Transistor Amplifier
An amplifier is a circuit designed to enhance the strength of a signal. It takes an input signal, which can be either a current or voltage, and produces an output signal that is a boosted or amplified version of the input. When an amplifier circuit relies exclusively on transistors for its operation, it is referred to as a transistor amplifier. These transistor-based amplifiers find widespread use in various applications, including RF (radio frequency), audio, and OFC (optic fiber communication). However, one of the most common everyday applications we encounter is using transistors as audio amplifiers.
It’s important to note that there are three commonly utilized transistor configurations: common base (CB), common collector (CC), and common emitter (CE). The common base configuration offers a gain lower than one, while the common collector configuration, also known as the emitter follower, provides a gain almost equal to one. In contrast, the common emitter configuration delivers a positive gain greater than one. Consequently, the common emitter configuration is the most prevalent choice for audio amplifier applications. This article will delve deeper into the world of transistor amplifiers, exploring their characteristics and applications.
Parameters
A good transistor amplifier must have the following parameters;
- High input impedance
- High band width
- High gain
- High slew rate
- High linearity
- High efficiency
- High stability etc.
The above given parameters are explained in the next section.
Input impedance:
Input impedance refers to the impedance experienced by the input voltage source when it is linked to the input of the transistor amplifier. To ensure that the transistor amplifier circuit does not burden or affect the input voltage source, it is imperative for the transistor amplifier circuit to possess a substantial input impedance.
Bandwidth.
The range of frequencies within which an amplifier can effectively amplify a signal is known as the bandwidth of that specific amplifier. Typically, bandwidth is determined by measuring the half-power points, which are the frequencies at which the output power drops to half of its peak value on the frequency versus output power graph. Put simply, bandwidth is the difference between the lower and upper half-power points. For a quality audio amplifier, the desirable bandwidth spans from 20 Hz to 20 KHz, as this corresponds to the audible frequency range for the human ear. In the provided figure (Fig 3), you can observe the frequency response of a single-stage RC coupled transistor amplifier, with points P1 and P2 identified as the lower and upper half-power points, respectively.
Gain.
The amplifier’s gain is the measure of the ratio between its output power and input power, signifying how effectively it magnifies an incoming signal. This gain can be conveyed either as a straightforward numerical value or in decibels (dB). In numerical terms, gain is calculated using the equation G = Pout / Pin, where Pout represents the output power, and Pin signifies the input power. In decibels, gain is denoted by the formula Gain in dB = 10 log (Pout / Pin).
Furthermore, gain can also be expressed concerning the output voltage relative to the input voltage or the output current relative to the input current. To articulate voltage gain in decibels, the equation Av in dB = 20 log (Vout / Vin) is employed, whereas for current gain in dB, the equation Ai = 20 log (Iout / Iin) is utilized.
Derivation of gain.
G = 10 log ( Pout / Pin)………(1)
Let Pout = Vout / Rout and Pin = Vin / Rin. Where Vout is the output voltage Vin is the input voltage, Pout is the output power, Pin is the input power, Rin is the input voltage and Routis the output resistance. Substituting this in equation 1 we have
G = 10log ( Vout²/Rout) / (Vin²/Rin)………….(2)
Let Rout = Rin, then the equation 2 becomes
G = 10log ( Vout² / Vin² )
i.e.
G = 20 log ( Vout / Vin )
Efficiency.
Amplifier efficiency gauges how effectively the amplifier utilizes the power it draws from the power supply. In simpler terms, it quantifies the proportion of power taken from the power supply that is effectively converted into the amplifier’s output. Efficiency is typically represented as a percentage, and the formula is expressed as ζ = (Pout / Ps) x 100, where ζ denotes the efficiency, Pout stands for the power output, and Ps represents the power drawn from the power supply.
