Want to create or adapt books like this? Learn more about how Pressbooks support
Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.
22 Class B Power Analysis
Learning Objective
The objective of this exercise is to examine large signal class B operation. A voltage follower will be investigated to determine output compliance, maximum load power, supplied DC power and efficiency. The effects of crossover distortion will be noted by comparing resistor and diode biasing schemes.
Theory Overview
The maximum output signal, or compliance, of a class B amplifier is determined by its AC load line. The peak to peak compliance is roughly equal to the total DC supply voltage(s). As two output devices are used, each conducting for half of the cycle, the quiescent current can remain low, unlike a class A amplifier. This results in vastly improved efficiency, theoretically up to 78.5%. The switchover from one transistor to the other is problematic and can result in crossover or notch distortion. To alleviate this, the transistors are given a small idle current so that each base-emitter junction is just about fully on. While resistors can be used to create this bias, trying to match the linear current-voltage characteristic of a resistor to the logarithmic characteristic of a PN junction is tricky. Consequently, another PN junction, namely a diode, is used instead. The diode will result in a more stable circuit which produces less notch distortion.
Equipment
(1) Dual adjustable DC power supply
model:
srn:
(1) DMM
model:
srn:
(1) Dual channel oscilloscope
model:
srn:
(1) Low distortion function generator
model:
srn:
(1) Distortion analyzer
model:
srn:
(1) Small signal NPN transistor (2N3904)
(1) Small signal PNP transistor (2N3906)
(2) Switching diodes (1N914 or 1N4148)
(1) 100 Ω resistor ¼ watt
actual:
(2) 220 Ω resistor ¼ watt
actual:
(2) 2.2 k Ω resistor ¼ watt
actual:
(2) 10 µF capacitor
actual:
(1) 100 µF capacitor
actual:
Schematics
Procedure
Resistor versus Diode Bias and Crossover Distortion
Consider the circuit of Figure 1 using Vcc = 6 volts, R1 = R2 = 2.2 kΩ, R3 = R4 = 220 Ω, Rload = 100 Ω, C1 = C2 = 10 µF and C3 = 100 µF. Ideally this circuit will produce a compliance of just under 6 volts peak-peak.
Build the circuit of Figure 1 using Vcc = 6 volts, R1 = R2 = 2.2 kΩ, R3 = R4 = 220 Ω, Rload = 100Ω, C1 = C2 = 10 µF and C3 = 100 µF. Disconnect the signal source and insert an ammeter into the collector of Q1. Record ICQ in Table 1.
Connect the signal source and apply a 1 kHz sine at 2 volts peak. Look at the load voltage and capture the oscilloscope image. There should be considerable notch or crossover distortion.
Cycle through the remaining supply voltages in Table 1, repeating steps 2 and 3. Only images of the first and last trials need be captured. As the bias current increases, the notch distortion should decrease.
Replace R3 and R4 with switching diodes, as shown in Figure 2. Repeat steps 2 through 4 using this circuit and Table 2. Overall, the superior matching of the di