Transistor amplifier circuit

Types of Transistor Amplifier Circuits

A transistor amplifier circuit is intended to make the output signal's amplitude greater than the input signal's amplitude without changing its strength. Aline, the purpose of a transistor amplifier is to increase the signal's power for further processing or transmission. Amplifiers are frequently employed in communication circuits, audio devices, and electronic test gear.

Different amplifier circuit types exist, each of which has advantages that make it suited for particular applications.

• Class A Amplifier: This amplifier uses transistors that are always active, even when there is no input signal. The positive and negative halves of the input signal are amplified equally for both AC current. Even though this circuit consumes too much current and is inefficient, it produces a clear output for low power.
• Class B Amplifier: It uses two complementary transistors, one of which conducts during the positive half and the other during the negative half of the input AC signal. The transistors function alternately, reducing the current drain.
• Class AB Amplifier: This is a hybrid amplifier of class A and class B. The two transistors conduct during the input signal's zero cross, just like in the class A amplifier. However, just like in the class B amplifier, it only uses one half of the AC cycle and is more power-saving. The power consumption of this amplifier is less than that of the class A amplifier, but it is greater than that of the class B amplifier. The output signal is, however, less distorted than in the class B amplifier. The biasing is stable for various temperature ranges.
• Inverter Amplifier: It generates an output signal that is the input signal's inverse or opposite. If the input signal is positive, the output will be negative, but if the input signal is negative, the output will be positive. The voltage gain is greater than unity for this circuit. It has at least one FET.
• Non-Inverter Amplifier: This circuit makes the output voltage and input voltage proportional by increasing the voltage level without changing the signal. If the input signal is positive, the output signal will also be positive, and if the input signal is negative, the output will be negative. For silicon transistors, the minimum voltage drop is 0.6 volts, and for germanium, it is 0.3 volts.
• Emitter Follower: It has both input and output signals. The output voltage follows the input voltage, thus its name. Class A is where it belongs. It is also known as a voltage amplifier because it raises the input voltage without raising the output voltage.

Function and features of transistor amplifier circuit

Transistor amplifiers are crucial for enhancing weak electrical signals in many electronic devices.

• Signal Amplification

  A transistor amplifier circuit uses transistors to boost a signal's power. By controlling a high-power output with a low-power input, transistors increase a signal's voltage, current, or both to enable further processing.

• Analog signal processing

  Transistor amplifiers can filter, mix and modulate analog signals in radios, televisions, audio equipment, and communication devices. They shape and transmit messages in analog form across different media.

• Switching applications

  A fast transistor amplifier circuit can switch electrical signals on and off rapidly to encode information in digital form. Transistors act as electronic switches in digital circuits, memory devices, computers, and all modern electronic systems.

• Oscillation generation

  Some transistor circuits can produce oscillating signals at certain frequencies. These circuit oscillators, pulse generators, and frequency modulators use transistors to create waves that generate radio, audio, or clock signals.

• Signal buffering

  Transistor amplifiers buffer signals from high-impedance sources like pickups and low-impedance loads such as power amplifiers. Buffering prevents loading effects that degrade the original signal during transmission between stages of processing.

Scenarios

• Audio and Music Applications:

  Transistor amplifier circuits are vital in enhancing weak audio signals to more potent signals for loudspeakers. This process allows users to enjoy music and sounds with more magnified volume and clarity.

• Broadcasting and Communication:

  These circuits also play a significant role in radio and television transmission. They help in the modulation and amplification of signals for the effective transmission of waves over vast distances. The circuit can also amplify signals in telecommunication devices, which makes it easy to encode and decode signals for clearer voice communication.

• Testing Instruments:

  Transistor amplifier circuits are useful in testing and measuring equipment. They boost signals in oscilloscopes, spectrum analyzers, and signal generators for more accurate readings and signal manipulation in electronic labs and testing facilities.

• Medical Equipment:

  These circuits are essential in medical gadgets like EEG. They help in the rectification and amplification of weak electrical signals from the heart for more accurate diagnosis and monitoring of patients.

• Consumer Electronics:

  Transistor amplifier circuits are found in various consumer electronic devices like televisions and smartphones. They enhance audio signals for better sound quality when users listen to music or during calls.

• Instrumentation and Control:

  Various industrial control systems use transistor amplifier circuits. These circuits amplify sensor signals for pressure, temperature, and humidity. The signals are then used for effective monitoring and control of industrial processes.

How to Choose Transistor Amplifier Circuits

Choosing the right transistor amplifier circuit to fit a specific application or requirement can be a challenge. However, by taking a few key factors into account, it's possible to make an informed decision.

• Gain Requirements: First and foremost, it is important to determine the gain that is needed for a specific application. The gain needed for different applications can be different; for instance, intercom systems may require a higher gain compared to the one used in a radio. Knowing the kind of application is critical as it influences the kind of circuit chosen.
• Input and Output Impedance: The input and output impedance of the source and load respectively must be considered. An input and output impedance that can be matched well with the source and load will ensure maximum power transfer and good signal integrity. To know the right transistor amplifier circuit to choose, one must determine the impedance of the source and load.
• Voltage and Current Requirements: It's important to identify the voltage and current requirements for the power supply and the load. This will help narrow down the options of available circuits.
• Power Supply Voltage: The amplifier circuit's power supply voltage must be compatible with that of the application. This is to say that the maximum voltage should not exceed the maximum voltage capacity of the transistor and the minimum voltage must not go below the transistor's cutoff voltage.
• Transistor Type: When choosing an amplifier circuit, the kind of transistor, i.e., BJTs or MOSFETs must be taken into account. The reason is that each has its own set of pros and cons. For example, bipolar junction transistors offer better current gain and are usually available in smaller packages. On the other hand, MOSFETs have higher input impedance and lower noise, making them a suitable choice for voltage-sensitive applications.
• Number of Stages: Amplifier circuits can come in one stage or multi-stage. Choosing between the two depends on the application requirement because a multi-stage circuit can provide the desired gain and sensitivity.
• Power Output: This can either be low, medium or high. A low power output means that the output power is lower than 1 watt, a medium output is between 1 and 10 watts, while a high output is anything higher than that. Knowing the kind of output power the application requires will help narrow down options.
• Other Features: When the need arises, some amplifier circuits come with additional features like tone control, volume control and filtering which are useful for an audio amplifier.

Q&A

Q: What is the purpose of a transistor in an amplifier?

A: Transistors are semiconductor devices that can amplify electric signals. They form the core of most modern amplifiers. Transistors increase the current of low-power input signals and drive the output stage.

Q: Why is feedback used in amplifier circuits?

A: Feedback circuits stabilize the amplifier by controlling gain and bandwidth. They improve performance and make circuits less sensitive to component variations.

Q: What is the purpose of coupling in amplifier circuits?

A: Coupling circuits allow signal variations to pass while blocking DC voltages. They connect different amplifier stages and prevent one stage's DC level from affecting the next.

Q: What kind of power supply is required for transistor amplifiers?

A: Transistor amplifier circuits usually run on low-voltage power supplies. These can be batteries or mains power supplies that have been stepped down to safe levels.