Class A transistor amplifiers exhibit an efficiency of up to 25%, Class AB amplifiers can achieve up to 55% efficiency, and Class C amplifiers can attain an impressive 90% efficiency. While Class A amplifiers excel in signal reproduction, their efficiency is notably low. Conversely, Class C amplifiers boast high efficiency but compromise signal reproduction quality. Class AB amplifiers strike a balance between the two, making them a common choice for audio amplifier applications.
Stability.
Stability refers to an amplifier’s ability to withstand and prevent unwanted oscillations. These oscillations can manifest as high-amplitude disturbances that obscure the desired signal or as extremely low-amplitude, high-frequency fluctuations across the spectrum. Typically, stability issues arise when operating at high frequencies, especially near 20KHz in the context of audio amplifiers. Enhancing stability can be achieved by incorporating measures such as adding a Zobel network at the output or implementing negative feedback.
Slew rate.
The slew rate of an amplifier is the highest rate at which the output changes over a given period. It signifies the amplifier’s capacity to respond swiftly to alterations in the input signal. In more straightforward terms, it indicates the speed or rapidity of the amplifier’s response. Typically, the slew rate is denoted in units of V/μS, and it is calculated using the equation SR = dVo/dt, where SR represents the slew rate, dVo stands for the change in output voltage, and dt is the change in time.
Linearity.
An amplifier is deemed linear when there exists a direct and consistent connection between the input power and the output power, signifying the constancy of its gain. Achieving complete 100% linearity is virtually unattainable because amplifiers employing active components such as BJTs, JFETs, or MOSFETs typically experience a loss of gain at higher frequencies due to internal parasitic capacitance. Additionally, the presence of input DC decoupling capacitors, which is a common feature in nearly all practical audio amplifier circuits, introduces a lower cutoff frequency.
Noise.
Noise encompasses undesired and unpredictable disruptions within a signal. In more straightforward terms, it can be described as undesired variations or irregular frequencies within a signal. These disturbances can originate from various sources, including design imperfections, component malfunctions, external interferences, interactions between multiple signals within a system, or specific components utilized in the circuit.
Output voltage swing.
Output voltage swing represents the utmost extent over which an amplifier’s output can oscillate. This measurement is taken from the highest positive peak to the lowest negative peak. In single-supply amplifiers, it is gauged from the highest positive peak to the ground reference. The range of output voltage swing is typically contingent on factors such as the supply voltage, biasing configuration, and component specifications.
Common emitter RC coupled amplifier.
The common emitter RC coupled amplifier stands as one of the most basic and fundamental transistor amplifiers that can be constructed. While it may not generate significant amplification, its primary role is pre-amplification, meaning it elevates feeble signals to a level suitable for subsequent processing or further amplification. When meticulously designed, this amplifier can deliver exceptional signal attributes. You can observe the schematic diagram of a single-stage common emitter RC coupled amplifier employing a transistor in Figure 1.
Capacitor Cin serves as the input DC decoupling component, effectively obstructing any potential DC component within the input signal from reaching the base of transistor Q1. If an external DC voltage manages to reach the base of Q1, it can disrupt the biasing conditions, thereby impacting the amplifier’s performance.
R1 and R2 collectively form the biasing resistor network, furnishing the base of transistor Q1 with the essential bias voltage required to drive it into the active region of operation. The transistor has distinct regions of operation: the cut-off region, where it is entirely turned off; the saturation region, where it behaves like a closed switch and is fully turned on; and the active region, which lies between cut-off and saturation. You can gain a clearer understanding of these regions by referring to Figure 2. To ensure proper functioning, a transistor amplifier must operate within the active region.
Consider a scenario without any biasing for the transistor. As commonly known, a silicon transistor necessitates 0.7 volts to turn on. This 0.7 V will be derived from the input audio signal by the transistor. Consequently, any portions of the input waveform with amplitudes ≤ 0.7V will be absent in the output waveform. Conversely, if a heavy bias is applied to the transistor’s base, it will enter saturation (fully on) and mimic a closed switch. In this state, further variations in the base current due to the input audio signal will no longer affect the output. At this juncture, the voltage across the collector and emitter will be 0.2V (Vce sat = 0.2V). This underscores the significance of appropriate biasing for the reliable operation of a transistor amplifier.
