Class A amplifier Theory
A Class A amplifier is the simplest and most fundamental type of power amplifier, widely recognized for its excellent linearity and low distortion.
It has poor efficiency, as the device draws a constant current and dissipates significant power even without a signal. Despite these limitations, Class A remains a cornerstone in amplifier design valued for its simplicity, and role in building foundational understanding of amplifier operation.
Definition and Working principle
A Class A amplifier is a type of electronic amplifier in which the output device (such as a transistor or tube) conducts current during the entire cycle (360° conduction angle) of the input signal. This means it amplifies the full waveform of the input signal without turning off at any point during the cycle. The operating point (Q-point) is carefully biased in the center of the load line, which ensures that the active element never enters cutoff or saturation during normal operation, resulting in minimal distortion and high linearity.
Bias Point and Load Line Analysis
In a Class A amplifier, the device is biased at approximately the midpoint of its I–V curve (Bias point = VCC/2), allowing for a highest symmetrical voltage swing. This allows to achieve maximum signal swing and efficiency. This provides a useful first-order estimate of the DC operating point (Q-point), helping ensure maximum undistorted output in the design.
Equations :
Current through Rload:
$$I_R=\cfrac{V_{CC}-V_{CE}}{R_{load}}$$
Current through Collector of the transistor:
$$I_C=I_Se^{V_{BE}/V_t}\left(1+\cfrac{V_{CE}}{V_A}\right)$$
VA is the Early voltage associated with BJTs, VCE is collector to emitter voltage, VBE is the base to emitter voltage, Vt is kT/q which is equal to 25.6mV at 25oC.
As per KCL, IL=IC. So both the equations, can be plotted together over VCE as shown in above figure.
Derivation of Gain formula
The small signal gain can be calculated by considering the small signal linear model of the above circuit. The small signal model is valid for other types of transistors as well (e.g., MOSFETs, JFETs) and gives a fairly close estimate. Key components are transconductance (gm) model of transistor and output impedance representing the load circuit (RL). The current through RL is gm x vi. The output voltage appearing is vo = gm x vi x RL. Therefore,
$$\text{Gain}=\cfrac{v_o}{v_i}=g_mR_{L}$$
This estimate is a good approximate for large signals as well as long as output voltage swing is within the linear operating range of the transistor.
Linearity and distortion
Unlike other amplifier classes (B and C) which have dead zone and cross over distortion, a Class A amplifier’s transistor remains constantly conducting, therefore no dead-zone, ensuring the entire input signal is amplified.
Load configurations and Efficiency calculations
Class A amplifier can be configured in following ways depending on the applications. They have different efficiencies:
- Class A amplifier with resistive load (maximum efficiency of 25%)
- Class A amplifier with inductive load (maximum efficiency of 50%)
- Class A amplifier with transformer load (maximum efficiency of 50%)
Class A amplifier with resistive load
Efficiency :
$$\eta{}=\cfrac{\text{Average signal power delivered to Load}}{\text{Average power drawn from supply}}$$
Average power delivered to load ():
$$P_{avg,load}=\cfrac{V_{CC}^2}{8R_L}$$
Average power drawn from the power supply ():
$$P_{avg,sup}=\cfrac{V_{CC}^2}{2R_L}$$
$$\eta{}=\cfrac{V_{CC}^2/8R_L}{V_{CC}^2/2R_L}=25\%$$
Class A amplifier with inductive loading
Average power delivered to load RL (Derivation 3):
$$P_{avg,load}=\cfrac{1}{2}I_L^2R_L$$
Average power drawn from the power supply (Derivation 4):
$$P_{avg,load}=V_{CC}I_L$$
$$\eta{}=\cfrac{I_L^2R_L/2}{V_{CC}I_L}=\cfrac{I_LR_L}{2V_{CC}}$$