Operation: The monostable multivibrator circuit includes one energy storage element, typically a capacitor. When triggered, the capacitor charges or discharges to change the state of the circuit. The output remains in the unstable state for a fixed duration determined by the values of external resistors and capacitors connected to it. Applications: Monostable multivibrators are used in applications requiring a precise time delay or pulse generation. They are used in timers, pulse generators, debouncing circuits, and time-delay relays. 2.3 Astable Multivibrators Astable multivibrators are free-running oscillators that continuously switch between two unstable states without any external triggering.
- 5. R G P V द े B u n k e r s Operation: Astable multivibrators do not have any stable state, unlike bistable and monostable multivibrators. The circuit continuously alternates between its two unstable states, generating a continuous square wave or pulse train at its output. Applications: Astable multivibrators are widely used in applications like clock generation, frequency division, tone generation, and pulse-width modulation (PWM) for motor control and power electronics. 3. Clippers and Clampers 3.1 Clippers Clippers, also known as limiters, are electronic circuits used to clip or remove a portion of an input signal that exceeds a specified voltage level. Positive and negative clippers can be employed to shape the waveform according to the desired output. Positive Clippers: A positive clipper removes the positive portion of the input signal that exceeds a predetermined voltage level. The output waveform will only show the negative part of the input signal below the clipping voltage. Negative Clippers: A negative clipper removes the negative portion of the input signal that falls below a specified voltage level. The output waveform will only display the positive part of the input signal above the clipping voltage. Applications: Clippers are widely used in audio and video signal processing, waveform shaping, and noise elimination in communication systems. 3.2 Clampers Clampers, also known as DC restorers or level shifters, are circuits used to add or “clamp” a DC level to an AC waveform. The purpose of clampers is to set a reference voltage level for the AC signal. This is particularly useful when the AC signal is to be further processed, and any DC offset needs to be removed. Positive Clampers: A positive clamper adds a positive DC level to the input signal. It shifts the entire waveform upwards, ensuring that the most negative point of the waveform aligns with the reference DC level.
- 6. R G P V द े B u n k e r s Negative Clampers: A negative clamper adds a negative DC level to the input signal. It shifts the entire waveform downwards, aligning the most positive point of the waveform with the reference DC level. Applications: Clampers are commonly used in display systems, where AC-coupled signals need to be restored to a specific voltage level for proper visualization. They are also used in communication systems and signal processing applications. 4. Differential Amplifier and CMRR Calculation 4.1 Differential Amplifier A differential amplifier is a type of electronic amplifier that amplifies the difference between two input signals while rejecting any signals that are common to both inputs (common-mode signals). Differential amplifiers are critical components in various electronic systems, especially in communication and instrumentation applications. Operation: A differential amplifier usually consists of two transistors, often implemented with BJTs or FETs, and resistors connected in a differential configuration. The input signals are applied to the bases (for BJTs) or gates (for FETs) of the transistors, and the amplified output is taken from the collectors (for BJTs) or drains (for FETs). The differential amplifier amplifies the voltage difference between the two input signals while attenuating any signal that is common to both inputs. Applications: Differential amplifiers are extensively used in balanced communication systems, audio signal processing, instrumentation amplifiers for precise measurements, and in various sensor interfaces. 4.2 Calculation of Differential Gain, Common Mode Gain, and CMRR using h-parameters In differential amplifier analysis, h-parameters (also known as hybrid parameters or two-port parameters) are commonly used to relate the input and output voltages and currents of the amplifier. Differential Gain: The differential gain, often denoted by Ad, is the ratio of the change in the output voltage to the change in the differential input voltage. It quantifies how effectively the amplifier amplifies the difference between the two input signals. Common Mode Gain: The common-mode gain, often denoted by Ac, is the ratio of the change in the output voltage to the change in the common-mode input voltage. It indicates how much
- 7. R G P V द े B u n k e r s the amplifier amplifies any input signal that appears simultaneously and identically at both inputs. Common Mode Rejection Ratio (CMRR): The CMRR is a crucial parameter that quantifies the differential amplifier’s ability to reject common-mode signals. It is calculated by dividing the differential gain (Ad) by the common-mode gain (Ac). CMRR is typically expressed in decibels (dB). Importance: CMRR is particularly crucial in applications where the differential amplifier needs to amplify small differential signals while rejecting common-mode noise effectively. High CMRR ensures accurate and noise-free measurements in various sensor and communication systems. By mastering these topics, students will gain a comprehensive understanding of electronic devices, their behavior, and practical applications in modern engineering. These fundamental concepts form the backbone of electronic circuit design and analysis and are essential for any aspiring computer science engineer.