Cout, serving as the output DC decoupling capacitor, serves the purpose of preventing any DC voltage from transferring from the current stage to the subsequent one. Its omission would result in the amplifier’s output (Vout) being constrained by the DC level existing at the transistor’s collector.
Rc denotes the collector resistor, while Re is the emitter resistor. The selection of Rc and Re values is carried out in a manner that ensures approximately 50% of Vcc drops across the collector and emitter of the transistor. This choice aims to position the operating point at the midpoint of the load line. Specifically, about 40% of Vcc is allocated to Rc, and roughly 10% of Vcc is allotted to Re. Maintaining a 10%Vcc voltage drop across Re is customary practice as a more substantial voltage drop across Re would diminish the output voltage swing.
Ce, acting as the emitter by-pass capacitor, plays a critical role in the amplifier’s functionality. Under zero signal conditions (i.e., no input), only the quiescent current, determined by the biasing resistors R1 and R2, flows through Re. This current is a direct current of a few milliamperes, and Ce remains inactive. However, when an input signal is introduced, the transistor amplifies it, leading to the emergence of a corresponding alternating current through Re. Ce’s function is to shunt this alternating component of the emitter current. Without Ce, the entire emitter current would traverse through Re, resulting in a substantial voltage drop across it. This voltage drop would then be added to the Vbe of the transistor, leading to a modification in the bias settings. Essentially, this scenario resembles imposing significant negative feedback, substantially reducing the amplifier’s gain.
Design of RC coupled amplifier.
The design of a single stage RC coupled amplifier is shown below.
The nominal vale of collector current Ic and hfe can be obtained from the datasheet of the transistor.
Design of Re and Ce.
Let voltage across Re; VRe = 10%Vcc ………….(1)
Voltage across Rc; VRc = 40% Vcc. ……………..(2)
The remaining 50% will drop across the collector-emitter .
From (1) and (2) Rc =0.4 (Vcc/Ic) and Re = 01(Vcc/Ic).
Design of R1 and R2.
Base current Ib = Ic/hfe.
Let Ic ≈ Ie .
Let current through R1; IR1 = 10Ib.
Also voltage across R2 ; VR2 must be equal to Vbe + VRe. From this VR2 can be found.
There fore VR1 = Vcc-VR2. Since VR1 ,VR2 and IR1 are found we can find R1 and R2 using the following equations.
R1 = VR1/IR1 and R2 = VR2/IR1.
Finding Ce.
Impedance of emitter by-pass capacitor should be one by tenth of Re.
i.e, XCe = 1/10 (Re) .
Also XCe = 1/2∏FCe.
F can be selected to be 100Hz.
From this Ce can be found.
Finding Cin.
Impedance of the input capacitor(Cin) should be one by tenth of the transistors input impedance (Rin).
i.e, XCin = 1/10 (Rin)
Rin = R1 parallel R2 parallel (1 + (hfe re))
re = 25mV/Ie.
Xcin = 1/2∏FCin.
From this Cin can be found.
Finding Cout.
Impedance of the output capacitor (Cout) must be one by tenth of the circuit’s output resistance (Rout).
i.e, XCout = 1/10 (Rout).
Rout = Rc.
XCout = 1/ 2∏FCout.
From this Cout can be found.
Setting the gain.
Introducing a suitable load resistor RL across the transistor’s collector and ground will set the gain. This is not shown in Fig1.
Expression for the voltage gain (Av) of a common emitter transistor amplifier is as follows.
Av = -(rc/re)
re = 25mV/Ie
and rc = Rc parallel RL
From this RL can be found.
Wrap Up!
So we’ve seen so much in detail about transistor amplifiers and how they function. We’ve also seend the theory part, underlying calculations and the concepts. Apply this in your learning curve.
